FluWatch report: May 19 to June 22, 2019 (Weeks 21 to 25)

Influenza/Influenza-like Illness Activity (geographic spread)

During week 25, the following influenza activity levels were reported (Figure 1):

• 3 regions reported localized activity: in Alta.(2), and N.B.(1).
• 19 regions reported sporadic activity: in Alta.(2), Sask.(2), Man.(2), Ont.(5), Que.(5), N.L.(1), and N.B.(2)
• 26 regions in 11 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	Sporadic
N.L.	Central	No Activity
N.L.	Western	No Activity
P.E.I.	Prince Edward Island	No Activity
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	Sporadic
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	Sporadic
N.B.	Region 7	Localized
Que.	Nord-est	Sporadic
Que.	Québec et Chaudieres-Appalaches	No Activity
Que.	Centre-du-Québec	Sporadic
Que.	Montréal et Laval	Sporadic
Que.	Ouest-du-Québec	Sporadic
Que.	Montérégie	Sporadic
Ont.	Central East	Sporadic
Ont.	Central West	Sporadic
Ont.	Eastern	Sporadic
Ont.	North East	No Activity
Ont.	North West	Sporadic
Ont.	South West	No Activity
Ont.	Toronto	Sporadic
Man.	Northern Regional	Sporadic
Man.	Prairie Mountain	No Activity
Man.	Interlake-Eastern	No Activity
Man.	Winnipeg	Sporadic
Man.	Southern Health	No Activity
Sask.	North	Sporadic
Sask.	Central	No Activity
Sask.	South	Sporadic
Alta.	North Zone	Sporadic
Alta.	Edmonton	Localized
Alta.	Central Zone	Sporadic
Alta.	Calgary	Localized
Alta.	South Zone	No Activity
B.C.	Interior	No Data
B.C.	Fraser	No Data
B.C.	Vancouver Coastal	No Data
B.C.	Vancouver Island	No Data
B.C.	Northern	No Data
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 21-25, 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 7.5% in week 21 to 3.9% in week 25. The percentage of positive tests crossed the seasonal threshold of 5%, indicating the end of the influenza season at the national level in week 21.
• A total of 864 laboratory detections of influenza were reported during these five weeks, of which 61% were influenza A. Influenza A(H3N2) accounted for 86% of subtyped influenza A detections during these five weeks.

To date this season, 48,432 laboratory-confirmed influenza detections have been reported:

• 95% have been influenza A. Overall, among the 16,426 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. The percentage of tests positive for influenza B was highest throughout the months of April and May (weeks 13-22) between 1.9% and 3.0%.

To date this season, detailed information on age and type/subtype has been received for 38,911 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	41	42	5	48
25	33	36	2	28

Figure 3 - Text equivalent

Reporting provincesTable Figure 3 - Footnote 1	Week (May 19 to June 22, 2019)					Cumulative (August 26, 2018 to June 22, 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	52	4	40	8	6	6354	3159	1592	1603	237	6591
AB	180	5	145	30	44	7218	3642	1356	2220	357	7575
SK	0	0	0	0	0	2401	1389	142	870	98	2499
MB	23	7	14	2	16	1193	334	131	728	73	1266
ON	82	18	34	30	46	6386	2103	1721	2562	295	6681
QC	109	0	0	109	163	17149	0	0	17149	961	18110
NB	33	6	3	24	43	2883	246	39	2598	365	3248
NS	2	0	0	2	3	786	0	0	786	22	808
PEI	0	0	0	0	0	278	221	56	1	2	280
NL	6	0	0	6	1	1035	1	0	1034	5	1040
YT	16	0	13	3	0	131	65	28	38	0	131
N.W.T	2	0	2	0	0	181	175	6	0	2	183
NU	0	0	0	0	0	20	20	0	0	0	20
Canada	505	40	251	214	322	46015	11355	5071	29589	2417	48432
PercentageTable Figure 3 - Footnote 2	61%	8%	50%	42%	39%	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 June 22, 2019)
	Influenza A				B	Influenza A and B
	A Total	A(H1N1) pdm09	A(H3N2)	A (UnS)Table 1 – Note 1	Total	#	%
0-4	6773	1693	271	4809	365	7138	18%
5-19	5186	1392	504	3290	620	5806	15%
20-44	6954	2027	647	4280	405	7359	19%
45-64	7089	1984	702	4403	139	7228	19%
65+	11107	1492	2880	6735	273	11380	29%
Total	37109	8588	5004	23517	1802	38911	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 25, 0.8% 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	1.8%	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	0.8%	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%

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 21 to 25, 22 new laboratory-confirmed influenza outbreaks were reported: long-term care facilities (LTCF) (10) acute care facilities (1) and other settings (11). All the influenza outbreaks were associated with influenza A, of which 15 were due to influenza A(H3N2) and the remainder were unsubtyped.

To date this season, 926 laboratory-confirmed influenza outbreaks have been reported (Figure 6): 

• 542 (59%) outbreaks were in LTCF, 32 were in schools and daycares, 129 in acute care facilities, and 223 were in other settings.
• Among the 841 outbreaks for which the influenza type was available, 98% (821) were associated with influenza A.
• Among the 385 outbreaks for which the influenza A subtype was available, 65% (251) were associated with influenza A(H3N2);

To date this season, 172 ILI outbreaks have been reported; 104 occurred in LTCF, 62 in schools, one in other settings and five in acute care facilities.

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	7	0	0
8	5	30	11	4	0
9	9	23	14	2	0
10	12	27	8	1	0
11	4	24	14	0	0
12	9	22	15	0	0
13	9	23	11	0	0
14	3	22	6	0	0
15	6	19	8	0	0
16	0	15	10	0	0
17	1	12	8	0	0
18	1	14	4	0	0
19	1	8	2	0	0
20	1	1	1	0	0
21	0	5	4	0	0
22	1	0	1	0	0
23	0	3	3	0	0
24	0	2	1	0	0
25	0	0	2	0	0

Severe Outcomes Influenza Surveillance

Provincial/Territorial Influenza Hospitalizations and Deaths

To date this season, 3,622 influenza-associated hospitalizations have been reported by participating provinces and territories^{Footnote 1}. Note that data from one participating jurisdiction has not been available since surveillance week 13, so these figures are an underestimate of cases for this season.

Hospitalizations (Table 2):

• 96% (3,495) were associated with influenza A
• Among the 2,122 cases for which the influenza subtype was available, 1,452 (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 614 ICU admissions and 209 deaths have been reported.
  • 39% (241) of reported ICU admissions were in adults aged 45-64 years.
  • 97% (597) 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 2019-25

Age Groups (years)	Cumulative (August 26, 2018 to June 22, 2019)
	Influenza A	Influenza B	Rate per 100,000 population
0-4	421	37	96,22
5-19	264	34	21,48
20-44	388	18	14,27
45-64	856	15	40,02
65+	1566	23	130.60
Total	3495	127
%	96.5%	3.5%
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, 1,336 pediatric hospitalizations have been reported (Figure 7 & 8):

• 66% of cases were in children under 5 years of age.
• 91% (1,213) of cases have been associated with influenza A.
• Among the 385 cases for which the influenza subtype was available, 306 (80%) were associated with A(H1N1)pdm09.

To date this season, 262 ICU admissions, and 10 deaths have been reported.

• 59% (155) of ICU admissions were in children under 5 years of age.
• All but 20 (92%) of the 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	46	15	3	28
50	51	23	4	47
51	84	32	4	72
52	89	47	7	92
1	91	40	5	75
2	62	35	4	62
3	64	38	4	67
4	67	35	7	47
5	49	40	11	59
6	45	38	15	79
7	45	40	17	120
8	44	47	25	139
9	32	50	13	153
10	44	48	17	135
11	43	41	18	118
12	42	33	13	89
13	29	32	14	67
14	34	24	12	56
15	28	23	13	56
16	34	20	10	41
17	29	16	8	37
18	23	14	4	28
19	19	10	5	18
20	16	10	4	19
21	10	5	2	10
22	9	4	1	8
23	6	2	0	5
24	7	2	0	5
25	4	1	0	3

Figure 8 - Text equivalent

Age Group	Total
0-5 mo	142
6-23 mo	329
2-4 yr	410
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 27 June, 2019, the National Microbiology Laboratory (NML) has characterized 2,479 influenza viruses (623 A(H3N2), 1,647 A(H1N1) and 209 B) that were received from Canadian laboratories. 

Genetic Characterization of Influenza A(H3N2):

262 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.
• 233 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):

• 200 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.
• 160 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.
• 135 influenza A(H3N2) viruses characterized belonged to genetic group 3C.2a1. 20 viruses belonged to genetic group 3C.2a and 200 to 3C.3a. Sequencing is pending for the remaining isolates.

Influenza A(H1N1):

• 1,603 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).

• 41 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.
• 142 viruses showed reduced titer with ferret antisera raised against cell culture-propagated B/Colorado/06/2017. Sequence analysis showed that 139 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:

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

• All 501 influenza A viruses were resistant to amantadine.

Antiviral Resistance – Oseltamivir:

1,387 influenza viruses (204 A(H3N2), 1,068 A(H1N1) and 115 B) were tested for resistance to oseltamivir and it was found that:

• All 204 A(H3N2) viruses were sensitive to oseltamivir.
• Of the 1,068 A(H1N1) viruses tested, 1,064 were sensitive to oseltamivir and four viruses were resistant to oseltamivir with a H275Y mutation.
• All 115 B viruses were sensitive to oseltamivir.

Antiviral Resistance – Zanamivir:

1,386 influenza viruses (204 A(H3N2), 1,067 H1N1 and 115 B) were tested for resistance to zanamivir and it was found that:

• All 1,386 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.