Table 1. Participant profile, influenza A(H3N2) test-positive cases and influenza test-negative controls, 2022/23 influenza vaccine effectiveness evaluation, SPSN, Canada, 1 November 2022–6 January 2023 (n = 1,451).

Characteristics	All participantsa (column %)							Proportion influenza vaccinatedb (row %)
	Overall		Influenza A(H3N2) cases		Influenza virus test- negative controls		p valuec	Overall		p valuec	Influenza A(H3N2) casesd		Influenza virus test- negative controlsd
	n	%	n	%	n	%		n	%		n	%	n	%
N (row %)	1,451	100	471	32	980	68	NA	436	30	NA	75	16	361	37
Age group (years)
1–19	528	36	216	46	312	32	< 0.001	106	20	< 0.001	25	12	81	26
20–64	704	49	212	45	492	50		184	26		29	14	155	32
≥ 65	219	15	43	9	176	18		146	67		21	49	125	71
Median (range)	33 (1–92)		23 (1–87)		37 (1–92)		< 0.001	50 (1–92)		< 0.001	39 (1–84)		53 (1–92)
Interquartile range	11–55		9–40		13–59		NA	22–70		NA	13–66		27–71
Sex
Female	888	61	278	59	610	62	0.19	286	32	0.022	47	17	239	39
Male	551	38	191	41	360	37		146	27		28	15	118	33
Unknown	12	1	2	0	10	1	NA	4	33	NA	0	0	4	40
Comorbiditye
No	1,068	74	365	77	703	72	0.008	280	26	< 0.001	45	12	235	33
Yes	299	21	78	17	221	23		133	44		22	28	111	50
Unknown	84	6	28	6	56	6	NA	23	27	NA	8	29	15	27
Province
Alberta	275	19	77	16	198	20	0.20	89	32	< 0.001	12	16	77	39
British Columbia	293	20	97	21	196	20		115	39		21	22	94	48
Ontario	656	45	213	45	443	45		183	28		33	16	150	34
Quebec	227	16	84	18	143	15		49	22		9	11	40	28
Weeks of specimen collection, 2022/23
44–45	131	9	44	9	87	9	< 0.001	30	23	< 0.001	6	14	24	28
46–47	329	23	153	32	176	18		56	17		21	14	35	20
48–49	407	28	166	35	241	25		111	27		25	15	86	36
50–51	382	26	91	19	291	30		132	35		21	23	111	38
52–1	202	14	17	4	185	19		107	53		2	12	105	57

NA: not applicable; SPSN: Sentinel Practitioner Surveillance Network.

^a Presenting within 7 days of influenza-like illness onset defined as acute onset of fever and cough and one other symptom including sore throat, myalgia, arthralgia or prostration. Fever was not a required symptom in adults ≥ 65 years.

^b Vaccination status based on patient self-report; defined as receipt of 2022/23 seasonal influenza vaccine at least 2 weeks before symptom onset. Patients vaccinated less than 2 weeks before onset of symptoms or with unknown vaccination status or timing were excluded.

^c p values for comparison between cases and controls or for the proportion vaccinated were derived by chi-squared test or Wilcoxon rank-sum test.

^d Without regard to time before illness onset, 101 of 497 (20%) cases and 406 of 1,025 (40%) controls were vaccinated (p < 0.001).

^e Includes chronic comorbidities that place individuals at higher risk of serious complications from influenza as defined by Canada’s National Advisory Committee on Immunization, including: heart, pulmonary (including asthma), renal, metabolic (such as diabetes), blood, cancer or immunocompromising conditions, conditions that compromise management of respiratory secretions and increase risk of aspiration, or morbid obesity (body mass index ≥ 40) [9].

Unless otherwise specified, values displayed in the columns represent the number of specimens per category and percentages are relative to the total.