Table 1 Characteristics of study participants by severity of manifestations from 2009 H1N1 Influenza, as reported to the Shandong Center for disease control and prevention, China, during the period from May 13, 2009 to March 31, 2010

Age (years) (%)

≤5

28 (8.2)

5 (1.5)

<0.001

>5 and <60

290 (84.5)

338 (98.5)

≥60

25 (7.3)

0

Gender (%)

Female

190 (55.4)

142 (41.4)

<0.001

Male

153 (44.6)

201 (58.6)

Ethnic groups (%)

Han Chinese

337 (98.2)

339 (98.8)

0.52

Others

6 (1.8)

4 (1.2)

Occupation (%)

Peasant

138 (40.2)

14 (4.1)

<0.001

Others

205 (59.8)

329 (95.9)

Body mass index (kg/m ² ) (%)

<18.5

45 (13.1)

85 (24.8)

<0.001

18.5~24.9

151 (44.0)

233 (67.9)

25.0~29.9

97 (28.3)

23 (6.7)

≥30

50 (14.6)

2 (0.6)

Chronic coexisting illness (yes, %)

Chronic respiratory disease

30 (8.9)

1 (0.3)

<0.001

Diabetes

20 (6.0)

1 (0.3)

<0.001

Chronic renal disease

10 (2.9)

0

0.002

Chronic liver disease

8 (2.4)

0

0.007

Cancer or tumor

6 (1.8)

0

0.03

Chronic cardiac disease

46 (13.5)

0

<0.001

Immunosuppressive disorder

9 (2.9)

0

0.004

Pregnant status (yes, %)

72 (21.0)

4 (1.2)

<0.001

Time from symptom onset to hospital admission (days) (%)

≤2

144 (42.0)

229 (66.8)

<0.001

>2

180 (52.5)

83 (24.2)

Missing

19 (5.5)

31 (9.0)

Receiving seasonal influenza vaccination one year before onset (yes, %)

3 (1.2)

5 (1.7)

0.48