Table 2. Univariate association between Salmonella enterica serovar frequency, cluster size, cluster density, and cluster being solved, Minnesota, USA, 2001–2007*.
| Characteristic | No. (%) solved clusters | No. unsolved clusters | Odds ratio (95% confidence interval) |
|---|---|---|---|
| Serovar | |||
| Very common† | 22 (10) | 203 | Referent |
| Common‡ | 11 (23) | 37 | 2.74 (1.23–6.13) |
| Uncommon§ | 10 (14) | 61 | 1.51 (0.68–3.37) |
| Total |
43 (13) |
301 |
|
| Cluster size¶ | |||
| 2 | 16 (8) | 194 | Referent |
| 3 | 8 (15) | 47 | 2.06 (0.83–5.11) |
| 4 | 7 (24) | 22 | 3.86# (1.43–10.40) |
| >5 | 12 (24) | 38 | 3.83 (1.68–8.74) |
| Total |
43 (13) |
301 |
|
| Cluster density, d** | |||
| 0 | 5 (71) | 2 | 25.8 (3.42–195.37) |
| 1–7 | 16 (33) | 33 | 5.01 (1.33–18.89) |
| 8–14 | 11 (22) | 40 | 2.84 (0.73–11.07) |
| >15 | 3 (9) | 31 | Referent |
| Total | 35 (25) | 106 |
*A solved cluster is one that results in identification of a confirmed outbreak. †S. enterica serovars Typhimurium and Enteritidis. ‡S. enterica serovars Newport, Heidelberg, and Montevideo. §All other serovars. ¶Significant Mantel-Haenszel χ2 test result for trend (p<0.001). #Clusters of 4 cases compared with clusters of 3 cases odds ratio 1.87, 95% confidence interval 0.52–6.66. **Cluster density measured as the number of days from receipt of first cluster case to third case received at the Minnesota Department of Health Public Health Laboratory.