Norovirus returns to pre-pandemic levels, CDC data show
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After a slump in reported norovirus outbreaks during the pandemic, numbers have rebounded to pre-pandemic levels as COVID-19 mitigation measures have eased around the country, CDC researchers reported Thursday.
Data from NoroSTAT — a network of 12 states that report norovirus outbreaks to the CDC — showed there were nearly triple the outbreaks from August 2021 through July 2022 compared with the same time period in 2020-2021.
The participating states reported 992 norovirus outbreaks in 2021-2022 — close to the 1,056 reported in 2019-2020 — after reporting just 343 during the 2020-2021 surveillance year.
Of the outbreaks in 2021-2022, 82% were due to person-to-person spread, with the majority — 59% — occurring in long-term care facilities. Among the laboratory-confirmed outbreaks — overall about 17% of the year’s outbreaks — 43% were the GII.4 Sydney strain, which has dominated U.S. outbreaks since it emerged in the 2015-2016 surveillance year, the researchers said.
They said nonpharmaceutical interventions implemented at the beginning of the COVID-19 pandemic were “likely effective in preventing outbreaks of other infectious diseases, including norovirus.”
That echoes a report published last March by Alicia N.M. Kraay, PhD, MPH, now an infectious disease epidemiologist at the Illinois University College of Applied Health Sciences, and colleagues, who said that a more than 80% decline in norovirus outbreaks between April and July 2020 was probably a result of interventions like social distancing, mask wearing and increased hand hygiene.
Kraay said the new MMWR report “makes sense and provides further support to this initial hypothesis.”
“In addition to increased contact, we know that norovirus can reinfect people many times during their life. After being infected, people are protected against getting sick for a while, but over time that protection gets weaker and may eventually disappear,” Kraay told Healio in an email.
“So, for norovirus, the fact that there were fewer cases for a while may have caused more people to be susceptible to getting norovirus, potentially making transmission more likely and future outbreaks more severe and contributing to the number of cases observed in the 2021-2022 surveillance year,” she said. “In other words, it’s not just about increased exposure as COVID mitigation measures decline, it’s about how this increased exposure interacts with population immunity.”
According to the new report, the 12 NoroSTAT states in pre-COVID-19 years reported numbers similar to 2019-2020 and 2021-2022, ranging from 1,219 outbreaks in the 2015-2016 surveillance year to 1,471 outbreaks in the 2018-2019 surveillance year.
The researchers said that declines in norovirus outbreaks in the U.S. mirrored declines seen in other countries at the beginning of the COVID-19 pandemic. They noted, however, that U.S. outbreak levels started rapidly increasing in January 2022 — just as mitigation measures were being eased in most parts of the country.
Norovirus transmission can be prevented by thorough hand-washing, avoiding food preparation for at least 2 days after symptoms end, and properly cleaning contaminated areas, according to the CDC.
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