COVID-19 restrictions have led to 86 percent drop in norovirus infections in US, finds study

The onset of the COVID-19 pandemic led to the imposition of several non-pharmaceutical interventions (NPIs) across the world. While citizens in the USA have been chafing over the advice to wear masks in public, restricted social interactions, and limited attendance at schools, colleges and workplaces, a new study published on the preprint server medRxiv* in November 2020 reports that these NPIs were linked to a steep reduction in the incidence of the non-respiratory pathogen, noroviruses.

Observers have already noted the reduction in the number of people seeking healthcare for many acute illnesses, even those not curbed by NPIs. For one, the number of norovirus outbreaks reported to the US Centers for Disease Control and Prevention (CDC) fell drastically from the spring of 2020.

This has caused some to wonder if this phenomenon was due to under-reporting of these diseases, including some potentially lethal conditions, or due to seasonal variations.

Tracking norovirus outbreaks

The current study was designed to be an interrupted time-series ecological analysis, covering nine US states in the North and East of the US, all of which had been part of the Norovirus Sentinel Testing and Tracking network (NoroSTAT) for 4 to 8 years. This system was set up to ensure rapid and comprehensive tracking of such cases, by mandating that all suspected or confirmed outbreaks be reported within 7 working days along with the setting and number of cases.

The information on the outbreaks covered the period from July 2012 to July 2020. The states included were from those with the highest number of reported cases per capital, and the data spans the period from the first year since the state joined the network, up until July 2020.

The researchers examined over 9,000 reports of acute gastroenteritis outbreaks where norovirus was suspected or confirmed to be the causative agent. These covered over 8 years of research. They focused on the monthly norovirus outbreak rate and average outbreak size.

The monthly outbreaks were compared before and after the introduction of NPIs, that is, from August 2012 to February 2020. The comparison was on a month-by-month basis, leaving out the transitional month of March.

They found that in all the nine states, there was a significant 86% decrease in the rate of outbreaks, when April-July 2020 was compared with all months before COVID-19 began. In terms of numbers, this meant a reduction in the number of monthly outbreaks, from around 6.2 to 0.78 on average, from the reference period to the post-NPI period.

These estimates remained robust even after adjusting for the expected decline in incidence during summer. Even if only the period between April to July each year was compared, the estimates remained unchanged. Moreover, the reduction was observed in many different settings, including nursing homes, child daycare centers, healthcare facilities and schools – though it was less remarkable in nursing homes.

From April onwards, when all NPIs were in place, reported monthly outbreaks fell almost to zero in all nine states and did not rise throughout the study period.

The average size of the outbreak was less by over 60% compared to earlier outbreaks. This indicates that the decrease is not only because less severe outbreaks are being overlooked by the overwhelm of surveillance systems. This is supported by information from state public health departments, confirming that all norovirus outbreaks are still being reported to the National Outbreak Reporting System (NORS).

The fact that the smallest drop in outbreak size and number was seen in nursing homes and healthcare facilities may be attributed to the fact that these could not remain closed, unlike daycare centers and schools. Yet, the implementation of preventive measures was associated with the lowering of incidence even in such centers.

Schools are typically hotspots of norovirus infection, and their closure may have led to the initial decline in the number of outbreaks. The contribution of this factor will be clearer as winter sets in with and when more schools and restaurants reopen, since it is modulated by NPIs still in force on the one side, and reduced population-wide immunity on the other.

These findings agree with norovirus trends in the UK in March and April. While influenza rates were observed to have declined dramatically, this is the first time that the incidence of a non-respiratory pathogen has been shown to go down, indicating the unexpected benefits of NPIs for other types of infectious disease, including those transmitted by the feco-oral route.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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