Influenza in Canada:

2006-2007 Season Update

Canada Communicable Disease Report

1 May 2007

Volume 33
Number 09

F Reyes, MHSc (1), S Aziz, MSc (1), JF Macey, MA, MSc (1), Y Li, PhD (2), B Winchester, MSc (1), P Zabchuk (1), S Wootton, MD (3), P Huston, MD, MPH (1), TWS Tam, MD, FRCPC (1)

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

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

  3. Data Center, Immunization Monitoring Program, Active (IMPACT), Vancouver, British Columbia

Introduction

Canada's national influenza surveillance system, FluWatch, now in its 11th year is coordinated through the Immunization and Respiratory Infections Division (IRID), Public Health Agency of Canada. The program collects data and information from various sources in order to provide a national picture of influenza activity.

This report provides a summary of the results on influenza activity in Canada during the current 2006-2007 season from 27 August 2006 up to and including 3 March 2007.

Methods

FluWatch reports on five main indicators of influenza activity: 1) sentinel laboratory-based influenza and other respiratory virus detections; 2) strain identification and antiviral resistance for circulating influenza viruses; 3) sentinel influenza-like illness (ILI) primary care consultation rates; 4) regional influenza activity levels as assigned by provincial and territorial FluWatch representatives; and 5) paediatric influenza-associated hospital admissions and mortality data. In addition, the FluWatch program also conducts an assessment of international influenza activity by monitoring reports from other influenza surveillance programs worldwide. Detailed surveillance methodology has been previously described(1).

Results

Respiratory Virus Detections
Since 27 August, 2006, 33 laboratories across Canada reported a total of 59,771 influenza tests, of which 4,423 (7.4%) have tested positive for influenza viruses. The provincial distribution of influenza tests conducted and the number positive are shown in Table 1.

Of the 4,423 influenza detections to date, 4,250 (96.1%) were influenza A and 173 (3.9%) were influenza B. Influenza A detections were reported across Canada, however, the majority detections to date were from Ontario (42.0% or 1,784/4,250), Alberta (20.2% or 858/4,250) and Quebec (17.3% or 734/4,250). Influenza B detections were reported in most provinces across Canada except in the Atlantic region and Manitoba. The majority of influenza B detections to date were from Quebec (67.1% or 116/173) and British Columbia (17.3% or 30/173).

To date detailed case-by-case epidemiological reports for 1,364/4,423 (30.8%) laboratory-confirmed influenza cases have been received from six provinces and three territories (Table 1). The majority of reports received were from western Canada; Alberta (50.3% or 686/1,364), British Columbia (24.4% or 333/1,364), and Saskatchewan (13.3% or 182/1,364). For this reason, case-by-case results should be interpreted with caution as the results are not representative of the overall Canadian situation.

Of the 1,364 cases, 1,329 (97.4%) were influenza A infections and 35 (2.6%) were influenza B infections. The majority of influenza A cases were reported in children <10 years of age (48% or 635/1,329), followed by adults between 25 to 44 years of age (17% or 224/1,329) (Figure 1). The high proportion of cases observed among children is reflected by the majority of outbreaks of ILI in schools being reported in Alberta and British Columbia, where the majority (74.5%) of the case-by-case reports were reported. Although 28% (10/35) of the influenza B cases were among children < 5 years of age, a large proportion of influenza B cases were in the younger adult and elderly age groups (20% each or 7/35 in the 25 to 44 and > 65 year age groups).

Table 1. Total number of influenza tests conducted and aggregate and case-by-case influenza data, by province/territory or region, Canada, 2006-2007

Province/ Territory or Region

Season to date: 27 August, 2006 - 3 March, 2007

Total number
of influenza tests

Aggregate detections

Case-by-case data

Number of positive tests

% of total positive tests

Number of cases

% of total cases

Nfld.

502

76

1.7

P.E.I

84

25

0.6

N.S.

633

49

1.1

53

3.9

N.B.

880

67

1.5

Atlantic

2,099

217

4.9

53

3.9

Que.

12,700

850

19.2

Ont.

20,661

1,794

40.6

71

5.2

Man.

1,544

28

0.6

32

2.3

Sask.

4,040

217

4.9

182

13.3

Alta.

15,489

873

19.7

686

50.3

Prairies

21,073

1,118

25.3

900

66.0

B.C.

3,238

444

10.0

333

24.4

Y.T.

4

0.3

N.W.T

2

0.1

Nun.

1

0.1

Territories

7

 

Total

59,771

4,423

1,364

 

 

Figure 1. Proportionate distributions of case-by-case data, by influenza type and by age group, Canada, 2006-2007

Figure 1. Proportionate distributions of case-by-case data, by influenza type and by age group, Canada, 2006-2007

Influenza Virus Strain Identification

To date, 614 influenza viruses have been antigenically characterized by the National Microbiology Laboratory (NML): 382 (62.2%) were A/Wisconsin/67/2005 (H3N2)-like; 202 (32.9%) were A/New Caledonia/20/1999 (H1N1)-like; seven (1.1%) were B/Malaysia/2506/2004-like; and 23 (3.7%) were B/Shanghai/361/2002-like. All but the B/Shanghai/ 361/2002-like strain is included in the composition of the 2006-2007 Canadian influenza vaccine. The majority of influenza A viruses identified early in the season were influenza A/New Caledonia/20/1999 (H1N1)-like; however, the number of influenza A/Wisconsin/67/2005 (H3N2)-like viruses have been increasing since mid-January and now represent the majority of influenza strains characterized so far this season.

Of the influenza A (H1N1) viruses characterized to date, 80% (162/202) were from the West; British Columbia (35% or 71/202), Alberta (25% or 51/202), and Saskatchewan (20% or 40/202). Of the influenza A (H3N2) viruses characterized, 59% (224/382) were from Ontario and 25% (96/382) were from British Columbia.

Antiviral Resistance

Since the start of the season, the NML has tested 541 influenza A isolates (167 H1N1, 374 H3N2) for amantadine resistance. None of the H1N1 isolates tested were resistant to amantadine; however, 27.8% (104/374) of the H3N2 isolates were resistant. The proportion of H3N2 isolates resistant to amantadine varied by province/territory and the results are as follows (proportion resistant, number of isolates resistant/number of isolates tested): Nova Scotia (100%, 4/4); Prince Edward Island (100%, 2/2); British Columbia (72%, 59/82); Alberta (43%, 6/14); Saskatchewan (29%, 2/7); Newfoundland and Labrador (25%, 1/4); New Brunswick (20%, 1/5); Ontario (12%, 28/236); Quebec (8%, 1/12); and Yukon (0%, 0/1).

Of the 498 isolates (170 influenza A (H1N1), 301 influenza A (H3N2), 26 influenza B) tested for oseltamivir resistance, none were found to be resistant.

At this time, there is no change to the November 2006 Public Health Agency of Canada recommendation that health care providers in Canada not prescribe amantadine to treat and prevent influenza during the current flu season(2).

ILI Consultations Reported by Sentinel Clinical Practices

Weekly ILI rates have remained within or below baseline levels since the beginning of the season. To date, the highest rate observed was in week 09 (late February-early March) with 51 consultations for ILI per 1,000 patient visits, which slightly exceeds the peak observed during the previous season (49 per 1,000 in week 09). Figure 2 shows the Canadian age-standardized, census-division weighted ILI consultation rates for the current season, compared to the mean rate and 95% confidence intervals for the previous 10 years of the FluWatch program. The highest ILI consultation rates were reported in children: 46/1,000 patients seen in the 0 to 4-year age group and 30/1,000 in those aged 5 to 19 years. So far this season, the median sentinel participation rate has been fairly good at 73% (range from 48% to 85%) per week.

Figure 2. Census-division weighted age-standardized ILI consultation rates, by influenza season and report week, Canada, 2006-2007, compared with seasons 1996-1997 to 2005-2006 (average with 95% confidence intervals)

Figure 2. Census-division weighted age-standardized ILI consultation rates, by influenza season and report week, Canada, 2006-2007, compared with seasons 1996-1997 to 2005-2006 (average with 95% confidence intervals)

Regional Influenza Activity Levels Assessed by Provincial and Territorial Epidemiologists

Influenza activity in Canada overall was relatively mild from September 2006 to mid-January 2007, except in some regions in Alberta and Ontario (where localized influenza activity was reported since early November 2006). Influenza activity increased across the country from late-January to early March; however, remained mild to moderate overall. Regional variations in the spread/timing and intensity of influenza activity were observed. Although sporadic activity was first reported by regions in British Columbia, Alberta and Ontario in early September 2006, activity remained low in British Columbia until mid-January 2007, unlike in Alberta and Ontario where influenza activity continued to increase. Increased influenza activity then spread west of Alberta to British Columbia and outwards from Ontario to Quebec and Manitoba in late January, followed by the Atlantic provinces in early to late- February. Influenza activity has remained low in Saskatchewan and the Territories since the start of the season.

Widespread influenza activity has been reported 23 times by 12 regions in five provinces since the start of the season, the majority of which were reported between early February to early March 2007 and mostly reported in Toronto, Ontario (35% or 8/23) and in several regions in British Columbia (35% or 8/23).

To date, 406 outbreaks of influenza or ILI have been reported: 112 (27.6%) in long-term care facilities (LTCF's); 9 (2.2%) in hospitals; 257 (63.3%) in schools; and 28 (6.9%) in other facilities. The majority of the LTCF outbreaks reported to date were from Ontario (37% or 41/112) and Quebec (22% or 25/112) while the majority of outbreaks of ILI in schools were from New Brunswick (30% or 78/257), British Columbia (28% or 72/257) and Alberta (19% or 49/257). There were more outbreaks reported to date this season compared to the same period in the previous season which was also considered a relatively mild season.

Influenza Hospitalizations in Children

To date this season, a total of 186 influenza-associated paediatric hospitalizations have been reported through the Immunization Monitoring Program ACTive (IMPACT) network, compared to 108 cases at the same time last season. The provincial distribution of cases is as follows: 28.0% (52/186) from Ontario; 27.4% (51/186) from Quebec; 16.1% (30/186) from Alberta; 13.4% (25/186) from British Columbia; 9.1% (17/186) from Nova Scotia; 2.7% (5/186) from Newfoundland and Labrador; and 1.6% (3/186) each from Manitoba and Saskatchewan. Influenza A was identified in 93.5% (174/186) of the cases and influenza B was identified in the remaining 6.5% (12/186).

Eighty-one percent (150/186) of the paediatric hospitalizations reported to date were hospitalized between week 5 and week 9 (28 January – 3 March, 2007), with the highest number of cases reported at 42 hospitalizations in week 9. The age distributions of the 186 cases are as follows: 30.6% (57/186) among 6 to 23 month olds; 23.1% (43/186) among 2 to 4 year olds; 21.0% (39/186) among 5 to 9 year olds; 14.0% (26/186) among 0 to 5 month olds; and 11.3% (21/186) among 10 to 16 year olds. To date, one influenza- associated paediatric death has been reported in Canada. The death, due to infection with an influenza A/New Caledonia/20/1999 (H1N1)-like virus, was reported in week 05 (late January to early February 2007) from BC Children's Hospital in Vancouver, British Columbia. The child was between 5 to 9 years of age, previously healthy and had not received influenza immunization.

International United States

In the United States, influenza activity overall remained low from October 2006 to mid-December 2006, steadily increased from January to February 2007, and decreased slightly by early March. Since late December 2006, the proportion of patient visits to sentinel providers for ILI has been at or above baseline levels. The proportion of deaths due to pneumonia and influenza has remained below the epidemic threshold since the start of the season. To date, the CDC has received reports of 25 influenza-associated paediatric deaths. Since October 2006, the CDC has tested a total of 119,002 specimens for influenza viruses of which 14,993 (12.6%) were positive, of which 12,438 (83.0%) were influenza A viruses and 2,555 (17.0%) were influenza B viruses. In addition, 303 influenza viruses were antigenically characterized: 200 (66.0%) influenza A (H1), 25 (8.3%) influenza A (H3) and 78 (25.7%) influenza B. Of the 225 influenza A viruses, 189 (84.0%) were A/New Caledonia/20/1999(H1N1)-like viruses, 11 (4.9%) were H1N1-like viruses showing reduced titres with antisera produced against A/New Caledonia/20/1999 (H1N1), 12 (5.3%) were A/Wisconsin/67/2005 (H3N2)-like viruses, and 13 (5.8%) were H3N2-like viruses showing reduced titres with antisera produced against A/Wisconsin/67/2005 (H3N2). Of the 78 influenza B viruses, 52 (66.7%) belonged to the B/Victoria/ 2/1987 lineage (29 B/Ohio/01/2005-like viruses and 23 showed somewhat reduced titers with antisera produced against B/Ohio/01/2005) and 26 (33.3%) belonged to the B/Yamagata/16/1988 lineage(3).

Worldwide

Between September 2006 and January 2007, influenza
activity was generally low compared with the same period in recent years(4). Influenza activity began in November in the northern hemisphere, which was late compared to previous years, and increased from January through mid-February but remained moderate in general. In the southern hemisphere, mild influenza activity continued in September and declined in October(5). Influenza A (H1N1) viruses circulated in the United States and in a few European countries, whereas influenza A (H3N2) viruses predominated in many European countries and in some Asian countries/areas. Influenza B circulated at low levels(4).

In Europe, influenza activity remained low up until late December 2006 when increased influenza activity was reported in some northern parts of Europe. By mid-January 2007, influenza activity started to increase for Europe as a whole, starting in the central and southern parts of Europe and then towards the north. Widespread influenza activity was reported across most of Europe by mid-February. Although influenza activity was still increasing in some countries towards the end of February, activity in the southern and western European countries started to decline. Based on characterization data to date this season, 70.9% (1,151/1,624) were A/Wisconsin/67/2005 (H3N2)-like, 17.6% (286/1,624) were A/California/7/2004 (H3N2)-like, 5.9% (96/1,624) were A/New Caledonia/20/1999 (H1N1)-like, 5.5% (90/1,624) were B/Malaysia/2506/2004-like, and <1% (1/1,624) was a B/Jiangsu/10/2003-like(6).

Acknowledgements

The authors gratefully acknowledge and thank all the Flu Watch surveillance partners who are participating in this year's influenza surveillance program, including the National Micro- biology Laboratory, laboratories reporting to the Respiratory Virus Detections Surveillance System (RVDSS), sentinel primary care practitioners, provincial ministries of health, and the Immunization Monitoring Program, ACTive (IMPACT) Network. A special thank you to Estelle Arsenault from the Immunization and Respiratory Infections Division, Public Health Agency of Canada for her help and effort in the translation of this report.


References

  1. Reyes F, Macey JF, Aziz S et al. Influenza in Canada : 2005-2006 season. CCDR 2007;33(3):21-41.

  2. Public Health Agency of Canada. Recommendation for use of Amantadine for treatment and prevention of influenza. November 2006. URL: http://www.phac-aspc.gc.ca/media/ nr-rp/2006/20061101-amantadine_e.htm.

  3. Centers for Disease Control and Prevention. Weekly report: Influenza summary update, week ending March 3, 2007 - week 9. URL: http://www.cdc.gov/flu/weekly/.

  4. World Health Organization. Recommended composition of influenza virus vaccines for use in the 2007-2008 influenza season. Weekly Epidemiological Record 2007;82(9):69-74.

  5. World Health Organization. Influenza. Weekly Epidemiological Record 2007;82(9): 74-6.

  6. European Influenza Surveillance Scheme. EISS weekly electronic bulletin, Influenza season 2006-2007. 9 March 2007, Issue No 217. URL: http://www.eiss.org/cgi-files/ bulletin_v2.cgi.

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: