Table 1. Norovirus outbreaks with exposed population size reported to the National Outbreak Reporting System, United States, 2009–2017*.
| Characteristics | No. (%) | Median attack rate, % (IQR) | Median final size, % (IQR) | Median R0 (IQR) |
|---|---|---|---|---|
| All outbreaks |
7,094 (100) |
22 (11–36) |
28 (16–47) |
2.75 (2.38–3.65) |
| Major setting | ||||
| Child day care | 272 (4) | 21 (13–36) | 18 (11–29) | 2.67 (2.39–3.60) |
| Hospital or healthcare facility | 271 (4) | 22 (11–38) | 19 (11–34) | 2.70 (2.33–3.59) |
| Long-term care or assisted living facility | 5,335 (75) | 23 (13–36) | 30 (17–47) | 2.81 (2.42–3.76) |
| Other | 350 (5) | 20 (10–36) | 24 (15–40) | 2.66 (2.35–3.60) |
| Private home or residence | 42 (1) | 66 (50–91) | 9 (6–16) | 3.80 (2.26–4.92) |
| Restaurant | 77 (1) | 50 (27–64) | 10 (6–16) | 3.12 (2.53–4.31) |
| School, college, or university |
747 (11) |
12 (6–24) |
42 (19–80) |
2.41 (2.24–2.92) |
| Season | ||||
| Winter | 4,016 (57) | 22 (12–36) | 30 (17–51) | 2.80 (2.40–3.77) |
| Fall | 808 (11) | 21 (11–37) | 26 (15–47) | 2.72 (2.36–3.63) |
| Spring | 1,964 (28) | 20 (11–35) | 27 (15–44) | 2.69 (2.37–3.57) |
| Summer |
306 (4) |
17 (9–33) |
19 (11–32) |
2.57 (2.29–3.33) |
| Outbreak status | ||||
| Confirmed | 3,114 (44) | 26 (15–40) | 35 (20–55) | 2.99 (2.51–4.22) |
| Suspected |
3,980 (56) |
18 (9–31) |
24 (14–40) |
2.59 (2.32–3.27) |
| Census region | ||||
| Northeast | 1,898 (27) | 17 (9–29) | 31 (17–53) | 2.58 (2.30–3.23) |
| Midwest | 2,205 (31) | 25 (13–39) | 26 (15–44) | 2.87 (2.44–3.98) |
| South | 2,224 (31) | 23 (12–38) | 29 (17–47) | 2.81 (2.39–3.93) |
| West |
767 (11) |
21 (13–34) |
28 (16–44) |
2.75 (2.42–3.57) |
| Year | ||||
| 2009 Jan–Jun† | 243 (3) | 28 (15–42) | 35 (20–55) | 3.09 (2.50-4.56) |
| 2009 Jul–2010 Jun | 275 (4) | 29 (15–45) | 35 (19–57) | 3.17 (2.51–4.77) |
| 2010 Jul–2011 Jun | 592 (8) | 29 (16–44) | 32 (19–54) | 3.12 (2.54–4.58) |
| 2011 Jul–2012 Jun | 679 (10) | 27 (15–40) | 35 (19–59) | 3.01 (2.52–4.29) |
| 2012 Jul–2013 Jun | 967 (14) | 21 (12–36) | 28 (16–46) | 2.73 (2.38–3.61) |
| 2013 Jul–2014 Jun | 913 (13) | 20 (11–33) | 29 (18–51) | 2.68 (2.38–3.45) |
| 2014 Jul–2015 Jun | 941 (13) | 21 (11–35) | 28 (16–46) | 2.74 (2.3–3.61) |
| 2015 Jul–2016 Jun | 1,007 (14) | 17 (9–32) | 25 (14–42) | 2.57 (2.31–3.29) |
| 2016 Jul–2017 Jun | 1,070 (15) | 19 (10–31) | 26 (14–42) | 2.63 (2.33–3.24) |
| 2017 Jul–Dec† | 407 (6) | 20 (10–34) | 22 (14–38) | 2.66 (2.33–3.55) |
*IQR, interquartile range. †Partial norovirus years included in this analysis. The National Outbreak Reporting System was established in January 2009, and the first year of this analysis is 2009 January–June. At the time of analysis, we received data through December 2017.