Table 2.
Clinical and Epidemiologic Characteristics Used to Discriminate Between Norovirus or Nonviral Etiologies With NORS-Reported Outbreaks From 2009–2012
| Clinical and Epidemiologic Characteristics | Confirmed Norovirus (%)a | Confirmed Nonviral (%)b | Likelihood Ratio | Sensitivity (95% CI), % | Specificity (95% CI), % | Positive Predictive Value (95% CI), % | Negative Predictive Value (95% CI), % |
|---|---|---|---|---|---|---|---|
| Median duration of illness, hc | |||||||
| 12–60 | 949 (79.6) | 11 (6.2) | 12.9 | 79.6 (77.2–81.8) | 93.8 (89.3–96.5) | 98.9 (98.0–99.4) | 40.7 (36.1–45.6) |
| Not 12–60 | 243 (20.4) | 167 (93.8) | |||||
| Total No. of outbreaks with all criteria | 1192 (40.6) | 178 (11.3) | |||||
| Proportion with vomitingc | |||||||
| ≥50% | 1857 (85.8) | 438 (41.7) | 2.1 | 85.8 (84.3–87.2) | 58.5 (55.5–61.4) | 80.9 (79.3–82.5) | 66.8 (63.7–69.7) |
| <50% | 307 (14.2) | 612 (58.3) | |||||
| Total No. of outbreaks with all criteria | 2164 (73.6) | 1050 (66.8) | |||||
| Median incubation period, hc | |||||||
| 24–48 | 117 (75.0) | 14 (41.2) | 1.8 | 75.0 (67.7–81.1) | 58.8 (42.2–73.6) | 89.3 (82.9–93.5) | 33.9 (23.1–46.6) |
| Not 24–48 | 39 (25.0) | 20 (58.8) | |||||
| Total No. of outbreaks with all criteria | 156 (5.3) | 34 (2.2) | |||||
| Fever-to-vomiting ratio | |||||||
| ≤1 | 1451 (97.8) | 343 (42.8) | 2.3 | 97.8 (96.9–98.4) | 57.2 (53.7–60.6) | 80.9 (79.0–82.6) | 93.3 (90.7–95.2) |
| >1 | 33 (2.2) | 458 (57.2) | |||||
| Total No. of outbreaks with all criteria | 1484 (50.5) | 801 (50.9) | |||||
| Diarrhea-to-vomiting ratio | |||||||
| <2.5 | 2050 (95.8) | 558 (61.3) | 1.6 | 95.8 (94.9–96.6) | 36.7 (35.6–41.9) | 78.6 (77.0–80.1) | 79.8 (75.8–83.3) |
| ≥2.5 | 89 (4.2) | 352 (38.7) | |||||
| Total No. of outbreaks with all criteria | 2139 (72.8) | 910 (57.9) | |||||
Abbreviations: CI, confidence interval; NORS, National Outbreak Reporting System.
aSingle-etiology outbreaks with norovirus reported as confirmed (laboratory confirmation of 2 or more cases, n = 2939).
bSingle-etiology outbreaks with a confirmed nonviral etiology (n = 1573) (Supplementary Table 2).
cKaplan criteria include vomiting ≥0.5 affected persons, median incubation period of 24–48 hours, and median duration of illness of 12–60 hours.