Table 1.
Distribution of Acute Gastroenteritis Clinical and Epidemiologic Characteristics Among Confirmed Norovirus, Suspected Norovirus, Confirmed Nonviral, and Unknown Etiology Outbreaksain the NORS 2009–2012
| Characteristic | No. (%) With Characteristic | Median | IQR (Q1, Q3)b | P c |
|---|---|---|---|---|
| Median incubation period, h | ||||
| Confirmed norovirus | 156 (5.3) | 30.0 | (24.0, 37.0) | Ref |
| Suspected norovirus | 43 (3.3) | 30.0 | (24.0–34.0) | .92 |
| Confirmed nonviral | 33 (2.1) | 60.0 | (48.0, 120.0) | <.0001 |
| Unknown | 200 (5.4) | 24.0 | (11.3, 34.0) | <.0001 |
| Median duration of illness, h | ||||
| Confirmed norovirus | 1192 (40.6) | 48.0 | (24.0, 48.0) | Ref |
| Suspected norovirus | 378 (28.6) | 42.5 | (24.0, 48.0) | .63 |
| Confirmed nonviral | 177 (11.5) | 144.0 | (96.0, 204.0) | <.0001 |
| Unknown | 1184 (32.1) | 36.0 | (24.0, 48.0) | <.0001 |
| Proportion of cases with bloody stools | ||||
| Confirmed norovirus | 759 (25.8) | 0.0 | (0.0, 0.0) | Ref |
| Suspected norovirus | 355 (26.9) | 0.0 | (0.0, 0.0) | .56 |
| Confirmed nonviral | 827 (53.6) | 0.3 | (0.04, 0.50) | <.0001 |
| Unknown | 911 (24.7) | 0.0 | (0.0, 0.0) | .29 |
| Proportion of cases with diarrhea | ||||
| Confirmed norovirus | 2200 (74.9) | 0.86 | (0.75, 0.98) | Ref |
| Suspected norovirus | 944 (71.5) | 0.88 | (0.74, 1.00) | .30 |
| Confirmed nonviral | 1261 (81.7) | 1.00 | (0.99, 1.00) | <.0001 |
| Unknown | 2506 (67.8) | 0.94 | (0.75, 1.00) | <.0001 |
| Proportion cases with fever | ||||
| Confirmed norovirus | 1542 (52.5) | 0.22 | (0.08, 0.40) | Ref |
| Suspected norovirus | 686 (51.9) | 0.26 | (0.09, 0.45) | .05 |
| Confirmed nonviral | 1012 (65.5) | 0.58 | (0.33, 0.83) | <.0001 |
| Unknown | 1473 (39.9) | 0.18 | (0.01, 0.43) | .006 |
| Proportion of cases with vomiting | ||||
| Confirmed norovirus | 2164 (73.6) | 0.72 | (0.58, 0.87) | Ref |
| Suspected norovirus | 919 (69.6) | 0.71 | (0.56, 0.89) | .69 |
| Confirmed nonviral | 1031 (66.8) | 0.39 | (0.22, 0.60) | <.0001 |
| Unknown | 2369 (64.1) | 0.75 | (0.49, 1.00) | .8 |
| Fever-to-vomiting ratio | ||||
| Confirmed norovirus | 1506 (51.2) | 0.31 | (0.12, 0.56) | Ref |
| Suspected norovirus | 665 (50.3) | 0.39 | (0.17, 0.64) | .002 |
| Confirmed nonviral | 795 (51.5) | 1.33 | (1.00, 2.00) | <.0001 |
| Unknown | 1329 (36.0) | 0.33 | (0.05, 0.71) | .84 |
| Diarrhea-to-vomiting ratio | ||||
| Confirmed norovirus | 2139 (72.8) | 1.33 | (1.00, 1.44) | Ref |
| Suspected norovirus | 908 (68.7) | 1.12 | (1.00, 1.50) | .8 |
| Confirmed nonviral | 899 (58.2) | 2.00 | (1.44, 3.40) | <.0001 |
| Unknown | 2214 (59.9) | 1.00 | (1.00, 1.67) | .88 |
Abbreviations: IQR, interquartile range; NORS, National Outbreak Reporting System.
aSingle-etiology outbreaks with norovirus reported as laboratory confirmed (laboratory confirmation of 2 or more cases, n = 2939), norovirus reported as suspected (laboratory confirmation in fewer than 2 cases, n = 1321), or a confirmed nonviral etiology (n = 1573) (Supplementary Table 2); unknown etiology outbreaks reported without any suspected or confirmed etiology (n = 3694).
bIQR (Q1, Q3) is the interquartile range, where Q1 is the 25th percentile and Q2 is the 75th percentile.
c P values were obtained by Kruskal-Wallis tests with post hoc Steel, Dwass comparisons with laboratory-confirmed norovirus outbreaks.