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Convalescent HI titers and case classification according to PRNT.
| HI titers | Cases | PRNT | |||
|---|---|---|---|---|---|
| Primary infection | Secondary infection | ||||
| Cases | % | Cases | % | ||
| ≤80 | 11 | 11 | 100.0% | 0 | 0.0% |
| 160 | 4 | 1 | 25.0% | 3 | 75.0% |
| 320 | 14 | 3 | 21.4% | 11 | 78.6% |
| 640 | 14 | 1 | 7.1% | 13 | 92.9% |
| 1280 | 23 | 0 | 0.0% | 23 | 100% |
| ≥2560 | 23 | 0 | 0.0% | 23 | 100% |
