Table 1. Participant profile, by influenza A(H1N1)pdm09 case and vaccination status, Canadian Sentinel Practitioner Surveillance Network, 4 November 2018–12 January 2019 (n = 1,442).

Characteristic	Overall		Distribution by case status (column %)					Vaccination coverage (row %)b
			A(H1N1)pdm09 cases		Negative controls		p valuea	A(H1N1)pdm09 cases		p valuec	Negative controls		p valuec
	n	%	n	%	n	%		n	%		n	%
n (row %)	1,442	100	585	41	857	59	NA	45	8	NA	234	27	NA
Age group (years)
1–8	282	20	163	28	119	14	< 0.001	3	2	< 0.001	21	18	< 0.001
9–19	134	9	54	9	80	9		2	4		9	11
20–49	632	44	253	43	379	44		19	8		78	21
50–64	256	18	90	15	166	19		12	13		55	33
≥ 65	138	10	25	4	113	13		9	36		71	63
Median age (range)	35 (1–97)		31 (1–82)		38 (1–97)		< 0.001	48 (2–82)		< 0.001	51.5 (1–97)		< 0.001
Sex
Female	869	60	328	56	541	63	0.005	29	9	0.25	170	31	< 0.001
Male	564	39	255	44	309	36		16	6		61	20
Unknown	9	1	2	0	7	1	NA	0	0	NA	3	43	NA
Comorbidityd
No	1,099	76	474	81	625	73	< 0.001	30	6	0.01	141	23	< 0.001
Yes	264	18	76	13	188	22		11	14		85	45
Unknown	79	5	35	6	44	5	NA	4	11	NA	8	18	NA
Province
Alberta	432	30	219	37	213	25	< 0.001	16	7	0.02	65	31	0.08
British Columbia	267	19	87	15	180	21		10	11		55	31
Ontario	546	38	179	31	367	43		18	10		97	26
Quebec	197	14	100	17	97	11		1	1		17	18
Specimen collection interval from ILI onset (days)e
≤ 4	1,066	74	476	81	590	69	< 0.001	38	8	0.58	150	25	0.07
5–7	376	26	109	19	267	31		7	6		84	31
Median interval (range)	3 (0–7)		3 (0–7)		3 (0–7)		< 0.001	3 (0–7)		0.61	4 (0–7)		0.04
Month of specimen collection
November	409	28	158	27	251	29	0.006	7	4	0.14	51	20	0.01
December	736	51	326	56	410	48		27	8		121	30
January	297	21	101	17	196	23		11	11		62	32
Vaccination status
Vaccination without regard to timingf	334/ 1,497	22	62/ 602	10	272/ 895	30	< 0.001	NA	NA	NA	NA	NA	NA
≥ 2 weeks before ILI onset	279	19	45	8	234	27	< 0.001	NA	NA	NA	NA	NA	NA

ILI: influenza-like illness; NA: not applicable.

Unless otherwise specified, values displayed in the columns represent the number of specimens per category and percentages are relative to the total. Where the denominator for the percentages differs from the total, fractions supporting the calculation of percentages are shown.

^a p values for comparison between cases and controls were derived by chi-squared test, Fisher’s exact test or Wilcoxon rank-sum test.

^b Vaccination status based on patients’ self-report; defined as receipt of 2018/19 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 of the proportion vaccinated were derived by chi-squared test, Fisher’s exact test or Wilcoxon rank-sum test.

^d 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).

^e Missing specimen collection dates were imputed as the date the specimen was received and processed at the laboratory minus 2 days, the average time between specimen collection date and laboratory received date among specimens with complete information for both values.

^f Participants who received seasonal 2018/19 influenza vaccine less than 2 weeks before ILI onset or for whom vaccination timing was unknown were excluded from the primary analysis. They are included here for assessing vaccination regardless of timing for comparison to other estimates of vaccination coverage.