Table 1.

Characteristics of Index Cases With Laboratory-Confirmed Influenza B Victoria- or Yamagata-Lineage Virus Infection and Their Household Contacts, Hong Kong, China, 2007–2011

Characteristic	All Households				P Value	Households Without Co–Index Cases				P Value
	Yamagata		Victoria			Yamagata		Victoria
	No.	%	No.	%		No.	%	No.	%
Index Cases
Total	75	100	34	100		63	100	28	100
Year(s)					0.001					<0.001
2007	17	23	0	0		16	25	0	0
2008	50	67	26	76		41	65	21	75
2009–2011	8	11	8	24		6	10	7	25
Age, years					0.48					0.61
≤5	11	15	4	12		8	13	3	11
6–15	36	48	22	65		31	49	18	64
16–49	24	32	7	21		21	33	6	21
≥50	4	5	1	3		3	5	1	4
Male sex	29	39	15	44	0.74	25	40	14	50	0.10
Interventiona					0.36					<0.01
Yes	38	51	19	56		29	46	18	64
No	37	49	15	44		34	54	10	36
Vaccination in the past yearb					0.32					0.15
No	68	92	29	85		58	92	24	86
Yes	6	8	5	15		5	8	4	14
Prescribed antiviral treatmentb										0.07
No	46	70	27	79	0.42	40	73	23	82
Yes	20	30	7	21		15	27	5	18
Household Contacts
Total	227	100	112	100		181	100	94	100
Year					<0.001					<0.001
2007	44	19	0	0		37	20	0	0
2008	157	69	86	77		124	69	71	76
2009–2011	26	11	26	23		20	11	23	24
Age, yearsb					0.83					0.78
0–5	13	6	6	5		7	4	4	4
6–15	25	11	12	11		21	12	10	11
16–49	143	63	75	67		111	63	62	66
≥50	42	19	19	17		38	21	18	19
Male sex	78	34	37	33	0.67	62	34	31	33	0.71
Interventiona					0.96					<0.001
Yes	122	54	60	54		87	48	57	61
No	105	46	52	46		94	52	37	39
Vaccination in the past year					0.09					0.38
No	197	87	101	90		156	86	83	88
Yes	30	13	11	10		25	14	11	12

^a In 2007, 2008, and the first half of 2009 (January through June), households were randomly allocated to 3 groups for estimation of the effectiveness of surgical face masks and hand hygiene in preventing influenza transmission within households.

^b Some index cases and household contacts had missing values for this characteristic.