Table 3. Pandemic influenza A (H1N1) 2009 cases and test-negative controls included in the study by patient characteristics.
| Patient Characteristics | Cases n = 918 | Test-Negative Controls n = 1,984 | p-Value |
| Median age, y | 12 | 27 | <0.001a |
| Age group, y, n /total n (%) | |||
| 0–4 | 180/917 (19.6) | 520/1,978 (26.3) | <0.001b |
| 5–14 | 326/917 (35.6) | 195/1,978 (9.9) | |
| 15–64 | 393/917 (42.9) | 1,069/1,978 (54.0) | |
| ≥65 | 18/917 (2.0) | 194/1,978 (9.8) | |
| Female sex, n /total n (%) | 485/912 (53.2) | 1,005/1,971 (51.0) | 0.279b |
| Symptoms, n /total n (%) | |||
| Fever | 903/918 (98.4) | 1,842/1,957 (94.1) | <0.001b |
| Headache | 611/907 (67.4) | 1,194/1,936 (61.7) | 0.003 |
| Cough | 869/914 (95.1) | 1,718/1,964 (87.5) | <0.001b |
| Sore throat | 539/914 (59.0) | 1,340/1,945 (68.9) | <0.001b |
| Days between onset of symptoms and swabbing, n /total n (%) | |||
| 0 | 111/918 (12.1) | 212/1,984 (10.7) | <0.001b |
| 1 | 512/918 (55.8) | 987/1,984 (49.7) | |
| 2 | 201/918 (21.9) | 454/1,984 (22.9) | |
| 3 | 70/918 (7.6) | 181/1,984 (9.1) | |
| 4 | 17/918 (1.9) | 77/1,984 (3.9) | |
| 5 | 6/918 (0.7) | 36/1,984 (1.8) | |
| 6 | 1/918 (0.1) | 21/1,984 (1.1) | |
| 7 | 0/918 (0.0) | 16/1,984 (0.8) | |
| Diabetes, n /total n (%) | 8/690 (1.2) | 72/1,670 (4.3) | <0.001b |
| Heart disease, n/total n (%) | 20/688 (2.9) | 198/1,670 (11.9) | <0.001b |
| Any hospitalization in the previous 12 mo for chronic diseases, n /total n (%) | 5/680 (0.7) | 37/1,739 (2.3) | 0.005b |
| Smoker, n /total n (%) | |||
| Current | 35/814 (4.3) | 176/1,739 (10.1) | <0.001b |
| Former | 80/814 (9.8) | 244/1,739 (14.0) | |
| Never | 699/814 (85.9) | 1319/1,739 (75.8) | |
| Pandemic vaccination, n /total n (%) | 12/895 (1.3) | 185/1,940 (9.5) | <0.001b |
| Seasonal vaccination, 2009–2010, n /total n (%) | 56/913 (6.1) | 240/1,975 (12.2) | <0.001b |
| Any influenza vaccination in the previous two seasons, n /total n (%) | 56/516 (10.9) | 213/1,316 (16.2) | 0.003b |
| Median number of GP visits in the previous 12 mo | 3 | 4 | <0.001a |
a
Nonparametric test of the median.
b
Two-sided Fisher exact test.