Norovirus outbreaks have increased to prepandemic levels

The number of norovirus outbreaks increased in the 2021 to 2022 surveillance year, returning to levels similar to prepandemic levels, according to research published in the Sept. 23 issue of the U.S. Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report.

Anita K. Kambhampati, M.P.H., from the CDC in Atlanta, and colleagues examined norovirus outbreaks using data from the Norovirus Sentinel Testing and Tracking Network (NoroSTAT), established in 2012 for 12 states.

The researchers found that during the 2021 to 2022 surveillance year, the 12 NoroSTAT participating states reported 992 outbreaks of norovirus to the CDC. In comparison, in 2019 to 2020 and 2020 to 2021 surveillance years, the same states reported 1,056 and 343 norovirus outbreaks, respectively. During the prepandemic surveillance years, the number of norovirus outbreaks varied from 1,219 (2015 to 2016) to 1,471 (2018 to 2019). The characteristics of norovirus outbreaks reported by NoroSTAT participating states during 2021 to 2022 were similar to those reported during the prepandemic years. Eighty-two percent of outbreaks were due to person-to-person spread. Fifty-nine percent of outbreaks occurred in long-term care facilities. Forty-three percent of the laboratory-confirmed outbreaks with typing information during 2021 to 2022 were GII.4 Sydney(P16).

“Nonpharmaceutical interventions implemented during the COVID-19 pandemic were likely effective in preventing outbreaks of other infectious diseases, including norovirus,” the authors write. “As the use of nonpharmaceutical interventions has relaxed, norovirus outbreak incidence has returned to levels similar to those during prepandemic surveillance years.”