Table 2. Vaccine effectiveness estimates against influenza A(H3N2), overall and variant-specific, SPSN, Canada, 1 November 2022–6 January 2023 (weeks 44–1) (n = 1,451).

	Total	Cases						Controls			Unadjusted VEa		Adjustedb VEa
	N	n vacc	N				%	n vacc	N	%	%	95% CI	%	95% CI
Primary analysis	1,451	75	471				16	361	980	37	68	57 to 75	54	38 to 66
Age-stratified
1–19 years	528	25				216	12	81	312	26	63	39 to 77	47	11 to 69
20–64 years	704	29				212	14	155	492	32	66	47 to 78	58	33 to 73
≥ 65 years	219	21				43	49	125	176	71	61	23 to 80	59	15 to 80
Variant-specific
All S156d viruses	1,164	27			184		15	361	980	37	71	55 to 81	53	25 to 70
All H156e viruses	1,187	38		207			18	361	980	37	61	44 to 74	50	24 to 67
H156 with T135f	1,120	22	140				16	361	980	37	68	49 to 80	52	19 to 71
H156 with K135g, all SPSN provinces	1,033	13	53				25	361	980	37	44	−6 to 71	46	−13 to 74
H156 with K135g, British Columbia only	243	13	47				28	94	196	48	59	17 to 79	44	−22 to 75

CI: confidence interval; ECDC: European Centre for Disease Prevention and Control; VE: vaccine effectiveness; OR: odds ratio; PLR: penalised logistic regression; SPSN: Sentinel Practitioner Surveillance Network; vac: vaccinated; VE: vaccine effectiveness.

^a OR compared influenza test positivity between vaccinated and unvaccinated participants using logistic regression, adjusting for confounders as specified. VE was calculated as (1 − OR) × 100%.

^b Adjusted for age group (1–19, 20–64, ≥ 65 years), province (Alberta, British Columbia, Ontario, Quebec) and calendar time based on bi-weekly categories (44–45, 46–47, 48–49, 50–51, 52–1). Age stratified estimates not further adjusted for finer age sub-categories.

^c 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.

^d 3C.2a1b.2a.2 viruses that are H156S (B) like the A/Darwin/9/2021 (egg-passaged) and A/Darwin/6/2021 (cell-passaged) vaccine strains (i.e. S156 ECDC subgroups ii-iv) [12] (Supplementary Table S1).

^e 3C.2a1b.2a.2 viruses that remain H156 (B) and with E50K (C) (as per H156 ECDC subgroup i).

^f 3C.2a1b.2a.2 viruses that remain H156 (B) and with E50K (C) (as per H156 ECDC subgroup i) but excluding viruses bearing K or A substitution at position 135 (i.e. T135 viruses, excluding viruses with T135K or T135A (A)(RBS)(−CHO) substitutions).

^g 3C.2a1b.2a.2 viruses that remain H156 (B) and with E50K (C) (as per H156 ECDC subgroup i) plus additional substitutions F79V + T135K (A)(RBS)(−CHO) + I140K (A) + G275D(C).