Table 4.
Mixed logistic regression of the binomial outcome that a herd test was positive for antibodies against influenza A(H1N1)pdm09 virus infection based on haemagglutination inhibition test
| No. of observations (positives) | Fixed effects* | OR | 95% CI | P value |
|---|---|---|---|---|
| Year of test (n = 5) | ||||
| 490 (204) | 2010 | 1 | ||
| 1288 (563) | 2011 | 1·02 | 0·78–1·33 | 0·89 |
| 1361 (641) | 2012 | 1·07 | 0·82–1·39 | 0·62 |
| 1401 (587) | 2013 | 0·82 | 0·64–1·07 | 0·14 |
| 1103 (478) | 2014 | 0·88 | 0·67–1·15 | 0·35 |
| Production class (n = 5) | ||||
| 305 (69) | Fattening herd | 1 | ||
| 362 (123) | Nucleus herd | 2·78 | 1·67–4·62 | <0·001 |
| 627 (297) | Multiplier herd | 4·72 | 3·05–7·30 | <0·001 |
| 4093 (1826) | Conventional herd | 2·63 | 1·87–3·70 | <0·001 |
| 256 (155) | Sow pool | 24·04 | 10·98–52·66 | <0·001 |
| Herd size in three quantiles (pigs) | ||||
| 1679 (601) | Small (<350 pigs) | 1 | ||
| 1909 (919) | Medium (350–665 pigs) | 1·85 | 1·52–2·26 | |
| 1851 (881) | Large (>665 pigs) | 1·63 | 1·33–1·99 | |
| Constant | 0·50 | 0·33–0·75 | ||
| Random effects† | ||||
| 5643 (2473) | County (n = 19) | |||
| Var(const.) | 4·45 | 2·16–9·15 | ||
| 5643 (2473) | County>herd id (n = 1567) | |||
| Var(const.) | 1·66 | 1·29–2·15 |
OR, Odds ratio; CI, confidence interval.
*
Three categorical fixed effects were: (1) year of test, (2) production class, and (3) herd size (based on national registry for subsidy).
†
County and herd ID were included as random effects to account for non-fixed effects associated with county and the individual herd.