Table 2.
Results of the univariate analysis for the vaccine exposure variables levels (IRR = incidence rate ratio)
|
Exposure level |
Total no. of flocks |
No. of positive flocks |
Incidence |
IRR |
|
|---|---|---|---|---|---|
| (new cases per 1000 farms) | (95%CI) | ||||
| Categorization of the number of vets or retailers who had bought the vaccine |
1 (0-1vet or retailer) |
71 216 |
47 |
0.66 |
Referent |
| |
|
|
|
|
|
| 2 (>1 vet or retailer) |
7247 |
10 |
1.4 |
2.1 (1.1-4.1) |
|
| |
|
|
|
|
|
| Categorization of the number of farms that had bought the vaccine |
1 (0–4 farms) |
49 849 |
25 |
0.50 (*) |
Referent |
| |
|
|
|
|
|
| 2 (5–10 farms) |
20 821 |
17 |
0.82 (*) |
1.6 (0.88-3.0) |
|
| |
|
|
|
|
|
| 3 (>10 farms) |
7793 |
15 |
1.9 (*) |
3.8 (2.0-7.3) |
|
| |
|
|
|
|
|
| Herd size (median for each province) |
|
78 463 |
57 |
|
1.005 |
| (1.001-1.01) |
Both exposure variables related to the vaccine [(1) categorization of the number of veterinarians or retailers who had purchased the vaccine and (2) categorization of the number of farms that had bought the vaccine] show a significant association with disease levels. The categorization of the number of farms that had bought the vaccine shows a statistically significant upward trend in risk. The median farm size for each province resulted as not having a health impact on the disease risk.
(*) Test for trend: χ2 = 2.92, P value = 0.003.