Table 3. Attack rate for pandemic (H1N1) 2009, including subclinical infection, by characteristics of index case-patients and close contacts, Beijing, People’s Republic of China.
| Characteristic | Attack rate, % (no. infected/total contacts) | χ2 p value |
|---|---|---|
| Overall |
2.4 (167/7,099) |
NA* |
| Index case-patient | ||
| Sex | ||
| M | 2.2 (91/4,192) | 0.225 |
| F | 2.6 (76/2,907) | |
| Age, y | ||
| 0–19 | 2.7 (113/4,144) | 0.022 |
| 20–50 | 1.9 (52/2,680) | |
| >50 | 0.7 (2/275) | |
| Infection source | ||
| Imported case | 2.5 (125/5,049) | 0.282 |
| Community-acquired case | 2.0 (42/2,050) | |
| By type of exposure | ||
| Exposure to symptomatic index case-patient in symptomatic phase | 3.1 (135/4,305) | <0.001 |
| Exposure to symptomatic index case-patient before illness onset | 1.2 (32/2,642) | |
| Exposure to subclinical index case-patient |
0 (0/149) |
|
| Close contacts | ||
| Sex | ||
| M | 2.3 (82/3,518) | 0.808 |
| F | 2.4 (85/3,514) | |
| Age, y | ||
| 0–19 | 4.5 (82/1,837) | <0.001 |
| 20–50 | 2.4 (78/3,299) | |
| >50 | 0.8 (7/843) | |
| Relationship to index case-patient | ||
| Spouse | 5.3 (4/75) | <0.001 |
| Other household member | 6.6 (52/786) | |
| Nonrelated roommate | 2.5 (9/367) | |
| Contact at workplace or school | 3.0 (49/1,610) | |
| Nonhousehold relative | 2.8 (5/177) | |
| Passenger on the same flight | 0.9 (28/3,129) | |
| Friend | 3.6 (19/523) | |
| Service person met at public place | 0.2 (1/432) | |
| Flight time for passenger on the same flight, h | ||
| <12 | 0.4 (8/1,846) | 0.001 |
| >12 | 1.6 (20/1,283) | |
| Exposure duration of nonpassenger close contact, h | ||
| <12 | 1.9 (38/2,054) | <0.001 |
| >12 | 5.3 (101/1,912) |
*Not available.