Influenza in Canada:

2005-2006 season

Canada Communicable Disease Report

1 February 2007

Volume 33
Number 03

F Reyes, MHSc (1), JF Macey, MSc (1), S Aziz, MSc (1), Y Li, PhD (2), K Watkins, MHSc (1), B Winchester, MSc (1), P Zabchuk (1), H Zheng, MD, MSc (1), P Huston, MD, MPH (1), TWS Tam, MD, FRCPC (1), T Hatchette, MD, FRCPC (3)

  1. Immunization and Respiratory Infections Division, CIDPC, Public Health Agency of Canada, Ottawa, Ontario

  2. Influenza and Respiratory Viruses Section, National Microbiology Laboratory, Public Health Agency of Canada, Winnipeg, Manitoba

  3. Division of Microbiology, Department of Pathology and Laboratory Medicine, QEII Health Science Center, Halifax, Nova Scotia

Introduction

The Immunization and Respiratory Infections Division (IRID), Centre for Infectious Disease Prevention and Control (CIDPC), Public Health Agency of Canada (PHAC), coordinates a national influenza surveillance network through the FluWatch program. The primary objectives of the FluWatch program are early detection of influenza activity in Canada and abroad; monitoring of circulating strains of influenza virus, including antigenic characterization, antiviral resistance and identification of new sub-types; and provision of virologic surveillance information to theWorld Health Organization (WHO) global surveillance initiative to contribute to decision-making for the following season's vaccine components. This report provides an epidemiologic and virologic summary of influenza activity in Canada during the 2005-2006 season.

Methods

The FluWatch program consists of a network of sentinel laboratories, sentinel primary care practices, provincial and territorial ministries of health, and pediatric tertiary care hospitals. Five main indicators of influenza activity are reported by the network on a weekly basis across Canada and throughout the season: (1) sentinel laboratory-based influenza and other respiratory virus detections; (2) strain identification and antiviral resistance for circulating influenza viruses; (3) sentinel primary care consultation rates of influenza-like illness (ILI); (4) regional influenza activity levels as assigned by provincial and territorial FluWatch representatives; and (5) pediatric influenza-associated hospital admissions and mortality data. In addition, the FluWatch program conducts an assessment of international influenza activity by monitoring reports from other influenza surveillance programs worldwide. A brief summary of influenza surveillance indicators is provided below.

Respiratory virus detections

Respiratory virus detections are reported through the sentinel laboratory-based Respiratory Virus Detections Surveillance System (RVDSS), which operates year-round from weeks 35 to 34 (early September to late August). Participating laboratories report the total number of influenza tests performed and the total number of tests positive for influenza by virus type (A or B) to IRID, PHAC, on a weekly basis (aggregate data). Samples from the Yukon, Northwest Territories and Nunavut are sent to reference laboratories in nearby provinces and are included in the aggregated provincial results from the laboratory conducting the test.

On a less timely basis (bi-monthly to monthly), a proportion of the participating RVDSS laboratories report case-by-case information. These data, representing a subset of the cumulative weekly detections, provide additional epidemiologic and laboratory information for each case, including date of onset, age of case, and virus type and subtype. Laboratory methods used for the detection of influenza in the 2005-2006 season were virus culture, virus antigen detection (including direct fluorescent antibody (DFA), point-of-care (POC) and rapid antigen POC methods), nucleic acid amplification and, in a small number of cases, seroconversion.

Influenza virus strain identification

The National Microbiology Laboratory (NML), CIDPC, PHAC, conducts national surveillance on human influenza virus strains in collaboration with provincial laboratories and other Canadian hospital- and university-based laboratories. A proportion of the weekly influenza detections across Canada are referred to the NML for strain characterization. NML virologic surveillance detects and describes antigenic changes as well as antiviral resistance in the circulating influenza virus strains. Information from Canadian influenza virus surveillance and representative strains is shared with the WHO collaborating centres for influenza. In this way, Canada contributes to global influenza monitoring and decision-making for vaccine recommendations for the upcoming season and assesses the antigenic match between circulating and vaccine strains.

ILI consultations reported by sentinel practitioners

FluWatch maintains a network of sentinel primary care practitioners across the country. The College of Family Physicians of Canada, National Research System (NaReS), is responsible for recruiting and managing the participation of sentinel physicians and nurses in seven provinces and three territories across Canada. In the other three provinces (British Columbia, Alberta and Saskatchewan), sentinel recruitment and reporting is managed by independent provincial programs. The FluWatch program objective is to have at least one sentinel recruited from each of the census divisions across Canada. In the case of Quebec, where there are 99 census divisions, representative recruitment is accomplished by coverage of health regions (n = 18) rather than by census division. In addition, for the more densely populated census divisions/health regions, the objective is to have at least one sentinel recruited per 250,000 population.

For one clinic day each week, sentinels are asked to report the total number of patients seen for any reason (denominator) and the total number of patients meeting a standard national case definition for ILI (numerator). Age group information is also collected to allow for calculation and monitoring of age-specific ILI rates. In Alberta, however, age group information is collected only for the numerator, and age group information for the denominator is calculated by applying the Canada-wide age-specific population distribution. Sentinel report forms are either returned by fax, or the information is conveyed by e-mail or telephone to IRID on a weekly basis for data collation, analysis and dissemination. The majority of sentinels recruited each year report weekly data year round; however, a proportion of sentinels report weekly data only during the active influenza season (weeks 40 to 20).

Data from sentinels are weighted by the estimated population for each of the census divisions being represented during the respective week. This is done in order to produce a summary ILI rate for the Canadian population. Each week the weights are recalculated on the basis of the actual census divisions with data to report. Weighted rates are summed to create a national ILI rate each week. The mean ILI consultation rates and 95% confidence intervals from the 1996-1997 to 2004-2005 seasons for weeks 40 through 18 were calculated and serve as a baseline comparison.

Regional influenza activity levels assessed by provincial and territorial epidemiologists

Provinces and territories are subdivided into influenza surveillance regions. Provincial and territorial FluWatch representatives assess the weekly influenza activity level in their respective jurisdictions according to laboratory reports of influenza detections, ILI rates and reports of outbreaks * occurring in long-term care facilities, hospitals, schools and/or worksites. Influenza activity levels are reported to IRID as meeting one of four standard categories: no activity reported, sporadic activity, localized activity or widespread activity .

Influenza-related hospitalizations in children

Since the 2003-2004 influenza season, hospital-based surveillance of influenza in children has been reported to FluWatch through the Immunization Monitoring Program ACTive (IMPACT) network of pediatric tertiary care hospitals. This national network involves 12 centres across Canada, which account for approximately 90,000 admissions every year and represent over 90% of all tertiary care pediatric beds in the country. A trained nurse monitor at each participating centre identifies laboratoryconfirmed cases of influenza requiring admission to hospital. Upon identification, detailed case report forms are completed based on information from the hospital chart. Case reports are sent to the IMPACT data centre in Vancouver, British Columbia, for review, data entry and analysis. Starting from the 2004-2005 season, aggregate data on age group, influenza type and death due to influenza in children are reported to IRID on a weekly basis.

International

The FluWatch program also monitors and reports on international influenza activity, as assessed from official information sources (e.g. the WHO, national ministries of health) and international influenza surveillance reports (e.g. US Centers for Disease Prevention and Control in Atlanta [CDC], European Influenza Surveillance Scheme and theWHO's FluNet). Over the past several years, international monitoring of influenza and emerging respiratory infections for the FluWatch program has included a summary and risk assessment of human infections with the H5N1 avian influenza virus.

Dissemination

FluWatch disseminates information through weekly reports during the active influenza season and biweekly reports during the low season (mid-May to September). These are available to health professionals and the public through a variety of media, including fax, e-mail, and PHAC FluWatch Web site, http://www. phac-aspc.gc.ca/fluwatch/index-eng.php.

In addition, summaries of laboratory detection data on influenza, respiratory syncytial virus (RSV), parainfluenza and adenovirus are made available weekly throughout the year on the RVDSS Web site, http://www.phac-aspc.gc.ca/bid-bmi/dsd-dsm/rvdi-divr/index.html. Summaries of worldwide influenza activity are included periodically in the CIDPC Infectious Diseases News Brief, http://www.phac-aspc.gc.ca/bid-bmi/dsd-dsm/nb-ab/index.html, and annual summaries and periodic updates on influenza surveillance in Canada are published in the Canada Communicable Disease Report,
http://www.phac-aspc.gc.ca/ publicat/ccdr-rmtc/index.html.

Results

Influenza virus detections from the RVDSS (aggregate data)

Between 28 August, 2005, and 26 August, 2006, 33 laboratories across Canada reported a total of 87,303 influenza tests. Approximately 8.5% of the total tests reported (7,422/87,303) were positive for influenza. Of these, 61% (4,527/7,422) were determined to be influenza A, and 39% (2,895/7,422) were influenza B.

Influenza virus detections from the RVDSS (case-by-case data)

Twenty-five of the 33 RVDSS participating laboratories reported a total of 6,590 case-by-case records to IRID. Total influenza detection reports (aggregate weekly data) relative to total case-bycase records are presented in Table 1 by reporting province (total detections) and province or territory from which the specimen originated (case-by-case records). Since specimens from the territories are sent to nearby provincial laboratories for testing, total detections from the territories are rolled into the provincial aggregate counts but are separated out for the case-by-case record totals, based on geographic information available in the case-bycase data. The largest number and proportion of cases were reported by Quebec (25.7% or 1,695/6,590), Ontario (24.6% or 1,619/6,590) and Alberta (19.7% or 1,300/6,590).

Table 1. Aggregate and case-by-case influenza data, by province/territory or region, Canada, 2005-2006

 

Aggregate detections

Case-by-case data

Province/Territory or Region

Number of cases

% of total

Number of cases

% of total

Nfld.

78

1.1

77

1.2

P.E.I.

8

0.1

9

0.1

N.S.

116

1.6

116

1.8

N.B.

279

3.8

166

2.5

Atlantic

481

6.5

368

5.6

Que.

1,715

23.1

1,695

25.7

Ont.

2,317

31.2

1,619

24.6

Man.

90

1.2

95

1.4

Sask.

651

8.8

645

9.8

Alta.

1,351

18.2

1,300

19.7

Prairies

2,092

28.2

2,040

31.0

B.C.

817

11.0

718

10.9

Y.T.

 

 

55

0.8

N.W.T.

 

 

34

0.5

Nun.

 

 

61

0.9

Territories

 

 

150

2.3

Total

7,422

 

6,590

 

Several methods of testing were used for the diagnosis of influenza in respiratory specimens, virus isolation accounting for the greatest proportion of positive influenza detections (49.1% or 3,236/6,590). Other reported methods of laboratory confirmation included virus antigen detection (38.3% or 2,522/6,590), nucleic acid amplification (12.5% or 824/6,590) and to a minor extent, serologic testing (0.1% or 8/6,590).

Table 2. Case-by-case influenza data, by province/territory, influenza type, Canada, 2005-2006

Province or Territory

Type A

Type B

Total

Influenza Type

% of influenza A among case-by-case data

% of influenza B among case-by-case data

Nfld.

38

39

77

49.4

50.6

P.E.I.

4

5

9

44.4

55.6

N.S.

28

88

116

24.1

75.9

N.B.

51

115

166

30.7

69.3

Que.

1,449

246

1,695

85.5

14.5

Ont.

806

813

1,619

49.8

50.2

Man.

67

28

95

70.5

29.5

Sask.

367

278

645

56.9

43.1

Alta.

717

583

1,300

55.2

44.8

B.C.

410

308

718

57.1

42.9

Y.T.

16

39

55

29.1

70.9

N.W.T.

21

13

34

61.8

38.2

Nun.

54

7

61

88.5

11.5

Total

4,028

2,562

6,590

61.1

38.9

 

Of the 6,590 total influenza viruses isolated during 2005-2006, 61.1% (4,028/6,590) were identified as influenza A and 38.9% (2,562/6,590) as influenza B. Influenza A cases were reported from across Canada during the 2005-2006 season, the greatest proportion from Quebec (36.0% or 1,449/4,028), Ontario (19.2% or 806/4,208) and Alberta (17.0% or 717/4,208). Of the influenza B cases identified, the greatest proportions were reported from Ontario (31.7% or 813/2,562) and Alberta (22.8% or 583/2,562) (Table 2).

Nationally, 72% of all influenza cases began in the 10-week period between week 5 and week 14 (late January to early April of 2006), and roughly 50% of the cases began during the peak period from week 8 to week 13 (mid-February to end of March 2006), with onsets peaking during week 12 (n = 613) (Figure 1). Regional peaks in case-by-case data were also evident in all provinces but not in the territories, where relatively few cases were reported. The first peak reflected in the case-by-case data occurred during week 5 in the Prairies, followed by British Columbia in week 7 and Ontario in week 8. The Atlantic Region and Quebec were the last to reach peak activity, and this occurred during week 12. Similar proportions of influenza A and B viruses were detected early in the season up until week 8 (late February of 2006), when influenza B detections peaked (n = 271) then declined thereafter. Influenza A detections continued to increase, peaking (n = 433) in week 12 (late March of 2006). Overall, influenza A predominated during the 2005-2006 season (61% of laboratory-confirmed cases). Regionally there was some variation in this pattern: influenza A accounted for 55% or more of the laboratory-confirmed cases in most regions of the country, except for Ontario and Atlantic Canada where influenza B cases predominated (accounting for between 58% and 67% of cases).

During the 2005-2006 season, the 0 to 4-year age group represented 20.7% of laboratory-confirmed case-by-case records (1,365/6,590), and the ≥ 65-year age group represented 16.7% (1,103/6,590). It was notable that the age groups 10 to 14 and 45 to 64 years represented the lowest proportion of laboratory confirmed cases at 10.8% (712/6,590) and 9.8% (643/6,590) respectively. Of the influenza A cases, the majority were in the ≥ 65-year group (24.6% or 989/4,028) and the 0 to 4 age group (21.7% or 874/4,028), whereas the majority of influenza B cases were in the younger age groups, with the highest proportion observed in the 15 to 24-year age group (19.8% or 507/2,562) and the lowest (4.4% or 114/2,562) in the ≥ 65-year group (Figure 2).

Figure 1. Case-by-case data by influenza type, region and week of onset, Canada, 2005-2006

Influenza in Canada 2005-2006
Influenza in the Atlantic Provinces 2005-2006
Influenza in Quebec 2005-2006
Influenza in Ontario 2005-2006
Influenza in Prairies 2005-2006
Influenza in British Colombia 2005-2006

 

Influenza in the Territories 2005-2006

Figure 2. Proportionate distributions of case-by-case data, by influenza type and by age group, Canada, 2005-2006

Figure 2. Proportionate distributions of case-by-case data, by influenza type and by age group, Canada, 2005-2006

Influenza virus strain identification, NML

During the period from 23 September, 2005, to 31 August, 2006, the NML antigenically characterized 1,030 influenza viruses received from provincial and hospital laboratories across Canada (Table 3): 44.6% (459/1,030) were influenza A (H3N2), 9.6% (99/1,030) were influenza A (H1N1), and 45.8% (472/1,030) were influenza B viruses. Of the 459 influenza A (H3N2) isolates characterized, 87.6% (402/459) were antigenically similar to A/California/7/2004, which was the influenza A (H3N2) component recommended for the 2005-2006 influenza vaccine, and 12.4% (57/459) were antigenically more closely related to A/Wisconsin/67/2005. This is an antigenic variant that evolved from A/California/7/2004. All 99 influenza A (H1N1) viruses antigenically characterized were similar to A/New Caledonia/ 20/1999, which was the influenza A (H1N1) component recommended for the 2005-2006 influenza vaccine. Influenza B viruses currently circulating can be divided into two antigenically distinct lineages represented by B/Yamagata/16/1988 and B/Victoria/2/1987 viruses. Of the 472 influenza B viruses characterized, 1.5% (7/472) belonged to the B/Yamagata lineage and were antigenically similar to the 2005-2006 vaccine strain B/Shanghai/361/2002, and 98.5% (465/472) belonged to the B/Victoria lineage. Of the 465 Victoria lineage viruses tested, 29.2% (136/465) were similar to B/Hong Kong/330/2001, and 70.8% (329/465) were antigenically more closely related to B/Malaysia/2506/2004, which is the influenza B component recommended by WHO for the 2006-2007 Northern Hemisphere influenza vaccine. Predominant strains for the 10 seasons from 1996 to 2006 in Canada are presented in Figure 3.

Table 3. Distribution of influenza strains, by virus type and province/territory, Canada, 2005-2006

Province or Territory A/New Caledonia/
20/1999
(H1N1)-like
A/
California/
07/2004
(H3N2)-like
A/
Wisconsin/
67/2005
(H3N2)-like*
Total
(Type A)
B/Hong Kong/
330/
2001
-like
B/
Malaysia/
2506/2004-like†
B/
Shanghai/
361/2002-like
Total
(Type B)
Total

Nfld.

 

10

 

10

1

6

 

7

17

P.E.I.

1

1

 

2

 

3

1

4

6

N.S.

1

2

1

4

2

12

 

14

18

N.B.

3

5

2

10

1

11

 

12

22

Que.

46

28

1

75

3

13

 

16

91

Ont.

35

165

5

205

14

169

2

185

390

Man.

 

5

 

5

1

 

 

1

6

Sask.

5

33

 

38

19

10

2

31

69

Alta.

1

70

5

76

52

17

1

70

146

B.C.

7

81

43

131

37

84

1

122

253

Y.T.

 

1

 

1

5

3

 

8

9

N.W.T.

 

1

 

1

1

 

 

1

2

Nun.

 

 

 

0

 

1

 

1

1

Total

99

402

57

558

136

329

7

472

1,030

*A/Wisconsin/67/2005(H3N2)-like is a new antigenic variant of the influenza A (H3N2) virus and is recommended as the H3 component for the 2006-2007 Northern Hemisphere vaccine.

†B/Malaysia/2506/2004-like is a new antigenic variant that belongs to the B/Victoria/02/87 lineage and is the recommended influenza B component for the Northern Hemisphere 2006-2007 influenza vaccine.

Figure 3. Seasonal distribution of case-by-case data by influenza type and week of onset, Canada, 1996-2006


click on the image to enlarge

Figure 3. Seasonal distribution of case-by-case data by influenza type and week of onset, Canada, 1996-2006

Drug susceptibility tests

A rapid assay has been established at the NML to test influenza A viruses for resistance to amantadine. Polymerase chain reaction (PCR) and sequence analysis were used to detect drug resistance markers among circulating influenza A viruses during the 2005-2006 season. A total of 608 influenza A isolates were screened for amantadine resistance, of which 16.8% (102/608) were influenza A (H1N1) and 83.2% (506/608) were influenza A (H3N2). Of the influenza A isolates tested, 76.5 % (465/608) were resistant to amantadine, and 23.5% (143/608) were sensitive (Table 4); 2% (2/102) of influenza A (H1N1) viruses and 91.5% (463/506) of influenza A (H3N2) viruses were resistant to amantadine.

Table 4. Amantadine susceptibility results, by province/territory and influenza A sub-type, Canada, 2005-2006

 

Influenza sub-type

Type A
(H1N1)

Type A
(H3N2)

Total
(type A)

S*

R*

S*

R*

S*

R*

Nfld.

 

 

 

13

0

13

P.E.I.

 

 

 

 

0

0

N.S.

 

 

 

1

0

1

N.B.

3

 

 

7

3

7

Que.

54

 

1

43

55

43

Ont.

34

1

12

163

46

164

Man.

 

 

 

5

0

5

Sask.

5

 

12

32

17

32

Alta.

1

 

6

77

7

77

B.C.

3

1

12

120

15

121

Y.T.

 

 

 

1

0

1

N.W.T.

 

 

 

1

0

1

Nun.

 

 

 

 

0

0

Total

100

2

43

463

143

465

*S = sensitive; R = resistant

 

ILI consultations reported by sentinel practitioners

Overall, FluWatch sentinel ILI site recruitment represented most of the well-populated urban and rural regions across Canada, except for Quebec. A total of nine sentinels (1/850,000 population) were recruited for the FluWatch program in seven (39%) of the 18 health regions in Quebec and resulted in under-representation of some regions within the province. In all other provinces and territories, 258 sentinels (1 per 100,000 population) were recruited for the FluWatch program and/or collaborating provincial sentinel programs, representing 71% of census divisions outside of Quebec (134/189). On average, IRID received weekly ILI data from 60% of FluWatch sentinels across Canada (160/267) for the entire 2005-2006 season and 69% (184/267) during the active season only. During the entire 2005-2006 season, 67% of the sentinels (179/267) provided ILI data for at least 50% of the reporting weeks (78% in the active season), and 11% (30/267) provided ILI data for at least 90% of the reporting weeks (29% in the active season).

During the 2005-2006 influenza season, ILI consultation rates peaked in week 9 (late February to early March) with a rate of 49 per 1,000 patient visits. Between week 40 and week 20 (early October 2005 to mid-May 2006), ILI rates remained within or below baseline values, with the exception of week 12 when the ILI rate was higher than baseline values. The rates decreased in week 13 (late March) and remained low for the rest of the season. During the active season (from October 2005 to mid-May 2006), an average of 15 ILI consultations were reported per 1,000 patients seen. The highest ILI consultation rates were reported in children: 66/1,000 patients seen in the 0 to 4-year age group and 40/1,000 in those aged 5 to 19 years.

Figure 4. ILI consultation rates, byweek, Canada, 2005-2006, compared to baseline (1996-1997 to 2004-2005 seasons)

Figure 4. ILI consultation rates, byweek, Canada, 2005-2006, compared to baseline (1996-1997 to 2004-2005 seasons)

Figure 5. Number of surveillance regions reporting localized or widespread influenza activity, by week, Canada, 2005-2006

Figure 5. Number of surveillance regions reporting localized or widespread influenza activity, by week, Canada, 2005-2006

Influenza activity level assessment

Saskatchewan was the first province to report localized influenza activity in one of its influenza surveillance regions, during week 39 (late September), followed by Manitoba in week 43 (late October). The number of provincial/territorial designated influenza surveillance regions (n = 76 across Canada) reporting localized activity increased gradually over the following weeks, with widespread activity first reported during week 51 in British Columbia. The proportion of influenza surveillance regions reporting localized or widespread activity peaked during week 13 at 29% (22/76). Most reporting of influenza activity level (64% of widespread activity reported and 48% of localized activity reported) occurred over an 8-week period from week 7 to week 14 (mid-February to early April of 2006). Fifteen or more of the 76 influenza surveillance regions across Canada reported localized or widespread activity during each of these weeks (Figure 5).

Provincial and territorial weekly participation in reporting of activity levels remained high throughout the season, with timely activity level assessments reported to IRID for all 76 provincial/ territorial designated influenza surveillance regions in 94% of the reporting weeks (49/52) during the 2005-2006 season.

During the 2005-2006 influenza season, nine provinces reported a total of 164 laboratory-confirmed influenza outbreaks in long-term care facilities (LTCF). Most of these (144 or 88%) were reported between week 2 and week 17 (mid-January to late April of 2006). The peak in outbreak reports occurred in week 10, when 17 LTCF outbreaks were reported in five provinces (Figure 6). Outbreaks of influenza or ILI were also reported in hospitals (n = 22, in five provinces and one territory), schools (n = 245, in seven provinces and two territories) and other types of facility (n = 30, in two provinces and one territory).

Influenza-associated hospitalizations in children

During the 2005-2006 influenza season, 12 IMPACT centres in eight provinces reported 374 influenza-associated hospitalizations. Of these, five resulted in deaths attributable to influenza or influenza-related complications (case fatality of 1.3%) and were reported by four different centres in four provinces. Three of the five deaths were attributed to influenza A infection (one death in a child between 6 and 23 months of age and two in children > 5 years), and two were attributed to influenza B infection (one in a child aged between 6 and 23 months and the other in a child > 5 years). Two of the five children who died had pre-existing conditions that put them at high risk of influenza-related complications. Of these, full or partial vaccination details were obtained for four cases, indicating that none had been vaccinated against influenza. On the basis of age and presence of high-risk conditions, three of the four had been eligible for vaccination. Influenza A represented 61.8% of the total number of hospitalized pediatric cases (231/374), and influenza B represented 38.0% of cases (142/374). One case (0.2% or 1/374) was infected with both influenza A and B. A similar distribution of influenza A- and influenza B-associated hospitalizations was reported up until week 8 (late February of 2006), after which influenza A infections accounted for the majority of hospitalizations for the remainder of the season. Hospitalizations due to influenza B infection peaked during week 10, whereas those due to influenza A infection peaked in week 12 (Figure 7).

Figure 6. Number of outbreaks reported in long-term care facilities (LTCF), by week, Canada, 2005-2006

Figure 6. Number of outbreaks reported in long-term care facilities (LTCF), by week, Canada, 2005-2006

Figure 7. Weekly pediatric admissions to IMPACT hospitals, by influenza type, Canada, 2005-2006

Figure 7. Weekly pediatric admissions to IMPACT hospitals, by influenza type, Canada, 2005-2006

Reporting of influenza hospitalizations in children began at different times in the 12 IMPACT hospitals. The first hospitals to report cases were in Ontario in week 47 (mid-September), from influenza A, and in Alberta and British Columbia in week 49 from influenza A and B. Six of the 12 centres had reported at least one case by week 1. The number of cases reported per week peaked in week 10 (early March) with 44 cases: 25 influenza A and 19 influenza B. The last cases of the season were reported by hospitals in Alberta (week 22 or end of May) and Quebec (week 24); both cases were infected with influenza A.

The distribution of hospitalized cases by age group was as follows: 14.2% of cases (53/374) were aged 0 to 5 months; 24.3% (91/374) were 6 to 23 months; 27.3% (102/374) were 2 to 4 years; and 34.2% (128/374) were ≥ 5 years. Full or partial vaccination details were obtained from 84.2% of cases (315/374), and the proportion of children immunized was found to be similar among the eligible age groups over 6 months of age (Table 5). The distribution of hospitalized cases by influenza type and by age group is presented in Figure 8.

Figure 8. Distribution of pediatric admissions to IMPACT hospitals, by type and age group, Canada, 2005-2006

Figure 8. Distribution of pediatric admissions to IMPACT hospitals, by type and age group, Canada, 2005-2006

Table 5: Influenza-associated pediatric admissions to IMPACT hospitals, by age and immunization status, Canada, 2005-2006

Age group

Number (%) of laboratory-confirmed influenza admissions

Number (%) of admissions with at least partial vaccination details

Number (%) immunized against influenza prior to admission

0-5 months

53 (14.2%)

53 (0%) -n/a*

0 (0%) n/a*

6-23 months

91 (24.3%)

79 (86.8%)

16 (20.3%)

2-4 years

102 (27.3%)

86 (84.3%)

19 (22.1%)

5+years

128 (34.2%)

97 (75.8%)

24 (24.7%)

Total

374

315

59

* According to NACI recommendations, immunization with currently available influenza vaccines is not recommended for infants < 6 months of age.

United States: CDC1

Between week 40 and week 25 (early October 2005 to mid-June 2006), 12.1% (17,977/148,636) of the specimens tested for influenza viruses by the WHO and the National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories in the United States were positive. Among the 17,977 influenza viruses tested, 79.7% (14,335/ 17,977) were influenza A viruses, and 20.3% (3,642/17,977) were influenza B viruses. Influenza activity in the United States peaked during week 10 (week ending 11 March, 2006).

Since 1 October, 2005, the CDC has antigenically characterized 828 influenza viruses: 503 influenza A (H3N2), 88 influenza A (H1N1) and 237 influenza B viruses. Of the 503 influenza A (H3N2) viruses, 75.7% (381/503) were characterized as A/California/07/2004-like, and 24.3% (122/503) demonstrated reduced titres with antisera produced against A/California/07/ 2004. Of the 122 low-reacting viruses, 96 were tested with antisera produced against A/Wisconsin/67/2005, and 70 were A/Wisconsin-like. The hemagglutinin proteins of 96.6% (85) of the 88 influenza A (H1N1) viruses were antigenically similar to the hemagglutinin of the A/New Caledonia/20/1999 strain, and 3.4% (3/88) showed reduced titres with antisera produced against A/New Caledonia/20/1999. Of the 237 influenza B viruses that have been characterized, 21.9% (52/237) belonged to the B/Yamagata lineage; eight were similar to B/Shanghai/361/2002, 43 were characterized as B/Florida/07/2004-like (a minor antigenic variant of B/Shanghai/361/2002), and one showed reduced titres with antisera produced against both B/Shanghai/361/2002 and B/Florida/07/2004. The majority of influenza B viruses characterized, 78.1% or 185/237, belonged to the B/Victoria lineage; 184 were similar to B/Ohio/1/2005; and one showed reduced titres with antisera produced against B/Ohio/1/2005. The B/Ohio/ 1/2005 strain is antigenically equivalent to the B/Malaysia/2506/ 2004 strain.

International: WHO2,3

Between September 2005 and September 2006, influenza activity was generally low compared with the same period in recent years. In North America and Asia, influenza activity began in October 2005, increased from December 2005 through March 2006 and declined in April 2006, except in Hong Kong Special Administrative Region (SAR), China, where outbreaks occurred until July 2006. In Europe, influenza activity remained low from October 2005 to January 2006, then increased in February, quickly reaching a peak before declining in April. In the southern hemisphere, influenza activity began in April 2006 but remained mild in general until an overall decline in September.

In North America and some Eastern European countries, influenza A (H3N2) viruses predominated and caused outbreaks, but in the rest of Europe influenza B viruses predominated. In Asia, influenza A (H1N1), A (H3N2) and B viruses co-circulated.With respect to the southern hemisphere, in South America influenza A (H1N1) viruses predominated and circulated locally.While outbreaks caused by influenza A (H3N2) viruses occurred in New Zealand and South Africa, activity remained low in other parts of Oceania and Africa.

Most recent influenza A (H3N2) viruses were antigenically similar to the vaccine viruses A/Wisconsin/67/2005(H3N2)-like and A/Hiroshima/52/2005(H3N2)-like. Although the majority of influenza A (H1N1) viruses were antigenically similar to the vaccine virus A/New Caledonia/20/1999(H1N1)-like, there was an observed increase in the number of recent virus isolates with low titres in hemagglutination tests against the vaccine antisera. The majority of influenza B viruses isolated during February through September 2006 belonged to the B/Victoria/2/87 lineage and were antigenically similar to the vaccine virus B/Malaysia/2506/2004.

Avian Influenza(4)

The first recorded cases of human infection with the avian influenza A (H5N1) virus occurred in Hong Kong in 1997; 18 persons were infected, of whom six died (33% fatality)(5). Since then, the H5N1 virus has crossed the species barrier on at least two other occasions: in Hong Kong (two cases including one death in February 2003) and in the current outbreaks that began in December 2003 in Southeast Asia(6). As the virus continues to circulate in avian populations through Asia, Europe, the Middle East and Africa, the geographic spread of human cases has extended from four to 10 countries. By the end of August 2006, the following countries reported human cases: Vietnam, Thailand, Cambodia, Indonesia, Azerbaijan, China, Djibouti, Egypt, Iraq and Turkey. The first case occurred in Vietnam (onset December 2003), and since then there have been 241 persons infected, of whom 141 have died (59% fatality). More than 50% of the cases have occurred in those < 20 years old; 90% have occurred in those < 40 years old(7). Human cases have occurred year round, but the highest incidence has been in the months of January to March. Almost all human cases have been linked to close contact with diseased household poultry flocks, and avian influenza H5N1 infection in humans remains a rare disease(6). To date, there has been no evidence of sustained human-to-human transmission, and the WHO influenza pandemic preparedness level remains at Phase 3(8).

Discussion

The 2005-2006 influenza season started later than usual in Canada and appeared to be milder overall (in terms of a lower proportion of positive laboratory tests for influenza, fewer LTCF outbreaks and fewer influenza surveillance regions reporting localized and widespread activity) than the previous two seasons. In contrast to the 2004-2005 season, which heavily affected the elderly residing in LTCFs, influenza in the 2005-2006 season had a greater impact on children. A mix of influenza A and B viruses circulated across Canada, the majority of the influenza A detections being A (H3N2) viruses (A/California/7/2004(H3N2)-like), and the majority of influenza B detections belonging to the B/Victoria/02/1987 lineage (B/Hong Kong/330/2001-like or B/Malaysia/2506/2004-like).

Influenza viruses began circulating late in Canada in the 2005-2006 season, as demonstrated by the late peak in laboratory-confirmed cases (week 12 or mid- to late-March) compared with the previous season (week 5 or early January). The first peak in laboratory-confirmed cases was observed in the west (the Prairies and British Columbia) in weeks 5 and 7, in Ontario in week 8, followed by peaks in Quebec and the Atlantic region in week 12. Influenza viruses were detected over a longer period of time in the Prairies (46/52 weeks), Ontario (44/52) and British Columbia (40/52) than the rest of the country.

The total number of influenza tests performed in Canada for the 2005-2006 season (n = 87,303) was lower than observed for the previous season (n = 101,258). Per capita testing rates for influenza have nonetheless increased over the past five seasons, from 174 influenza tests per 100,000 population in 2001-2002 to 320 tests per 100,000 population in 2005-2006. This increase in testing may be a response to increased awareness of the importance of influenza, present concerns over emerging viruses (e.g. human infection with avian influenza) and pandemic preparedness, as well as the increasing ease and use of rapid tests for influenza diagnosis.

The overall percentage of positive tests in the 2005-2006 season (8.5%) was lower than in the 2004-2005 and 2003-2004 seasons (12.7% and 12.3% respectively). This is consistent with seasons in which there is a co-circulation of influenza A and B or only influenza B. Over the 10 seasons from 1996-1997 to 2005-2006, the percentage of positive influenza tests has been below 10% (range 5.8% to 8.7%) when influenza seasons were typically characterized by mixed influenza A and B activity or a predominance of influenza B activity. During the 2005-2006 season, the peak in percentage of positive influenza tests was 20.4% (649/3,181 in week 11) compared with 24.5% (1,015/4,146 in week 5) in 2004-2005.

Of the total positive influenza identifications, 61.0% were influenza A viruses and 39% influenza B. Both influenza A and B viruses were detected up until week 8, after which influenza A detections predominated. In contrast, during the 2004-2005 season, influenza A detections predominated first, and only later in the season (after week 11) did the proportion of influenza B detections exceed the proportion of influenza A.

The NML characterized 1,030 influenza viruses, of which influenza A (H3N2) accounted for 44.6% (459/1,030) and influenza B for 45.8% (472/1,030). A lower proportion of influenza A (H1N1) was detected (9.6% or 99/1,030), the majority of them isolated in Quebec and Ontario. The majority of influenza A (H3N2) and A (H1N1) viruses characterized during the 2005- 2006 season were identified as A/California/7/2004(H3N2)-like and A/New Caledonia/20/1999(H1N1)-like viruses respectively, which were the recommended influenza A components included in the 2005-2006 influenza vaccine. After A/Wisconsin/67/2005 (H3N2)-like reagents became available, the majority of influenza A (H3N2) isolates characterized belonged to this new antigenic variant of A/California/7/2004(H3N2). The A/Wisconsin/67/2005 (H3N2)-like strain was subsequently selected as the A (H3N2) component of the 2006-2007 influenza vaccine. Only 1.5 % of all influenza B isolates characterized (7/472) matched the B/Shanghai/361/2002-like strain, which was the recommended influenza B component for the 2005-2006 vaccine. The other 98.5% (465/472) belonged to the B/Victoria lineage, of which 70.8% (329/465) were B/Malaysia/2506/2004-like, the strain selected as the influenza B component for the 2006-2007 influenza vaccine. While influenza A (H3N2) viruses predominated overall both in Canada and the United States, influenza B viruses accounted for the majority of detections in Europe overall. In contrast to the trend seen in Canada, influenza B viruses were more frequently isolated late in the season in the United States than influenza A viruses. TheWHO influenza surveillance system indicated a mild season globally compared to with the previous season, with a late start, and a predominance of influenza A (H3N2).

During the 2005-2006 influenza season, the emergence of a high level of resistance to the antiviral amantadine was detected among the circulating influenza A (H3N2) viruses. Of the 608 influenza A viruses (H3N2 and H1N1) tested by the NML, 91.5% of the influenza A (H3N2) (463/506) and 2% of the influenza A (H1N1) (2/102) viruses were resistant to amantadine. Accordingly, PHAC made an interim recommendation against the use of amantadine for the treatment and prevention of influenza for the remainder of the 2005-2006 influenza season in Canada(9). On the basis of amantadine resistance patterns observed to date, PHAC continues to recommend against the use of amantadine for the treatment and prevention of influenza(10). Globally, the rate of viruses resistant to amantadine has also been increasing(11-13). In addition to amantadine resistance testing, the NML has recently implemented a method for monitoring resistance to neuraminidase inhibitors, which will become part of its routine surveillance during the 2006-2007 influenza season. Ongoing surveillance of antiviral resistance patterns of circulating influenza strains will be an important activity for informing appropriate use of antiviral medications to reduce the impact of influenza.

Of the laboratory case-by-case data reported this season, the age group accounting for the greatest proportion of cases was the 0 to 14 age group (45%), as compared with the 2004-2005 season in which 46% of the cases were represented by the ≥ 65-year age group. The age distribution of cases varied by influenza type, with the majority of influenza A cases (54%) occurring in the adult population ≥ 25 years of age and the majority of influenza B cases (58%) occurring in children 0 to 14 years of age.

ILI consultation rates remained below the mean rate for the previous nine seasons for most weeks of the influenza season, except from week 9 to week 15, when ILI rates peaked. Peak ILI consultations also coincided with peak numbers of laboratoryconfirmed influenza (weeks 8 to 13). The peak ILI consultation rate was 49 ILI consultations per 1,000 patient visits, similar to that of the previous season. The overall ILI rate during the active influenza season for 2005-2006 (15 cases of ILI per 1,000 patients seen) was lower than the rate for 2004-2005 (20 per 1,000 patients seen). In both seasons the highest rates of ILI were observed in children: 36 per 1,000 in the 0 to 4-year age group and 32 per 1,000 in the 5 to 19 age group in 2005-2006 compared with 49 per 1,000 and 36 per 1,000 respectively in 2004-2005.

Consistent with the late season, the number of influenza surveillance regions reporting localized or widespread activity peaked during week 13, as compared with week 6 in the previous season. The decrease in the number of localized and widespread activity reports may be partly explained by a significant decrease in the number of outbreak reports occurring in LTCFs in 2005-2006 (164 in 2005-2006 compared with 762 in 2004-2005).

While the 2005-2006 season appeared to have a greater impact on children than the elderly, the number of pediatric hospitalizations reported by IMPACT sites was similar to that in the previous season (374 hospitalizations in 2005-2006 compared with 391 in 2004-2005). Influenza B accounted for a greater percentage of hospitalizations in 2005-2006 than in the previous season (38.2% versus 30.7%).

Limitations

Results from the influenza surveillance system should be interpreted with caution for several reasons:

  1. While case-by-case data were available for 89% of the aggregate influenza detections reported across Canada (6,590 case-by-case records submitted for 7,422 total laboratory confirmed influenza cases) by the RVDSS in 2005-2006, some provinces and territories are under-represented. Of the 11% (832/7,422) of case-by-case records not captured in the national database, most of the missing reports were from Ontario (30% or 698/2,317 Ontario cases not captured in the national database) and New Brunswick (40% or 113/279 New Brunswick cases not captured), indicating significant under-representation in the case-by-case data for these provinces. Further more, variation in the numbers of case-by-case data reported over time and their distribution by province/territory are likely to reflect differences in population size and distribution, testing and reporting practices and criteria, and availability of diagnostic services, which vary across the regions and thus should also be interpreted with caution.

  2. The predominant testing methods used for influenza detection vary considerably by province and/or RVDSS participating laboratories. During the 2005-2006 season, Alberta and Ontario laboratories reported the majority of influenza detections by DFA testing (70% of all DFA detections reported in Canada were from these provinces), whereas Quebec reported the most detections by enzyme immunoassay (93% of all EIA detections), and Alberta reported the majority of influenza detections by NAAT (85% of all NAAT detections). Over the past three seasons, the majority of laboratory-confirmed influenza cases have been detected using virus isolation; however, the use of this detection method has been decreasing (from 59% of total tests in 2003-2004 to 54% in 2004-2005 and 49% in 2005-2006), while the use of NAAT testing has been increasing (from 0.2% in 2004-2005 to 12.5% in 2005-2006). The proportion of cases confirmed by virus antigen detection has remained stable, around 40%, over the same period (41% in 2003- 2004, 45% in 2004-2005 and 38% in 2005-2006).

    Although the current notifiable disease case definition for laboratory-confirmed influenza does not yet include confirmation by NAAT methods(14) these cases were included in the RVDSS case counts for the 2005-2006 season for the following reasons: (a) this method has been recommended by the Canadian Public Health Laboratory Network to be included in the revision of the national notifiable disease case definition for laboratory-confirmed influenza; (b) exclusion of these cases would bias the national results (i.e. influenza detections in Alberta would be under-represented); (c) NAAT use is increasing and regarded as a highly sensitive and specific testing method for the detection of influenza infection(15); (d) the use of this method is validated and supported by the Canadian Pandemic Influenza Plan for the Health Sector(16); and (e) use of this method is expected to continue for influenza detection, and therefore monitoring and interpretation of its use at the outset are required for comparisons over time.

    Caution should be used when interpreting results from laboratory detections as the difference in findings may be due to variation in the sensitivity and specificity of the various laboratory tests used for influenza detection and the differences in testing practices across jurisdictions and over time. The sensitivity and specificity of the various laboratory methods for influenza diagnosis vary, depending on the specific type of test or kit used and factors such as the patient's age, timing of specimen collection (i.e. when prevalence of influenza in the community is high), specimen type and quality, and the technical expertise available in interpreting results.

  3. Age-specific data should also be interpreted with caution, as these data may be a result of biases in health care utilization and physician testing behaviour. For example, ILI surveillance does not capture influenza activity occurring in the elderly in LTCFs, children who visit pediatricians or the majority of consultations that occur in emergency departments and after-hours clinics. Also, ILI consultation rates across time may vary with sentinel participation and coverage rates. For example, although there were more sentinels recruited and slightly higher representation in census divisions outside of Quebec in the 2005-2006 season compared with the previous season (267 versus 239 sentinels and 71% versus 66% coverage respectively), overall sentinel participation was lower in 2005-2006 than in 2004-2005.


References

  1. Centers for Disease Control and Prevention. 2005-06 U.S. influenza season summary. Atlanta: CDC, 2006. URL: <http://www.cdc.gov/flu/weekly/weeklyarchives2005-2006/0 5-06summary.htm >. Accessed 23 November, 2006.

  2. World Health Organization. Influenza in the world, September 2005-January 2006. Wkly Epidemiol Rec 2006;81:94-6.

  3. World Health Organization. Influenza in the world, February- September 2006. Wkly Epidemiol Rec 2006;81:404-7.

  4. World Health Organization. Avian influenza. URL: <http://www.who.int/csr/disease/avian_influenza/en/ >. Accessed 28 November, 2006.

  5. World Health Organization. Avian influenza frequently asked questions. URL: <http://www.who.int/csr/disease/avian_influenza/ avian_faqs/en/index.html >. Accessed 28 November, 2006.

  6. World Health Organization. Avian influenza fact sheet. URL: < http://www.who.int/mediacentre/factsheets/avian_influenza/ en/index.html >. Accessed 28 November, 2006.

  7. World Health Organization. Epidemiology of WHO-confirmed human cases of avian A (H5N1) infection. Wkly Epidemiol Rec 2006;81:249-60.

  8. World Health Organization. Current WHO phase of pandemic alert. URL: < http://www.who.int/csr/disease/avian_influenza/ phase/en/index.html >. Accessed 28 November, 2006.

  9. Public Health Agency of Canada. Interim recommendation for use of amantadine for influenza. URL: < http://www.phac-aspc. gc.ca/media/advisories_avis/2006/statment060115.html >. Accessed 24 November, 2006.

  10. Public Health Agency of Canada. Recommendation for use of amantadine for treatment and prevention of influenza. URL: < http://www.phac-aspc.gc.ca/media/nr-rp/2006/20061101-a mantadine_e.html >. Accessed 20 December, 2006.

  11. Bright RA, Medina MJ, Xu X et al. Incidence of adamantane resistance among influenza A (H3N2) viruses isolated worldwide from 1994 to 2005: A cause for concern. Lancet 2005;366:1175-81.

  12. Li Y. 1999-2000 influenza season: Canadian laboratory diagnoses and strain characterization. CCDR 2000;26(22):185-9.

  13. Bright RA, Shay DK, Shu B et al. Adamantane resistance among influenza A viruses isolated early during the 2005-2006 influenza season in the United States. JAMA 2006;295(8):891-4.

  14. Health Canada. Case definitions for diseases under national surveillance. CCDR 2000;26(S3).

  15. Petric M, Comanor L, Petti CA. Role of the laboratory in diagnosis of influenza during seasonal epidemics and potential pandemics. J Infect Dis 2006;194(S2):S98-110.

  16. Public Health Agency of Canada. The Canadian pandemic influenza plan for the health sector. URL: < http://www. phac-aspc.gc.ca/cpip-pclcpi/index.html >. Accessed 18 December, 2006.



* Outbreak definitions. Residential institutions: Two or more cases of ILI within a 7-day period, including at least one laboratory-confirmed case. Institutional outbreaks should be reported to provincial/territorial health authorities within 24 hours of identification; Schools and worksites: > 10% absenteeism on any day most likely due to ILI.

† Activity level definitions.

  • 1 = No activity reported.

  • 2 = Sporadic - sporadically occurring ILI and confirmed influenza in the surveillance region at any time within the prior 4 weeks with no outbreaks detected in the influenza surveillance region.

  • 3 = Localized - sporadically occurring ILI and confirmed influenza in the surveillance region at any time within the prior 4 weeks together with outbreaks of ILI in schools and worksites or laboratory confirmed influenza in residential institutions occurring in < 50% of the influenza surveillance region(s).

  • 4 = Widespread - sporadically occurring ILI and confirmed influenza in the surveillance region at any time within the prior 4 weeks together with outbreaks of ILI in schools and worksites or laboratory confirmed influenza in residential institutions occurring in ≥ 50% of the influenza surveillance region(s).


Report a problem or mistake on this page
Please select all that apply:

Thank you for your help!

You will not receive a reply. For enquiries, contact us.

Date modified: