FluWatch report: June 23, 2019 to July 20, 2019 (Weeks 26 to 29)

Influenza/Influenza-like Illness Activity (geographic spread)

During week 29, the following influenza activity levels were reported (Figure 1):

• One region in Alta. reported localized activity
• 13 regions reported sporadic activity: B.C.(3), Alta.(2), Ont.(5), Que.(2), and P.E.I. (1)
• 38 regions in 12 provinces and territories reported no activity.

Figure 1 - Text equivalent

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

Laboratory-Confirmed Influenza Detections

During weeks 26-29, the following results were reported from sentinel laboratories across Canada (Figures 2 and 3):

• The percentage of tests positive for influenza continued to decrease from 3.1% in week 26 to 1.6% in week 29.
• A total of 181 laboratory detections of influenza were reported during these four weeks, of which 71% were influenza A. Influenza A(H3N2) accounted for 93% of subtyped influenza A detections during these five weeks.

To date this season, 48,694 laboratory-confirmed influenza detections have been reported:

• 95% have been influenza A. Overall, among the 16,528 influenza A viruses subtyped this season, 69% have been A(H1N1)pdm09.
• In the earlier part of the season (weeks 43-7) influenza A(H1N1)pdm09 was the predominant circulating subtype, followed by a smaller late-season wave of influenza A(H3N2) (weeks 8-21).
• Fewer influenza B detections have been reported this season compared to recent seasons.

To date this season, detailed information on age and type/subtype has been received for 39,111 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.

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	614	277	42	175
17	448	244	55	194
18	265	160	29	148
19	266	154	33	151
20	137	163	27	136
21	89	79	8	122
22	43	53	17	75
23	33	42	8	60
24	39	44	5	48
25	29	41	4	31
26	23	30	0	27
27	15	23	3	12
28	10	18	2	11
29	9	16	1	9

Figure 3 - Text equivalent

Reporting provincesTable Figure 3 - Footnote 1	Week (June 23 to July 20, 2019)					Cumulative (August 26, 2018 to July 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	67	3	49	15	5	6429	3162	1646	1621	242	6671
AB	30	0	27	3	8	7249	3642	1384	2223	365	7614
SK	0	0	0	0	0	2457	1389	142	926	109	2566
MB	0	0	0	0	3	1193	335	131	727	77	1270
ON	12	2	7	3	10	6400	2106	1729	2565	307	6707
QC	12	0	0	12	17	17161	0	0	17161	978	18139
NB	0	0	0	0	1	2883	246	39	2598	366	3249
NS	0	0	0	0	0	786	0	0	786	22	808
PEI	1	0	0	1	0	279	221	56	2	2	281
NL	0	0	0	0	8	1035	1	0	1034	13	1048
YT	6	1	4	1	0	137	66	32	39	0	137
N.W.T	0	0	0	0	1	181	175	6	0	3	184
NU	0	0	0	0	0	20	20	0	0	0	20
Canada	128	6	87	35	53	46210	11363	5165	29682	2484	48694
PercentageTable Figure 3 - Footnote 2	71%	5%	68%	27%	29%	95%	25%	11%	64%	5%	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 2019-25

Age groups (years)	Cumulative (August 26, 2018 to July 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	6790	1694	275	4821	376	7166	18%
5-19	5196	1393	505	3298	631	5827	15%
20-44	6973	2028	655	4290	428	7401	19%
45-64	7118	1985	716	4417	142	7260	19%
65+	11183	1494	2924	6765	274	11457	29%
Total	37260	8594	5075	23591	1851	39111	100%
Table 1 Footnote 1 UnS: unsubtyped: The specimen was typed as influenza A, but no result for subtyping was available. Return to Table 1 footnote 1 referrer

Syndromic / Influenza-like Illness Surveillance

Healthcare Professionals Sentinel Syndromic Surveillance

In week 29, 0.3% of visits to healthcare professionals were due to influenza-like illness (ILI) (Figure 4).

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	1.7%	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	1.1%	1.4%	1.0%	1.9%
43	1.0%	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	1.1%	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	1.4%	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	0.9%	2.4%	1.7%	3.0%
8	1.0%	2.3%	1.9%	2.7%
9	1.1%	2.2%	1.8%	2.7%
10	1.1%	2.1%	1.8%	2.7%
11	0.6%	2.1%	1.7%	2.7%
12	1.1%	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	0.8%	1.6%	0.9%	1.9%
16	0.7%	1.5%	1.0%	1.9%
17	0.5%	1.5%	0.9%	2.2%
18	0.7%	1.2%	0.8%	1.6%
19	0.8%	1.3%	1.0%	1.7%
20	0.8%	1.2%	0.9%	1.4%
21	0.9%	0.9%	0.3%	1.3%
22	0.9%	0.8%	0.6%	1.0%
23	0.9%	0.9%	0.6%	1.2%
24	0.9%	0.7%	0.4%	1.0%
25	0.8%	1.2%	0.5%	1.6%
26	0.5%	0.8%	0.4%	1.4%
27	0.5%	0.8%	0.4%	1.5%
28	1.0%	1.0%	0.6%	1.9%
29	0.3%	0.8%	0.5%	1.1%

Participatory Syndromic Surveillance

FluWatchers surveillance has ended for the 2018-19 influenza season. On average 2,097 participants reported to FluWatchers each week, resulting in 64,672 questionnaires completed this season. The proportion of participants reporting fever and cough peaked in week 51 at 3.9% (Figure 5). Approximately 63% of FluWatchers participants reported being vaccinated for influenza in the 2018-19 season.

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%
17	2.3%
18	1.9%

Influenza Outbreak Surveillance

In weeks 26 to 29, two new laboratory-confirmed influenza outbreaks were reported: one in a long-term care facility (LTCF) (1) and one in an acute care facility(1). Both outbreaks were associated with influenza A.

To date this season, 931 laboratory-confirmed influenza outbreaks have been reported (Figure 6):

• 545 (59%) outbreaks were in LTCF, 32 were in schools and daycares, 130 in acute care facilities, and 224 were in other settings.
• Among the 844 outbreaks for which the influenza type was available, 98% (824) were associated with influenza A.
• Among the 387 outbreaks for which the influenza A subtype was available, 65% (253) were associated with influenza A(H3N2);

To date this season, 176 ILI outbreaks have been reported; 108 occurred in LTCF, 62 in schools, five in acute care facilities, and one in other settings.

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	6	24	6	0	0
6	9	23	7	0	0
7	6	20	8	0	0
8	5	31	11	4	0
9	9	23	15	2	0
10	12	28	8	1	0
11	5	26	15	0	0
12	9	23	14	0	0
13	9	24	11	0	0
14	6	26	8	0	0
15	7	26	12	1	0
16	0	18	13	0	0
17	1	15	13	0	0
18	2	17	5	0	0
19	1	11	2	0	0
20	1	2	4	1	0
21	0	5	4	0	0
22	1	0	1	0	0
23	1	3	3	0	0
24	0	2	1	0	0
25	0	0	2	0	0
26	0	1	0	0	0
27	0	0	0	0	0
28	0	0	0	0	0
29	1	0	0	0	0

Severe Outcomes Influenza Surveillance

Provincial/Territorial Influenza Hospitalizations and Deaths

To date this season, 3,634 influenza-associated hospitalizations have been reported by participating provinces and territories^{Footnote 1}.

Hospitalizations (Table 2):

• 96% (3,503) were associated with influenza A
• Among the 2,135 cases for which the influenza subtype was available, 1,453 (68%) 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 613 ICU admissions and 221 deaths have been reported.
  • 38% (241) of reported ICU admissions were in adults aged 45-64 years.
  • 97% (595) of ICU admissions were associated with influenza A.
  • All but four 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 territories^{Footnote 1}, Canada, weeks 2018-35 to 2019-29

Age Groups (years)	Cumulative (August 26, 2018 to July 20, 2019)
	Influenza A	Influenza B	Rate per 100,000 population
0-4	422	39	96.85
5-19	266	34	21.62
20-44	386	20	14.27
45-64	857	15	40.07
65+	1572	23	131.10
Total	3503	131
%	96.4%	3.6%
Table 2 Footnote 1 Influenza-associated hospitalizations are reported by N.L., P.E.I, N.S., N.B., Man., Alb., Y.T. and N.W.T. Only hospitalizations that require intensive medical care are reported by Sask. The cumulative rate of hospitalizations is calculated using the population by age-group in participating provinces and territories. Table 2 footnote 1 referrer

Pediatric Influenza Hospitalizations and Deaths

To date this season, 1,340 pediatric hospitalizations have been reported (Figure 7 & 8):

• 66% of cases were in children under 5 years of age.
• 91% (1,217) of cases have been associated with influenza A.
• Among the 384 cases for which the influenza subtype was available, 307 (80%) were associated with A(H1N1)pdm09.

To date this season, 265 ICU admissions, and 10 deaths have been reported.

• 60% (158) of ICU admissions were in children under 5 years of age.
• 92% (244) of ICU admissions have been associated with influenza A; 81% of the 108 cases for which the influenza A subtype was available were associated with A(H1N1)pdm09.
• 80% (8) of deaths occurred in children 2 to 4 years of age.
• All deaths have been associated with influenza A.

The shaded area represents the maximum and minimum number of cases reported by week from seasons 2010-11 to 2017-19

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	37	4	0	9
48	40	9	2	23
49	47	15	3	28
50	51	23	4	47
51	84	32	4	72
52	89	47	7	92
1	91	40	5	75
2	63	35	4	62
3	64	38	4	67
4	67	35	7	47
5	50	40	11	59
6	45	38	15	79
7	45	40	17	120
8	44	47	25	139
9	33	50	13	153
10	45	48	17	135
11	43	41	18	118
12	41	33	13	89
13	28	32	14	67
14	31	24	12	56
15	28	23	13	56
16	33	20	10	41
17	33	16	8	37
18	22	14	4	28
19	19	10	5	18
20	17	10	4	19
21	10	5	2	10
22	8	4	1	8
23	6	2	0	5
24	7	2	0	5
25	5	1	0	3
26	2	1	0	3
27	1	1	0	2
28	0	1	0	2
29	0	0	0	2

Figure 8 - Text equivalent

Age Group	Total
0-5 mo	143
6-23 mo	329
2-4 yr	413
5-9 yr	301
10-16 yr	154

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 and ended  May 30th for the 2018-19 season.

This season, 950 hospitalizations, 127 ICU admissions and 57 deaths have been reported (Figure 9):

• 876 (92%) 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 208 cases for which the influenza subtype was available, 107 (51%) were associated with A(H1N1)pdm09.
• 88% of hospitalized cases reported more than one type of comorbid condition.  
• The most commonly reported comorbidity was endocrine disorders, which were reported in 88% of hospitalized cases.

Figure 9 - Text equivalent

Age Group	Total
20-34 yr	93
35-49 yr	85
50-64 yr	199
65-79 yr	273
80+ yr	300

Influenza Strain Characterizations

From September 1, 2018 to July 20, 2019, the National Microbiology Laboratory (NML) has characterized 2,526 influenza viruses (639 A(H3N2), 1,652 A(H1N1) and 235 B) that were received from Canadian laboratories.

Genetic Characterization of Influenza A(H3N2):

270 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.
• 241 viruses belonged to subclade 3C.2a1.
• Eight 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):

• 207 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.
• 162 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.
• 200 influenza A(H3N2) viruses characterized belonged to genetic group 3C.3a, 140 viruses to 3C.2a1. and 20 viruses belonged to genetic group 3C.2a . Sequencing is pending for the remaining isolates.

Influenza A(H1N1):

• 1,608 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.
• 44 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).

• 45 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.
• 164 viruses showed reduced titer with ferret antisera raised against cell culture-propagated B/Colorado/06/2017. Sequence analysis showed that 158 viruses that showed reduced titer had a three amino acid deletion (162-164) in the HA gene.
• 26 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:

519 influenza A (121 A(H3N2) and 380 A(H1N1)) viruses were tested for resistance to amantadine and it was found that:

• All 519 influenza A viruses were resistant to amantadine.

Antiviral Resistance – Oseltamivir:

1,431 influenza viruses (214 A(H3N2), 1,080 A(H1N1) and 137 B) were tested for resistance to oseltamivir and it was found that:

• All 214 A(H3N2) viruses were sensitive to oseltamivir.
• Of the 1,080 A(H1N1) viruses tested, 1,076 were sensitive to oseltamivir and four viruses were resistant to oseltamivir with a H275Y mutation.
• All 137 B viruses were sensitive to oseltamivir.

Antiviral Resistance – Zanamivir:

1,429 influenza viruses (214 A(H3N2), 1,078 H1N1 and 137 B) were tested for resistance to zanamivir and it was found that:

• All 1,429 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 disease^{Table 3 – Note 1}, Seasonal Influenza Vaccination Coverage Survey, Canada, October 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 Footnote 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. Table 3 footnote 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 30^th, 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.