FluWatch report: April 14 to April 20, 2019 (Week 16)

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Organization: Public Health Agency of Canada

Date published: 2019-04-26

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Overall Summary

  • Influenza continues to circulate in many regions across the country.
  • The second smaller wave of influenza activity, dominated by A(H3N2), decreased this week.
  • Detections of influenza A(H3N2) have been steadily increasing since mid-January and account for 89% of subtyped influenza A detections this week; however, influenza A(H1N1)pdm09 remains the predominant subtype to date this season.
  • Detections of influenza A continue to be greater than those of influenza B. There is currently very little influenza B circulation compared to previous seasons.

Influenza/Influenza-like Illness Activity (geographic spread)

During week 16, the following influenza activity levels were reported (Figure 1):

  • 14 regions reported localized activity: in Man.(1), Ont.(7), Que.(2), N.B.(1), N.S.(2), and Y.T.(1).
  • 22 regions reported sporadic activity: in B.C.(5), Man.(3), Que.(4), N.S.(2), N.B.(6), P.E.I (1) and N.W.T.(1)
  • Two regions: in Man.(1) and N.W.T.(1) reported no activity.

Figure 1 – Map of overall influenza/ILI activity by province and territory, Canada, week 2019-16

Figure 1. Text equivalent follows.
Figure 1 - Text equivalent
Province Influenza Surveillance Region Activity Level
N.L Eastern No Data
N.L Labrador-Grenfell No Data
N.L Central No Data
N.L Western No Data
P.E.I. Prince Edward Island Sporadic
N.S. Zone 1 - Western Sporadic
N.S. Zone 2 - Northern Sporadic
N.S. Zone 3 - Eastern Localized
N.S. Zone 4 - Central Localized
N.B. Region 1 Localized
N.B. Region 2 Sporadic
N.B. Region 3 Sporadic
N.B. Region 4 Sporadic
N.B. Region 5 Sporadic
N.B. Region 6 Sporadic
N.B. Region 7 Sporadic
Que. Nord-est Sporadic
Que. Québec et Chaudieres-Appalaches Sporadic
Que. Centre-du-Québec Sporadic
Que. Montréal et Laval Sporadic
Que. Ouest-du-Québec Localized
Que. Montérégie Localized
Ont. Central East Localized
Ont. Central West Localized
Ont. Eastern Localized
Ont. North East Localized
Ont. North West Localized
Ont. South West Localized
Ont. Toronto Localized
Man. Northern Regional No Activity
Man. Prairie Mountain Localized
Man. Interlake-Eastern Sporadic
Man. Winnipeg Sporadic
Man. Southern Health Sporadic
Sask. North No Data
Sask. Central No Data
Sask. South No Data
Alta. North Zone No Data
Alta. Edmonton No Data
Alta. Central Zone No Data
Alta. Calgary No Data
Alta. South Zone No Data
B.C. Interior Sporadic
B.C. Fraser Sporadic
B.C. Vancouver Coastal Sporadic
B.C. Vancouver Island Sporadic
B.C. Northern Sporadic
Y.T. Yukon Localized
N.W.T. North No Activity
N.W.T. South Sporadic
Nvt. Qikiqtaaluk No Data
Nvt. Kivalliq No Data
Nvt. Kitimeot No Data

Laboratory-Confirmed Influenza Detections

In week 16, the following results were reported from sentinel laboratories across Canada (Figures 2 and 3):

  • The percentage of tests positive for influenza decreased to 17% in week 16.
  • A total of 1,024 laboratory detections of influenza were reported, of which 85% were influenza A.
  • Influenza A(H3N2) accounted for 89% of subtyped influenza A detections.

To date this season, 44,970 laboratory-confirmed influenza detections have been reported:

  • 97% have been influenza A.
  • Among the 15,235 influenza A viruses subtyped, 73% have been A(H1N1)pdm09.
  • Influenza B often circulates later in the season in Canada (Feb-Apr). Fewer influenza B detections have been reported this season compared to recent seasons at this time of year.

To date this season, detailed information on age and type/subtype has been received for 36,036 laboratory-confirmed influenza cases (Table 1):

  • 83% of all influenza A(H1N1)pdm09 detections have been reported in individuals younger than 65 years of age.
  • 58% of all influenza A(H3N2) detections have been reported in adults 65 years of age and older.

For more detailed weekly and cumulative influenza data, see the text descriptions for Figures 2 and 3 or the Respiratory Virus Detections in Canada Report.

Figure 2 – Number of positive influenza tests and percentage of tests positive, by type, subtype and report week, Canada, weeks 2018-35 to 2019-16

Figure 2. Text equivalent follows.

The shaded area indicates weeks where the positivity rate was at least 5% and a minimum of 15 positive tests were observed, signalling the period of seasonal influenza activity.

Data for week 14 excludes subtyping results from one jurisdiction due to batch reporting of subtype information. The results for week 14 should be interpreted with caution.

Figure 2 - Text equivalent
Surveillance Week A(Unsubtyped) A(H3N2) A(H1N1)pdm09 Influenza B
35 3 2 7 0
36 4 7 4 0
37 3 2 3 1
38 6 3 2 3
39 11 5 1 3
40 16 7 29 3
41 27 6 21 3
42 40 19 55 2
43 83 23 128 4
44 169 13 214 6
45 244 18 295 15
46 346 10 404 9
47 449 17 507 8
48 679 29 632 10
49 851 35 785 16
50 1368 35 828 14
51 1890 54 953 21
52 2292 55 903 26
1 2387 58 893 31
2 1985 93 1061 43
3 1613 101 585 40
4 1483 93 532 48
5 1449 112 433 43
6 1234 118 315 42
7 1116 128 244 33
8 1066 236 216 40
9 1172 285 213 64
10 1085 335 268 71
11 990 357 197 90
12 959 380 155 131
13 939 387 128 167
14 1091 292 41 169
15 756 514 121 157
16 618 261 38 166

Figure 3 – Cumulative numbers of positive influenza specimens by type/subtype and province/territory, Canada, weeks 2018-35 to 2019-16

Figure 3. Text equivalent follows.
Figure 3 - Text equivalent
Reporting
provincesTable Figure 3 - Footnote 1
Week (April 7 to April 20, 2019) Cumulative (August 26, 2018 to April 20, 2019)
Influenza A B Influenza A B A & B
Total
A
Total
A
(H1N1)pdm09
A
(H3N2)
A(UnS)Table Figure 3 - Footnote 3 B
Total
A
Total
A
(H1N1)pdm09
A
(H3N2)
A(UnS)Table Figure 3 - Footnote 3 B
Total
BC 117 17 48 52 13 6160 3124 1426 1610 208 6368
AB 144 10 101 33 18 6596 3598 903 2095 228 6824
SK 0 0 0 0 0 2333 1378 104 851 75 2408
MB 12 1 6 5 5 1107 312 71 724 25 1132
ON 159 3 93 63 18 5953 2052 1469 2432 190 6143
QC 301 0 0 301 61 16345 0 0 16345 502 16847
NB 59 0 0 59 35 2722 230 34 2458 209 2931
NS 21 0 0 21 1 759 0 0 759 15 774
PEI 2 0 2 0 0 273 220 52 1 0 273
NL 52 0 0 52 1 971 1 0 970 4 975
YT 4 0 0 4 0 99 54 12 33 0 99
N.W.T 1 1 0 0 0 175 173 2 0 1 176
NU 0 0 0 0 0 20 20 0 0 0 20
Canada 872 32 250 590 152 43513 11162 4073 28278 1457 44970
PercentageTable Figure 3 - Footnote 2 85% 4% 29% 68% 15% 97% 26% 9% 65% 3% 100%
Table Figure 3 - Footnote 1

Specimens from NWT, YT, and Nvt are sent to reference laboratories in other provinces.

Return to Table Figure 3 - Footnote 1 referrer

Table Figure 3 - Footnote 2

Percentage of tests positive for sub-types of influenza A are a percentage of all influenza A detections.

Return to Table Figure 3 - Footnote 2 referrer

Table Figure 3 - Footnote 3

Unsubtyped: The specimen was typed as influenza A, but no result for subtyping was available.

Return to first Table Figure 3 - Footnote 3 referrer

Discrepancies in values in Figures 2 and 3 may be attributable to differing data sources.

Cumulative data includes updates to previous weeks.

Table 1 – Cumulative numbers of positive influenza specimens by type, subtype and age-group reported through case-based laboratory reporting, Canada, weeks 2018-35 to 2019-16
Age groups (years) Cumulative (August 26, 2018 to April 20, 2019)
Influenza A B Influenza A and B
A Total A(H1N1) pdm09 A(H3N2) A (UnS)Table 1 – Note 1 Total # %
0-4 6566 1679 213 4674 195 6761 19%
5-19 4967 1387 416 3164 330 5297 15%
20-44 6661 1990 536 4135 230 6891 19%
45-64 6757 1950 549 4258 92 6849 19%
65+ 10027 1452 2341 6234 211 10238 28%
Total 34978 8458 4055 22465 1058 36036 100%
Table 1 – Note 1

UnS: unsubtyped: The specimen was typed as influenza A, but no result for subtyping was available.

Return to Table 1 – Note 1 referrer.

Syndromic / Influenza-like Illness Surveillance

Healthcare Professionals Sentinel Syndromic Surveillance

In week 16, 0.8% of visits to healthcare professionals were due to influenza-like illness (ILI) (Figure 4).

Figure 4 – Percentage of visits for ILI reported by sentinels by report week, Canada, weeks 2018-35 to 2019-16

Number of Sentinels Reporting in Week 16: 91

Figure 4. Text equivalent follows.

The shaded area represents the maximum and minimum percentage of visits for ILI reported by week from seasons 2013-2014 to 2017-2018

Figure 4 - Text equivalent
Surveillance Week 2018-19 Average Min Max
35 0.6% 0.8% 0.5% 1.2%
36 0.7% 0.8% 0.7% 1.0%
37 0.6% 0.9% 0.7% 1.0%
38 0.5% 1.1% 1.0% 1.4%
39 1.8% 1.1% 0.9% 1.4%
40 0.6% 1.3% 0.9% 1.6%
41 1.1% 1.4% 0.9% 2.4%
42 1.0% 1.4% 1.0% 1.9%
43 0.9% 1.3% 1.0% 1.5%
44 0.8% 1.3% 0.9% 1.6%
45 1.3% 1.3% 1.2% 1.5%
46 1.3% 1.6% 1.0% 2.0%
47 1.3% 1.5% 1.1% 1.9%
48 1.5% 1.6% 0.8% 2.1%
49 1.6% 1.5% 1.0% 2.5%
50 1.5% 2.3% 1.3% 3.7%
51 1.0% 2.5% 1.6% 4.1%
52 3.2% 4.5% 1.7% 7.1%
1 2.8% 3.7% 1.7% 5.2%
2 1.8% 3.0% 1.1% 4.5%
3 1.1% 2.5% 1.3% 3.6%
4 1.3% 2.4% 1.7% 3.5%
5 1.4% 2.7% 2.0% 4.4%
6 0.9% 2.7% 2.1% 3.3%
7 0.8% 2.4% 1.7% 3.0%
8 0.9% 2.3% 1.9% 2.7%
9 1.0% 2.2% 1.8% 2.7%
10 1.1% 2.1% 1.8% 2.7%
11 0.5% 2.1% 1.7% 2.7%
12 1.0% 1.8% 1.1% 2.7%
13 1.1% 1.7% 1.2% 2.6%
14 1.0% 1.8% 1.3% 2.4%
15 0.9% 1.6% 0.9% 1.9%
16 0.8% 1.5% 1.0% 1.9%

Participatory Syndromic Surveillance

In week 16, 1,904 participants reported to FluWatchers, of which 46 (2.4%) reported symptoms of cough and fever (Figure 5).

Among the 46 participants who reported fever and cough:

  • 13% consulted a healthcare professional;
  • 65% reported days missed from work or school, resulting in a combined total of 92 missed days of work or school.

Figure 5 - Percentage of participants reporting cough and fever, Canada, weeks 2018-40 to 2019-16

Number of Participants Reporting in Week 16: 1,904

Figure 5. Text equivalent follows.
Figure 5 - Text equivalent
Surveillance Week % cough and fever
40 2.6%
41 2.5%
42 1.7%
43 1.5%
44 1.3%
45 1.7%
46 2.0%
47 1.3%
48 2.4%
49 2.0%
50 3.7%
51 3.9%
52 3.6%
1 3.4%
2 2.8%
3 2.5%
4 2.9%
5 3.0%
6 2.6%
7 2.7%
8 2.5%
9 2.7%
10 3.1%
11 2.6%
12 2.4%
13 2.4%
14 2.3%
15 1.8%
16 2.4%

Influenza Outbreak Surveillance

In week 16, 21 new laboratory-confirmed influenza outbreaks were reported: long-term care facilities (LTCF) (12) and other settings (9). Three new ILI outbreaks were reported: LTCF (2) and schools (1). Among the outbreaks with available subtype information (7), all were associated with influenza A(H3N2).

To date this season, 753 laboratory-confirmed influenza outbreaks have been reported (Figure 6):

  • 441 (59%) outbreaks were in LTCF, 30 were in schools and daycares, 113 in acute care facilities, and 169 were in other settings.
  • Among the 662 outbreaks for which the influenza type was available, 98% (648) were associated with influenza A.
  • Among the 281 outbreaks for which the influenza A subtype was available, 54% (151) were associated with influenza A(H3N2);

To date this season, 151 ILI outbreaks have been reported; 85 occurred in LTCF, 61 in schools, one in other settings and four in acute care facilities.

Figure 6 – Number of new outbreaks of laboratory-confirmed influenza by report week, Canada, weeks 2018-35 to 2019-16

Figure 6. Text equivalent follows.
Figure 6 - Text equivalent
Surveillance Week Acute Care Facilities Long Term Care Facilities Other Schools and Daycares Remote and/or Isolated Communities
35 0 0 0 0 0
36 0 0 0 0 0
37 0 0 0 0 0
38 0 0 0 0 0
39 0 0 0 0 0
40 0 2 0 1 0
41 0 0 0 0 0
42 0 2 1 0 0
43 0 3 1 0 0
44 0 1 1 2 0
45 0 2 1 3 0
46 2 5 1 3 0
47 2 4 0 1 0
48 2 6 1 5 0
49 1 2 3 1 0
50 3 9 5 4 0
51 3 12 4 2 0
52 4 25 5 0 0
1 5 40 11 0 0
2 8 38 7 0 0
3 3 27 10 0 0
4 6 19 10 1 0
5 5 23 7 0 0
6 9 21 7 0 0
7 6 20 7 0 0
8 5 30 11 4 0
9 9 23 13 2 0
10 12 26 8 1 0
11 3 23 14 0 0
12 8 22 13 0 0
13 8 23 11 0 0
14 3 18 4 0 0
15 6 15 7 0 0
16 0 12 9 0 0

Severe Outcomes Influenza Surveillance

Provincial/Territorial Influenza Hospitalizations and Deaths

To date this season, 2,932 influenza-associated hospitalizations have been reported by participating provinces and territoriesFootnote 1.

Hospitalizations (Table 2):

  • 98.4% (2,886) were associated with influenza A
  • Among the 1,670 cases for which the influenza subtype was available, 1,404 (84%) were associated with A(H1N1)pdm09.
  • The highest estimated rate of hospitalization is among adults over 65 years of age.

Intensive Care Unit (ICU) cases and deaths:

  • To date this season 536 ICU admissions and 165 deaths have been reported.
    • 41% (221) of reported ICU admissions were in adults aged 45-64 years.
    • All but seven ICU admissions were associated with influenza A.
    • All but one of the deaths were associated with influenza A.
Table 2 – Cumulative number and estimated rate of hospitalizations by age-group reported by participating provinces and territoriesFootnote 1, Canada, weeks 2018-35 to 2019-16
Age Groups (years) Cumulative (August 26, 2018 to April 20, 2019)
Influenza A Influenza B Rate per 100,000 population
0-4 383 17 84.04
5-19 231 15 17.73
20-44 345 4 12.27
45-64 748 3 34.51
65+ 1179 7 97.48
Total 2886 46  
% 98.4% 1.6%  
Table 2 – Note 1

Influenza-associated hospitalizations are reported by Alberta, Manitoba, New Brunswick, Newfoundland and Labrador, Northwest Territories, Nova Scotia, Prince Edward Island and Yukon. Only hospitalizations that require intensive medical care are reported by Saskatchewan. The cumulative rate of hospitalizations is calculated using the total population by age-group in participating provinces and territories.

Return to Table 2 – Note 1

Pediatric Influenza Hospitalizations and Deaths

To date this season, 1,217 pediatric hospitalizations have been reported (Figure 7 & 8):

  • 66% of cases were in children under 5 years of age.
  • 95% (1,152) of cases have been associated with influenza A.
  • Among the 358 cases for which the influenza subtype was available, 299 (84%) were associated with A(H1N1)pdm09.

To date this season, 229 ICU admissions, and 10 deaths have been reported.

  • 59% (135) of ICU admissions were in children under 5 years of age.
  • All but nine of the ICU admissions have been associated with influenza A.
  • 80% (8) of deaths occurred in children 2 to 4 years of age.
  • All deaths have been associated with influenza A.

Figure 7 – Number of pediatric (≤16 years of age) hospitalizations reported by the IMPACT network, by week, Canada, weeks 2018-35 to 2019-16

Figure 7. Text equivalent follows.

The shaded area represents the maximum and minimum number of cases reported by week from seasons 2010-11 to 2017-18

Figure 7 - Text equivalent
Surveillance week 2018-19 Average Min Max
35 0 0 0 0
36 0 0 0 1
37 0 0 0 2
38 0 0 0 2
39 0 1 0 3
40 0 0 0 2
41 1 1 0 2
42 5 1 0 4
43 12 1 0 3
44 15 3 1 6
45 37 3 2 4
46 41 5 1 13
47 36 4 0 9
48 40 9 2 23
49 46 15 3 28
50 51 23 4 47
51 84 32 4 72
52 89 47 7 92
1 92 40 5 75
2 61 35 4 62
3 65 38 4 67
4 66 35 7 47
5 49 40 11 59
6 47 38 15 79
7 46 40 17 120
8 45 47 25 139
9 35 50 13 153
10 46 48 17 135
11 41 41 18 118
12 42 33 13 89
13 29 32 14 67
14 34 24 12 56
15 28 23 13 56
16 33 20 10 41

Figure 8 - Cumulative numbers of pediatric hospitalizations (≤16 years of age) with influenza by age-group reported by the IMPACT network, Canada, weeks 2018-35 to 2019-16

Figure 8. Text equivalent follows.
Figure 8 - Text equivalent
Age Group Total
0-5 mo 130
6-23 mo 308
2-4 yr 367
5-9 yr 270
10-16 yr 142

Adult Influenza Hospitalizations and Deaths

Surveillance of laboratory-confirmed influenza-associated adult (≥16 years of age) hospitalizations by the Canadian Immunization Research Network (CIRN) Serious Outcomes Surveillance (SOS) network began on November 1st for the 2018-19 season.

To date this season, 901 hospitalizations, 105 ICU admissions and 50 deaths have been reported (Figure 9):

  • 840 (93%) hospitalizations were associated with influenza A.
  • A greater proportion of hospitalizations have been reported among adults ≥65 years of age (60%) compared to adults <65 years of age (40%).
  • Among the 194 cases for which the influenza subtype was available, 106 (55%) were associated with A(H1N1)pdm09.
  • 89% of hospitalized cases reported more than one type of comorbid condition.
  • The most commonly reported comorbidity was endocrine disorders, which were reported in 95% of hospitalized cases.

Figure 9 - Cumulative numbers of adult hospitalizations (>20 years of age) with influenza by age-group reported by CIRN, Canada, 2018-19, weeks 2018-44 to 2019-16

Figure 9. Text equivalent follows.
Figure 9 - Text equivalent
Age Group Total
20-34 yr 86
35-49 yr 82
50-64 yr 190
65-79 yr 263
80+ yr 280

Influenza Strain Characterizations

Since September 1, 2018, the National Microbiology Laboratory (NML) has characterized 2,006 influenza viruses (353 A(H3N2), 1,553 A(H1N1) and 100 B) that were received from Canadian laboratories.

Genetic Characterization of Influenza A(H3N2):

164 influenza A(H3N2) viruses did not grow to sufficient hemagglutination titer for antigenic characterization by hemagglutination inhibition (HI) assay. Therefore, NML has performed genetic characterization to determine the genetic group identity of these viruses.

Sequence analysis of the HA gene of the viruses showed that:

  • 20 viruses belonged to genetic group 3C.2a.
  • 139 viruses belonged to subclade 3C.2a1.
  • Four viruses belonged to 3C.3a.
  • One isolate could not be sequenced.

A/Singapore/INFIMH-16-0019/2016-like virus belongs to genetic group 3C.2a1 and is the influenza A(H3N2) component of the 2018-19 Northern Hemisphere influenza vaccine.

Antigenic Characterization:

Influenza A (H3N2):

  • 111 influenza A(H3N2) viruses were antigenically characterized as A/Singapore/INFIMH-16-0019/2016-like by HI testing using antiserum raised against egg-propagated A/Singapore/INFIMH-16-0019/2016.
  • 78 viruses showed reduced titer with ferret antisera raised against egg-propagated A/Singapore/INFIMH-16-0019/2016.
  • A/Singapore/INFIMH-16-0019/2016-like virus is the influenza A(H3N2) component of the 2018-19 Northern Hemisphere influenza vaccine.
  • 71 influenza A(H3N2) viruses characterized belonged to genetic group 3C.2a1. 18 viruses belonged to genetic group 3C.2a and 29 to 3C.3a. Sequencing is pending for the remaining isolates.

Influenza A(H1N1):

  • 1,510 A(H1N1) viruses characterized were antigenically similar to A/Michigan/45/2015, which is the influenza A(H1N1) component of the 2018-19 Northern Hemisphere influenza vaccine.
  • 43 viruses showed reduced titer with ferret antisera raised against cell culture-propagated A/Michigan/45/2015

Influenza B:

Influenza B viruses can be divided into two antigenically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87 viruses. The recommended influenza B components for the 2018-19 Northern Hemisphere influenza vaccine are B/Colorado/06/2017 (Victoria lineage) and B/Phuket/3073/2013 (Yamagata lineage).

  • 16 influenza B viruses were characterized as B/Colorado/06/2017, which belong to the Victoria lineage and are included as an influenza B component of the 2018-19 Northern Hemisphere influenza vaccine.
  • 61 viruses showed reduced titer with ferret antisera raised against cell culture-propagated B/Colorado/06/2017.
  • 23 influenza B viruses were characterized as B/Phuket/3073/2013-like, which belongs to the Yamagata lineage and is included as an influenza B component of the 2018-19 Northern Hemisphere quadrivalent influenza vaccine.

Antiviral Resistance

Antiviral Resistance – Amantadine:

428 influenza A (77 A(H3N2) and 351 A(H1N1)) viruses were tested for resistance to amantadine and it was found that:

  • All 428 influenza A viruses were resistant to amantadine.

Antiviral Resistance – Oseltamivir:

1,183 influenza viruses (134 A(H3N2), 989 A(H1N1) and 60 B) were tested for resistance to oseltamivir and it was found that:

  • All 134 A(H3N2) viruses were sensitive to oseltamivir.
  • Of the 989 A(H1N1) viruses tested, 985 were sensitive to oseltamivir and four viruses were resistant to oseltamivir with a H275Y mutation.
  • All 60 B viruses were sensitive to oseltamivir.

Antiviral Resistance – Zanamivir:

1,182 influenza viruses (134 A(H3N2), 988 H1N1 and 60 B) were tested for resistance to zanamivir and it was found that:

  • All 1,182 influenza viruses were sensitive to zanamivir.

Vaccine Monitoring

Vaccine monitoring refers to activities related to the monitoring of influenza vaccine coverage and effectiveness.

Vaccine Coverage

The Seasonal Influenza Immunization Coverage Survey is an annual telephone survey conducted between January and February that collects information related to the influenza vaccine in Canada. This survey measures vaccine coverage, which is the percentage of people who received the annual seasonal influenza vaccine in a specific influenza season.

In the 2018-19 influenza season, coverage was:

  • 34% among adults aged 18 to 64 years.
    • 31% among adults aged 18-64 wihout chronic diseases.
    • 43% among adults aged 18 to 64 years with chronic diseases.
  • 70% among seniors (aged 65 years and older).
Table 3 – Influenza vaccine coverage among adults (≥ 18 years of age) by age group, gender and chronic diseaseTable 3 – Note 1, Seasonal Influenza Vaccination Coverage Survey, Canada, September 2018-February 2019
Age group (years) Male Female Combined
Total Vaccine Coverage (%) Total Vaccine Coverage (%) Total Vaccine Coverage (%)
All adults (≥18) 1568 36.6 2150 46.8 3726 41.8
18-64 1252 28.6 1640 39.9 2898 34.3
without chronic
diseases
948 25.8 1171 36.1 2124 30.8
with chronic
diseases
304 36.3 465 48.5 770 42.8
≥65 316 69.0 510 70.9 828 69.9
Table 3 – Note 1

Excluded from stratified analysis: eight people who did not disclose their gender and four people (18-64 years old) who did disclose whether they had any chronic diseases.

Return to Table 3 – Note 1 referrer.

Vaccine Effectiveness

Vaccine effectiveness (VE) is a measure of how well the influenza vaccine is able to prevent influenza illness. Throughout the influenza season, surveillance networks estimate how well the influenza vaccine is working. Estimates can vary depending on several factors such as the study methods; the population, setting and outcomes that are being studied; the type and mix of vaccine products; the stage of the season and the kinds of influenza viruses that are circulating when the study is conducted.

The community-based Canadian Sentinel Practitioner Surveillance Network (SPSN) published an interim VE estimate in January 2019 for A(H1N1)pdm09. Subsequently, given an atypical late-season wave of influenza A(H3N2),SPSN has undertaken an additional interim analysis to assess effectiveness of the 2018/19 influenza vaccine against medically-attended outpatient A(H3N2) illness. Vaccine effectiveness (VE) monitoring methods and results are available at the SPSN website.

Based on data collected as of March 30th, 2019 from more than 2800 patients from B.C., Alta., Ont., and Que., the 2018/19 northern hemisphere vaccine effectiveness has varied depending on the strain.

  • A(H1N1)pdm09: In the first interim analysis, VE against A(H1N1)pdm09 was 72% (95% CI: 60 to 81) overall, with substantial protection observed in all age groups. In the most recent analysis as of March 30th, estimates against A(H1N1)pmd09 have remained stable at approximately 70%.
  • A(H3N2): In the most recent analysis, VE against A(H3N2) was 23% (95% CI: -9 to 46) overall. As the confidence interval crosses zero, this estimate does not provide evidence of vaccine protection against medically-attended outpatient A(H3N2) illness.

The SPSN continues to monitor and will further update VE estimates at end of season.

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