TABLE 1.

Demographic characteristics for all invited (N = 1324) and for participants included in the study (N = 357) and by serological response in a subsample (n = 72)^a

Variable	All invited (N = 1324)b		All participants (N = 357)		Serologically tested cases (n = 72)
					Total with blood test	Positive H1N1 (n = 34)		Negative H1N1 (n = 38)		P-value
	N	%	n	%	n	n	%	n	%
Age										NS
0–17 years	359	29	106	30	0*	–	–	–	–
18–40 years	577	44	127	36	32	18	56	14	44
>40 years	275	27	122	34	40	16	40	24	60
Missing			2		0
Gender										NS
Male	577	46	146	41	24	12	50	12	50
Female	665	54	210	59	48	22	46	26	54
Missing			1		0	0
Smoking status										NS
Smoker			71	20	19	7	37	12	63
Non-smoker			286	80	53	27	51	26	49
Level of education										NS
Primary and secondary school			57	16	7	2	29	5	71
High school			84	23	22	11	50	11	50
Higher education <4 years			61	17	18	7	39	11	61
Higher education >4 years			78	22	25	14	56	11	44
Missing			77	22	0

Confirmed H1N1 was defined as a serum HAI titre of ≥40 and negative H1N1 (<10) as HAI titres = 5. NS, not statistically significant. Number (n) and proportion of responders. *, not applicable. Only cases ≥18 years old were asked to provide blood samples.

^aPercentage is calculated vertically (within the characteristic) for all invited and all participants, while it is calculated horizontally (within each level of the characteristic) for the serologically tested subsample.

^bEighty-two patients missing due to lack of data on gender and age.