Rates of sudden infant deaths increased during the covid-19 pandemic, a new analysis suggests – and altered transmission of illnesses such as respiratory syncytial virus (RSV) might be linked to the rise.

Writing in JAMA Network Open, researchers analyzed U.S. mortality data from 2018 through 2021. In that period, there were 14,308 cases of sudden unexpected infant death (SUID) – an umbrella term that describes the unexpected death of a child under age 1. Infants who died of sudden infant death syndrome (SIDS), which occurs during sleep, were included in the count.

Starting in July 2020, the rate of sudden infant deaths began rising, and researchers noted a “pronounced epidemiologic shift” in the deaths between June and December 2021. During those months, the monthly rate of SUID rose between 10 and 14% compared with pre-pandemic levels.

It’s unclear how the covid-19 pandemic affected SUID risk, the researchers write. COVID-19 itself could have impacted SUID rates, though coronavirus hospitalizations were not clearly associated. Perhaps the increased rates fit with the “unintended cumulative outcomes” of pandemic mitigation strategies that may have limited health-care access, disrupted social networks and affected child care nationwide.

Or there could be another explanation: Pandemic mitigation strategies that decreased the risk of other infections may have affected SUID rates, the researchers write.

Precautions put in place early in the pandemic are thought to have reduced the spread of respiratory diseases. But in 2021, many pandemic restrictions were lifted, and the rate of RSV cases began rising even in the disease’s typical offseason, a rise that “closely” mirrored the shift in summer 2021.

“We don’t know what makes babies who die from SUID or SIDS more vulnerable, whether it’s genetics or something else. It could be that infections like RSV amplify those factors and make them more vulnerable,” co-author Erich Batra, associate professor of pediatrics and family and community medicine at Penn State College of Medicine, said in a news release.

The researchers call for more study of potential links between respiratory infections and sudden infant death. They also remind parents to use safe sleeping practices, including putting babies to sleep on their backs, avoiding soft bedding and not sharing a bed with an infant. Research earlier this year suggests that more than three-quarters of sudden infant deaths involve multiple unsafe sleeping practices.