FluWatch report: April 21 to April 27, 2024 (week 17)

On this page

• Influenza/influenza-like illness activity - Geographic spread
• Laboratory-confirmed influenza detections
• Syndromic / influenza-like illness surveillance
• Influenza outbreak surveillance
• Influenza severe outcomes surveillance
• Influenza strain characterization
• Antiviral resistance
• Influenza vaccine monitoring
• Provincial and international surveillance links

Influenza/influenza-like illness activity - Geographic spread

In week 17, 10 regions in three provinces reported localized influenza activity (Ont., Sask., and B.C.) and 33 regions across Canada reported sporadic influenza activity. Two regions in one territory reported no activity this week (Nvt.). (Figure 1).

Number of Regions Reporting in week 17: 45 out of 53

Figure 1 - Text description

Province	Influenza Surveillance Region	Activity Level
N.L.	Eastern	Sporadic
N.L.	Labrador-Grenfell	Sporadic
N.L.	Central	Sporadic
N.L.	Western	Sporadic
P.E.I.	Prince Edward Island	Sporadic
N.S.	Zone 1 - Western	Sporadic
N.S.	Zone 2 - Northern	Sporadic
N.S.	Zone 3 - Eastern	Sporadic
N.S.	Zone 4 - Central	Sporadic
N.B.	Region 1	Sporadic
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	Sporadic
Que.	Montérégie	Sporadic
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 Data
Man.	Prairie Mountain	No Data
Man.	Interlake-Eastern	No Data
Man.	Winnipeg	No Data
Man.	Southern Health	No Data
Sask.	North	Sporadic
Sask.	Central	Sporadic
Sask.	South	Localized
Alta.	North Zone	Sporadic
Alta.	Edmonton	Sporadic
Alta.	Central Zone	Sporadic
Alta.	Calgary	Sporadic
Alta.	South Zone	Sporadic
B.C.	Interior	Sporadic
B.C.	Fraser	Localized
B.C.	Vancouver Coastal	Sporadic
B.C.	Vancouver Island	Localized
B.C.	Northern	Sporadic
Y.T.	Yukon	Sporadic
N.W.T.	North	No Activity
N.W.T.	South	No Activity
Nvt.	Qikiqtaaluk	No Data
Nvt.	Kivalliq	No Data
Nvt.	Kitimeot	No Data

Laboratory-confirmed influenza detections

Influenza percent positivity continues to decrease (1,523 detections, 5.9%). The following results were reported from sentinel laboratories across Canada in week 16 (Figures 2 and 3):

• A total of 1,523 laboratory detections were reported (300 influenza A detections and 1,223 influenza B detections).
• Among subtyped influenza A detections (121), 59% (82) were influenza A(H1N1).
• Age information was reported for 901 detections. Children aged between 0 and 19 years of age reported the highest proportion of detections, 49%.

To date this season (August 27, 2023 to April 20, 2024):

• 97,652 influenza detections were reported, of which 80% (77,909) were influenza A and among subtyped influenza A detections (25,787), influenza A(H1N1) accounted for 85% (21,848) of detections.
• 76,521 laboratory-confirmed influenza detections with age information were reported, of which 23,953 (31%) were in individuals aged 0-19 years old. Across adult age groups, adults over 65 years reported the highest detections, 28%, while similar proportions are being observed in adults 20-44 years old, 19% and adults 45-64 years, 22% (Figure 4).

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.

Number of Laboratories Reporting in Week 17: 33 out of 35

Figure 2 - Text description

Surveillance Week	A(Unsubtyped)	A(H3N2)	A(H1N1)	Influenza B	Percent Positive A	Percent Positive B
35	19	27	51	9	0.7	0.1
36	12	11	50	9	0.4	0.1
37	28	14	46	6	0.5	0.0
38	28	15	57	4	0.5	0.0
39	18	10	74	20	0.5	0.1
40	39	20	80	13	0.6	0.1
41	42	9	51	7	0.5	0.0
42	59	15	96	19	0.7	0.1
43	99	29	136	23	1.2	0.1
44	291	52	315	25	2.6	0.1
45	684	61	541	37	4.9	0.1
46	1032	103	961	47	7.2	0.2
47	1147	134	1321	35	8.8	0.1
48	1668	156	1901	85	11.6	0.3
49	2277	194	2327	99	14.7	0.3
50	2899	236	2662	139	15.3	0.4
51	3880	330	2650	184	16.8	0.5
52	4851	395	2465	220	18.2	0.5
1	4111	442	1771	259	14.5	0.6
2	3204	337	872	225	11.2	0.6
3	2643	222	636	279	10.3	0.8
4	3058	111	469	380	11.1	1.1
5	3273	146	360	524	11.3	1.5
6	2910	116	335	593	10.4	1.7
7	2680	118	253	740	9.4	2.2
8	2226	78	224	1054	8.0	3.2
9	1740	113	204	1311	6.7	4.0
10	1396	91	187	1622	5.6	5.1
11	995	59	167	1656	4.4	5.4
12	712	58	146	1782	3.3	5.8
13	588	42	122	1971	2.7	6.6
14	494	56	96	2040	2.3	6.8
15	368	55	76	1775	1.8	6.0
16	241	45	64	1328	1.3	5.1
17	162	39	82	1223	1.2	4.7
18	0	0	0	0	0.0	0.0
19	0	0	0	0	0.0	0.0
20	0	0	0	0	0.0	0.0
21	0	0	0	0	0.0	0.0
22	0	0	0	0	0.0	0.0
23	0	0	0	0	0.0	0.0
24	0	0	0	0	0.0	0.0
25	0	0	0	0	0.0	0.0
26	0	0	0	0	0.0	0.0
27	0	0	0	0	0.0	0.0
28	0	0	0	0	0.0	0.0
29	0	0	0	0	0.0	0.0
30	0	0	0	0	0.0	0.0
31	0	0	0	0	0.0	0.0
32	0	0	0	0	0.0	0.0
33	0	0	0	0	0.0	0.0
34	0	0	0	0	0.0	0.0

The shaded area represents the maximum and minimum number of influenza tests or percentage of tests positive reported by week from seasons 2014-2015 to 2019-2020. Data from week 11 of the 2019-2020 season onwards are excluded from the historical comparison due to the COVID-19 pandemic.

The epidemic threshold is 5% tests positive for influenza. When it is exceeded, and a minimum of 15 weekly influenza detections are reported, a seasonal influenza epidemic is declared.

Figure 3 - Text description

Surveillance Week	Percentage of tests positive, 2023-2024	Percentage of tests positive, 2022-2023	Percentage of tests positive, 2021-2022	Maximum Percentage of Tests Positive	Minimum Percentage of Tests Positive	Average Percentage of Tests Positive
35	0.8	0.2	0.0	1.9	0.1	0.8
36	0.5	0.2	0.0	2.3	0.3	1.1
37	0.5	0.3	0.0	1.8	0.4	1.0
38	0.5	0.5	0.0	2.4	0.6	1.3
39	0.6	0.7	0.0	3.0	0.6	1.7
40	0.7	1.0	0.0	2.3	1.1	1.7
41	0.5	1.4	0.1	3.0	1.3	1.7
42	0.8	2.4	0.1	3.4	0.9	2.2
43	1.3	5.5	0.1	5.3	0.8	2.8
44	2.7	10.8	0.2	8.4	1.2	3.7
45	5.1	16.2	0.1	10.1	1.4	4.6
46	7.4	20.3	0.2	14.1	1.5	6.0
47	8.9	24.3	0.2	15.4	1.4	7.7
48	11.9	24.1	0.3	18.2	0.8	10.6
49	15.0	21.2	0.3	19.3	1.6	12.9
50	15.7	17.4	0.3	27.0	2.4	16.4
51	17.2	12.5	0.2	29.1	3.3	20.0
52	18.7	8.0	0.1	34.5	4.3	24.4
1	15.1	4.6	0.1	31.7	5.9	23.5
2	11.7	2.3	0.1	29.1	7.1	23.0
3	11.1	1.5	0.1	30.1	12.2	23.7
4	12.2	1.1	0.1	29.5	15.9	24.0
5	12.8	1.0	0.0	30.6	19.6	24.9
6	12.1	1.0	0.0	32.4	17.9	25.0
7	11.6	0.9	0.0	32.5	16.3	25.1
8	11.2	1.1	0.1	32.9	17.6	25.2
9	10.8	1.3	0.1	34.3	16.8	24.4
10	10.7	1.4	0.2	36.0	16.1	23.3
11	9.8	1.7	0.3	31.4	16.2	21.4
12	9.1	1.9	0.9	30.0	15.0	20.1
13	9.3	2.4	1.5	28.3	14.5	19.6
14	9.0	2.2	2.5	26.2	12.7	19.1
15	7.8	2.5	3.9	20.7	12.0	16.3
16	6.4	2.4	7.0	18.5	11.6	14.5
17	5.9	2.4	9.7	17.3	9.8	12.8
18	-	2.3	11.3	13.0	7.9	10.3
19	-	2.1	12.6	12.0	5.0	9.0
20	-	2.1	10.4	9.1	3.2	7.2
21	-	1.6	9.8	7.5	3.0	5.6
22	-	1.6	8.4	4.9	2.2	3.9
23	-	1.3	7.0	4.4	0.9	2.9
24	-	1.1	5.0	4.4	0.8	2.3
25	-	1.0	3.0	4.0	0.6	1.9
26	-	0.7	2.3	3.0	0.7	1.8
27	-	0.6	1.2	2.8	0.4	1.4
28	-	0.8	0.8	1.8	0.4	0.9
29	-	0.5	0.7	1.5	0.5	1.1
30	-	0.5	0.4	1.5	0.5	0.9
31	-	0.5	0.3	1.9	0.6	1.1
32	-	0.7	0.2	1.2	0.5	0.9
33	-	0.7	0.2	1.7	0.4	0.9
34	-	0.6	0.2	1.6	0.4	0.9

Laboratory data notes:

Testing for influenza and other respiratory viruses has been influenced by the COVID-19 pandemic. Changes in laboratory testing practices may affect the comparability of data to previous seasons.

Due to different testing protocols of laboratories across Canada, some influenza A subtype detection counts may not be included in total influenza A detection counts and percent positivity calculations.

Figure 4 - Text description

a) Proportion of influenza A and B by age-group

Age Group	Proportion of Influenza A	Proportion of Influenza B	Total Number Influenza Detections
0 to 4	79.9%	20.1%	10711
5 to 19	58.7%	41.3%	13242
20 to 44	70.9%	29.1%	16809
45 to 64	91.3%	8.7%	14620
65+	96.8%	3.2%	21139

b) Proportion of subtyped influenza A(H1N1) and A(H3N2) by age-group

Age Group	Proportion of A(H1N1)	Proportion of A(H3N2)	Total Number of Subtyped Influenza Detections
0 to 4	89.8%	10.2%	2862
5 to 19	87.4%	12.6%	2872
20 to 44	82.9%	17.1%	4049
45 to 64	91.2%	8.8%	4120
65+	82.1%	17.9%	5454

Syndromic / influenza-like illness surveillance

Healthcare practitioners sentinel surveillance

Sentinel practitioner ILI surveillance data will not be updated further due to the limited number of reporting sentinels.

ILI symptoms are not specific to any one respiratory pathogen and can be due to influenza, or other respiratory viruses, including respiratory syncytial virus and SARS-CoV-2, the virus that causes COVID-19. This makes the percentage of visits for ILI an important indicator of overall respiratory illness morbidity in the community in the presence of co-circulating viruses.

This indicator should be interpreted with caution as there have been a smaller number of sentinels reporting compared to previous seasons.

The shaded area represents the maximum and minimum percentage of visits for ILI reported by week from seasons 2014-2015 to 2019-2020. Data from week 11 of the 2019-2020 season onwards are excluded from the historical comparison due to the COVID-19 pandemic.

Figure 5 - Text description

Surveillance Week	2023-2024	2022-2023	2021-2022	Average	Min	Max
35	0.5%	0.6%	0.1%	0.6%	0.4%	0.9%
36	0.4%	1.2%	0.2%	0.6%	0.4%	0.9%
37	0.8%	0.6%	0.4%	0.7%	0.5%	1.0%
38	0.8%	0.9%	0.3%	0.7%	0.6%	1.0%
39	0.9%	1.0%	0.4%	0.9%	0.5%	1.2%
40	0.6%	0.5%	0.2%	1.2%	0.8%	1.7%
41	1.6%	1.1%	0.4%	1.7%	0.8%	2.8%
42	1.0%	1.2%	0.5%	1.6%	1.2%	2.1%
43	0.8%	0.9%	0.3%	1.2%	0.8%	1.7%
44	1.3%	0.6%	0.3%	1.2%	0.7%	1.7%
45	1.2%	1.0%	0.4%	1.2%	0.9%	1.5%
46	2.0%	0.9%	0.8%	1.4%	1.2%	1.8%
47	1.7%	0.7%	0.3%	1.6%	1.1%	2.2%
48	2.0%	1.1%	0.5%	1.5%	1.1%	2.2%
49	2.0%	0.9%	0.5%	1.7%	1.0%	2.8%
50	2.2%	1.1%	0.4%	1.5%	1.1%	1.7%
51	2.0%	1.7%	0.5%	1.9%	1.4%	2.7%
52	1.8%	1.5%	0.5%	2.0%	1.0%	3.1%
1	1.8%	2.1%	0.7%	3.4%	1.9%	5.4%
2	2.4%	1.6%	0.4%	3.4%	1.8%	5.7%
3	1.3%	1.4%	0.2%	2.3%	1.3%	3.7%
4	1.6%	0.9%	0.3%	2.0%	1.1%	2.9%
5	2.1%	1.0%	0.2%	2.1%	1.4%	3.1%
6	1.7%	0.7%	0.5%	2.4%	1.4%	4.0%
7	1.6%	0.7%	0.3%	2.4%	0.9%	3.5%
8	1.3%	0.8%	0.2%	2.3%	0.8%	3.4%
9	1.4%	0.7%	0.2%	2.3%	0.9%	3.1%
10	1.8%	0.9%	0.2%	2.0%	1.0%	2.8%
11	1.6%	0.6%	0.2%	1.9%	1.1%	2.8%
12	2.3%	0.8%	0.3%	1.6%	0.6%	2.6%
13	-	1.2%	0.4%	1.6%	1.1%	2.6%
14	-	1.1%	0.2%	1.7%	1.1%	3.0%
15	-	1.2%	0.3%	1.3%	0.9%	1.9%
16	-	1.5%	0.3%	1.2%	0.8%	1.7%
17	-	1.3%	0.4%	1.2%	0.7%	1.7%
18	-	1.8%	0.5%	1.3%	0.5%	2.0%
19	-	1.7%	0.3%	0.9%	0.6%	1.3%
20	-	1.5%	0.5%	1.1%	0.6%	1.5%
21	-	1.4%	0.3%	0.9%	0.5%	1.3%
22	-	1.0%	0.3%	0.7%	0.3%	1.0%
23	-	1.1%	0.2%	0.8%	0.6%	1.0%
24	-	1.3%	0.2%	0.7%	0.6%	1.0%
25	-	1.0%	0.2%	0.6%	0.4%	0.8%
26	-	0.8%	0.2%	0.8%	0.5%	1.4%
27	-	1.7%	0.3%	0.6%	0.5%	0.7%
28	-	1.0%	0.1%	0.7%	0.5%	1.3%
29	-	0.9%	0.3%	0.9%	0.6%	1.7%
30	-	1.3%	0.2%	0.6%	0.2%	0.9%
31	-	0.9%	0.2%	0.4%	0.2%	0.6%
32	-	0.9%	0.2%	0.8%	0.3%	1.2%
33	-	0.8%	0.3%	0.7%	0.4%	1.3%
34	-	1.0%	0.5%	0.7%	0.4%	1.5%

FluWatchers

In week 17, 8,645 participants reported to FluWatchers, of which 1.2% reported symptoms of cough and fever (Figure 6). The percentage of FluWatchers reporting cough and fever remains below expected levels for this time of year.

The reports of cough and fever are not specific to any one respiratory pathogen and can be due to influenza, or other respiratory viruses, including respiratory syncytial virus, rhinovirus, and SARS-CoV-2, the virus that causes COVID-19. This makes the proportion of individuals reporting cough and fever an important indicator of overall respiratory illness activity in the community in the presence of co-circulating viruses.

FluWatchers reporting is not impacted by changes in health services or health seeking behaviours.

Among the 106 participants who reported cough and fever:

• 21% consulted a healthcare professional.
• 68% reported days missed from work or school, resulting in an average of 2.5 missed days from work or school among those 72 participants.

Northwest Territories had the highest participation rate this week (49 participants per 100,000 population) and the neighbourhood with postal code K0A had the highest number of participants (110). See what is happening in your neighbourhood! Downloadable datasets are also available on Open Maps.

If you are interested in becoming a FluWatcher, sign up today.

Number of Participants Reporting in Week 17: 8,645

The shaded area represents the maximum and minimum percentage of percentage of participants reporting cough and fever by week, from seasons 2014-2015 to 2019-2020. Data from week 11 of the 2019-2020 season onwards are excluded from the historical comparison due to the COVID-19 pandemic

Figure 6 - Text description

Surveillance Week	2023-2024	2022-2023	2021-2022	Average	Min	Max
35	1.1%	1.3%	0.2%	-	-	-
36	1.2%	1.2%	0.3%	-	-	-
37	1.5%	1.6%	0.5%	-	-	-
38	2.0%	1.8%	0.5%	-	-	-
39	2.3%	2.3%	0.5%	-	-	-
40	1.8%	2.4%	0.6%	2.5%	2.2%	2.7%
41	1.6%	2.0%	0.5%	2.2%	1.8%	2.6%
42	1.7%	2.0%	0.4%	1.8%	1.6%	2.0%
43	1.7%	2.3%	0.5%	1.9%	1.5%	2.2%
44	1.9%	2.6%	0.5%	1.6%	1.4%	1.8%
45	1.9%	3.0%	0.5%	1.8%	1.4%	2.3%
46	1.9%	2.9%	0.4%	1.8%	1.2%	2.2%
47	2.0%	3.1%	0.6%	1.9%	1.4%	2.4%
48	2.2%	3.1%	0.5%	2.4%	1.7%	3.4%
49	2.2%	2.7%	0.4%	2.6%	2.0%	3.2%
50	2.5%	2.1%	0.6%	2.9%	2.1%	3.8%
51	2.4%	2.4%	1.0%	3.2%	2.5%	3.9%
52	3.0%	2.1%	1.5%	4.0%	2.8%	5.4%
1	2.2%	1.7%	1.1%	3.8%	2.9%	4.8%
2	1.4%	1.3%	1.0%	2.9%	1.9%	3.9%
3	1.4%	1.3%	0.8%	3.3%	2.3%	4.8%
4	1.5%	1.2%	0.6%	3.1%	2.1%	4.2%
5	1.5%	1.1%	0.6%	3.2%	2.6%	3.6%
6	1.6%	1.4%	0.5%	3.5%	2.8%	4.3%
7	1.6%	1.3%	0.4%	3.2%	2.6%	3.8%
8	1.4%	1.3%	0.5%	3.1%	2.5%	3.6%
9	1.3%	1.4%	0.5%	2.8%	2.4%	3.5%
10	1.3%	1.5%	0.6%	2.6%	2.1%	3.1%
11	1.3%	1.5%	0.9%	2.3%	1.9%	2.6%
12	1.2%	1.3%	1.2%	2.6%	2.5%	2.8%
13	1.5%	1.5%	1.8%	2.5%	2.0%	3.1%
14	1.4%	1.3%	2.3%	2.1%	1.3%	2.6%
15	1.0%	1.4%	1.9%	1.8%	1.6%	1.9%
16	1.2%	1.2%	1.9%	2.0%	1.5%	2.4%
17	1.2%	1.2%	1.6%	1.7%	1.4%	2.3%
18	-	1.1%	1.4%	1.5%	1.2%	2.1%
19	-	0.9%	1.3%	-	-	-
20	-	1.0%	1.2%	-	-	-
21	-	0.9%	1.2%	-	-	-
22	-	0.7%	1.2%	-	-	-
23	-	0.9%	1.2%	-	-	-
24	-	0.9%	1.3%	-	-	-
25	-	0.7%	1.3%	-	-	-
26	-	0.9%	1.8%	-	-	-
27	-	0.8%	2.0%	-	-	-
28	-	0.6%	1.9%	-	-	-
29	-	0.6%	1.8%	-	-	-
30	-	0.7%	1.6%	-	-	-
31	-	1.0%	1.3%	-	-	-
32	-	0.9%	1.2%	-	-	-
33	-	0.9%	1.4%	-	-	-
34	-	1.2%	1.3%	-	-	-

Influenza outbreak surveillance

In week 17, 9 laboratory-confirmed influenza outbreaks were reported. The majority were associated with influenza A (n=7, 78%).

To date this season (August 27, 2023 to April 27, 2024):

• 1,193 laboratory-confirmed influenza outbreaks have been reported
  • 633 were in LTC facilities (53%)
  • 323 were in acute care facilities (27%)
  • 236 were in a facility categorized as ‘other’ (20%)
  • 1 was in a school or daycare (<1%)
  • 1,161 outbreaks were due to influenza A and 23 outbreaks were due to influenza B; an additional 6 outbreaks were due to a mix of influenza A and influenza B and 3 outbreaks were untyped.
  • Among outbreaks with subtyping information (236), influenza A(H1N1) was detected in 86% of the outbreaks
• 90 ILI outbreaks have been reported
  • All ILI outbreaks have been reported in schools and/or daycares

Outbreaks of ILI are not specific to any one respiratory pathogen and can be due influenza, or other respiratory viruses, including respiratory syncytial virus, rhinovirus, COVID-19, or a mixture of viruses.

Number of provinces and territories^{Footnote 1} reporting in week 17: 11 out of 13

Figure 7 - Text description

Surveillance Week	Acute Care Facilities	Long Term Care Facilities	Other	Schools and Daycares	Remote and/or Isolated Communities
35	0	1	1	0	0
36	0	0	0	0	0
37	0	2	0	0	0
38	0	2	0	0	0
39	0	1	0	0	0
40	0	2	0	0	0
41	0	1	0	0	0
42	0	2	0	0	0
43	0	3	1	0	0
44	2	6	2	0	0
45	1	2	5	0	0
46	8	4	1	0	0
47	6	5	7	0	0
48	8	12	6	0	0
49	12	23	10	0	0
50	13	23	19	0	0
51	11	24	18	0	0
52	6	50	18	0	0
1	31	67	22	0	0
2	27	60	22	0	0
3	22	35	11	0	0
4	53	64	12	0	0
5	26	40	13	0	0
6	17	27	9	0	0
7	13	27	11	0	0
8	13	28	8	1	0
9	18	23	9	0	0
10	8	23	5	0	0
11	5	15	4	0	0
12	6	14	2	0	0
13	4	18	10	0	0
14	6	11	5	0	0
15	2	7	1	0	0
16	2	5	4	0	0
17	3	6	0	0	0
18	0	0	0	0	0
19	0	0	0	0	0
20	0	0	0	0	0
21	0	0	0	0	0
22	0	0	0	0	0
23	0	0	0	0	0
24	0	0	0	0	0
25	0	0	0	0	0
26	0	0	0	0	0
27	0	0	0	0	0
28	0	0	0	0	0
29	0	0	0	0	0
30	0	0	0	0	0
31	0	0	0	0	0
32	0	0	0	0	0
33	0	0	0	0	0
34	0	0	0	0	0

Influenza severe outcomes surveillance

Provincial/territorial influenza hospitalizations and deaths

In week 17, 41 influenza-associated hospitalizations, less than 5 ICU admissions, and less than 5 influenza-associated deaths were reported by participating provinces and territories^{Footnote 2}.

To date this season (August 27, 2023 to April 27, 2024), 4,310 influenza-associated hospitalizations were reported by participating provinces and territories:

• 89% of the hospitalizations were associated with influenza A.
• Of the cases with subtype information (2,841), 94% were associated with influenza A(H1N1).
• Adults aged 65 years of age and older accounted for 46% of reported hospitalizations. The highest cumulative hospitalization rates were among adults 65 years of age and older (133/100,000) and children under 5 years of age (96/100,000).

To date this season (August 27, 2023 to April 27, 2024), 430 ICU admissions and 243 influenza-associated deaths were reported.

• Adults aged 45-64 years of age and 65 years of age and older accounted for 37% and 30% of reported ICU admissions respectively.
• Adults aged 65 years of age and older accounted for 72% of reported deaths.

Number of provinces and territories reporting in week 17: 5 out of 5

Figure 8 - Text description

Surveillance Week	0-4 yrs	5-19 yrs	20-44 yrs	45-64 yrs	65+ yrs	Overall
35	-	0.1	-	0.0	0.2	0.0
36	-	0.1	-	0.0	0.4	0.1
37	-	0.1	-	0.2	0.7	0.2
38	-	0.1	0.0	0.2	0.8	0.2
39	0.4	0.1	0.1	0.3	1.1	0.3
40	0.4	0.1	0.1	0.4	1.6	0.5
41	0.4	0.3	0.2	0.5	1.9	0.6
42	0.9	0.4	0.2	0.6	2.1	0.7
43	0.9	0.7	0.3	0.9	2.9	1.0
44	2.7	1.1	0.5	1.4	4.7	1.7
45	4.3	1.7	0.9	2.5	7.8	2.8
46	9.2	2.4	1.6	4.5	13.1	4.9
47	16.4	3.7	2.6	7.7	19.5	7.7
48	24.4	5.7	4.2	12.1	28.2	11.7
49	36.6	7.4	6.3	17.2	41.9	17.0
50	44.9	9.5	8.3	22.6	57.4	22.6
51	52.5	11.8	9.8	28.0	71.5	27.7
52	59.0	13.2	11.3	32.8	87.0	32.7
1	62.4	13.9	12.2	36.5	99.6	36.4
2	65.7	14.2	12.7	38.1	105.0	38.2
3	67.5	14.4	13.0	39.4	109.9	39.6
4	69.1	15.1	13.4	40.3	113.4	40.8
5	70.2	15.6	13.6	41.2	117.0	41.8
6	71.8	16.0	14.0	42.0	118.9	42.6
7	72.2	16.4	14.2	42.4	120.1	43.1
8	74.0	17.0	14.5	43.0	121.6	43.8
9	76.5	17.5	14.9	43.3	123.2	44.6
10	78.7	18.5	15.4	43.8	124.8	45.4
11	81.0	19.7	15.8	44.2	126.1	46.2
12	84.8	20.6	16.2	44.7	127.1	47.0
13	86.8	21.5	16.6	45.1	128.3	47.7
14	89.0	22.5	17.3	45.6	129.8	48.6
15	92.0	23.0	17.8	46.1	130.9	49.4
16	94.0	23.5	18.0	46.5	131.5	49.8
17	96.3	23.9	18.5	47.0	132.7	50.5
18	-	-	-	-	-	-
19	-	-	-	-	-	-
20	-	-	-	-	-	-
21	-	-	-	-	-	-
22	-	-	-	-	-	-
23	-	-	-	-	-	-
24	-	-	-	-	-	-
25	-	-	-	-	-	-
26	-	-	-	-	-	-
27	-	-	-	-	-	-
28	-	-	-	-	-	-
29	-	-	-	-	-	-
30	-	-	-	-	-	-
31	-	-	-	-	-	-
32	-	-	-	-	-	-
33	-	-	-	-	-	-
34	-	-	-	-	-	-

Sentinel Pediatric Influenza Severe Outcomes

For the 2023-2024 season, data on pediatric influenza associated severe outcomes are provided by the Surveillance Program for Rapid Identification and Tracking of Infectious Diseases in kids (SPRINT-KIDS) Network^{Footnote 3}. The SPRINT-KIDS sentinel pediatric (≤18 years) hospital network provides severe outcome monitoring in both the emergency department and inpatient facilities and consists of 15 pediatric hospitals across 8 provinces in Canada (all provinces with the exception of New Brunswick and Prince Edward Island).

Emergency and Inpatient Influenza Testing

In week 17, 1,775 tests were conducted for influenza in emergency departments and inpatient wards from 11/15 sites:

• 140 tests (7.9%) were positive for influenza.
• The majority were influenza B (n=125, 89%).

To date this season (October 1, 2023 to April 27, 2024):

• 56,601 tests have been conducted for influenza across 13 sites^{Footnote 4}.
• 6,562 tests were positive for influenza.
• The majority were influenza A (n=4,430, 68%).

Hospitalizations

In week 17, 28 influenza-associated pediatric hospitalizations were reported from a total of 133 positive influenza tests from 10/15 sites:

• The majority were influenza B (n=33, 82%).

To date this season (October 1, 2023 to April 27, 2024):

• 1,084 influenza-associated pediatric hospitalizations were reported from a total of 4,435 positive influenza tests across 10 sites^{Footnote 4}.
• The majority were influenza A (n=801, 74%).

Influenza strain characterization

Since September 1, 2023, the National Microbiology Laboratory Branch (NMLB) has characterized 1,554 influenza viruses (256 A(H3N2), 797 A(H1N1), and 501 influenza B) received from Canadian laboratories.

Antigenic Characterization

Changes in circulating influenza viruses are monitored by antigenic characterization. Antigenic characterization results show how similar the circulating viruses are to reference viruses. Reference viruses represent strains included in the current seasonal influenza vaccine.

Influenza A(H1N1)

A/Wisconsin/67/2022 is the influenza A(H1N1) component of the 2023-2024 Northern Hemisphere influenza vaccine.

• 784 influenza A(H1N1) viruses were characterized as antigenically similar to A/Wisconsin/67/2022-like with antisera produced against cell-grown A/Wisconsin/67/2022.
• 13 influenza A(H1N1) showed reduced titer with antisera raised against cell-grown A/Wisconsin/67/2022.

Influenza A(H3N2)

A/Darwin/6/2021 (H3N2)-like virus is the influenza A(H3N2) component of the 2023-2024 Northern Hemisphere influenza vaccine.

• 244 influenza A(H3N2) were antigenically similar to A/Darwin/6/2021 (H3N2)-like virus using antisera raised against cell-grown A/Darwin/6/2021 (H3N2)-like virus.
• 12 influenza A(H3N2) showed reduced titer with antisera raised against cell-grown A/Darwin/6/2021 (H3N2)-like virus.

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 2023-2024 Northern Hemisphere influenza vaccine are B/Austria/1359417/2021 (Victoria lineage) and B/Phuket/3073/2013 (Yamagata lineage)

• 501 viruses characterized were antigenically similar to B/Austria/1359417/2021.

Genetic Characterization

Genetic characterization is used to determine how similar gene sequences of circulating influenza viruses are to the sequences of the vaccine components used in the current seasonal influenza vaccine.

Since September 1, 2023, NML has genetically characterized 1,564 influenza viruses.

Antiviral resistance

The National Microbiology Laboratory has not yet reported antiviral resistance results for influenza viruses collected during the 2023-2024 season.

Oseltamivir

1,237 influenza viruses (197 H3N2, 735 H1N1 and 305 influenza B) were tested for resistance to oseltamivir.

• Three of the 735 influenza A(H1N1) viruses was resistant to oseltamivir
• All influenza A(H3N2) viruses and B viruses were sensitive to oseltamivir.

Zanamivir

1,237 influenza viruses (197 H3N2, 735 H1N1 and 305 influenza B) were tested for resistance to zanamivir.

• All influenza viruses were sensitive to zanamivir.

Influenza vaccine monitoring

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

Vaccination coverage

The Seasonal Influenza Immunization Vaccination Coverage Survey is an annual telephone survey conducted between January and February that collects information from Canadians on whether they received the annual seasonal influenza vaccine that season. Vaccination coverage is measured as the percentage of people who reported receiving the influenza vaccine in a specific influenza season.

In the 2023-2024 influenza season, coverage was similar to previous seasons (2022-2023; 2019-2020):

• 42% among all adults aged 18 years and older.
  • 29% among adults aged 18-64 without chronic medical conditions.
  • 44% among adults aged 18-64 with chronic medical conditions.
• 73% among seniors (aged 65 years and older).

Vaccine effectiveness

The Canadian Sentinel Practitioner Surveillance Network (SPSN) provided mid-season 2023/24 vaccine effectiveness (VE) estimates for the multivalent influenza and monovalent Omicron XBB.1.5 vaccines in preventing medically-attended illness due to laboratory-confirmed influenza and COVID-19 among Canadians.

During the analysis period, influenza A made up >95% of influenza viruses. Among influenza A viruses, 80% were H1N1 and 20% were H3N2. The SPSN applied whole genome sequencing (WGS) to genetically characterize about 70% of all contributing influenza viruses. Among characterized H1N1 viruses they found a roughly equal proportion of vaccine matched clade 5a.2a.1 and alternate 5a.2a viruses. Among H3N2 detections, virtually all belonged to clade 2a.3a.1, genetically distinct from the clade 2a vaccine strain.

Based on data collected between October 29, 2023 and January 13, 2024, VE was estimated to be 63% (95% CI: 51-72) overall against influenza A(H1N1) and 40% (95% C:I 5-61) against influenza A(H3N2). VE was lower for H1N1 viruses belonging to clade 5a.2a.1 than 5a.2a and the SPSN provides possible reasons for that in the publication. Influenza A (H1N1) VE estimates were higher in children <20 years at 68% (95% CI: 42 to 83) and older adults 65 years and older at 72% (95% CI: 47 to 85), compared to adults 20-64 years at 56% (95% CI: 38 to 69); however, confidence intervals in age-stratified analyses broadly overlapped. Owing to limited detection of influenza A(H3N2) or influenza B viruses, age-stratified A(H3N2) and influenza B VE estimates were not reported in mid-season analyses.

The SPSN interim estimates are published and available online. Updated influenza VE estimates, inclusive of further age stratification and for influenza B will be published, if feasible, at the end of the 2023-2024 influenza season.

Provincial and international surveillance links

See Influenza surveillance resources

Notes

The data in the FluWatch report represent surveillance data available at the time of writing. All data are preliminary and may change as updates are received.

To learn more about the FluWatch program, see the Overview of influenza monitoring in Canada page.

For more information on the flu, see our Flu (influenza) web page.

We would like to thank all the FluWatch surveillance partners participating in this year's influenza surveillance program.

This report is available on the Government of Canada Influenza webpage.

Ce rapport est disponible dans les deux langues officielles.