Table 3. Serologic evidence for influenza infections during the 24 months of follow-up.
| Period | N |
>4-fold increase |
|||||||
|---|---|---|---|---|---|---|---|---|---|
| Swine influenza (H1N1) |
Swine influenza (H1N2) |
Human influenza (H1N1) |
|||||||
| n | Reported ILI,* n (%) | n | Reported ILI,* n (%) | n | Reported ILI,* n (%) | ||||
| Enrollment to 12-mo follow-up | 658 | 26 | 3 (11.5) | 17 | 7 (41.2) | 10 | 1 (10) | ||
| 12- to 24--mo follow-up | 586 | 109 | 18 (16.5) | 16 | 2 (12.5) | 19 | 3 (15.8) | ||
| Enrollment to 24-mo follow-up | 654 | 141 | 31 (22) | 23 | 2 (8.7) | 20 | 3 (15) | ||
| Any increase between pairs of serum samples† | 726 | 180 | 38 (21.1) | 37 | 9 (24.3) | 32 | 4 (12.5) | ||
*Percentage of the participants who demonstrated a ≥4-fold increase in titer who also self-reported an influenza-like illness (ILI) during follow–up. †From enrollment to 12 mo, 12 to 24 mo, or enrollment to 24 mo, among participants who permitted serum sample collections at least 2 times during the study.