Aug. 31, 2023 – Close and prolonged contact with someone with COVID-19 can more than quadruple the risk of getting the virus, a new study confirms. A higher – but somewhat reduced – risk persists even among people who have been vaccinated, had a prior infection, or both.
Led by public health researchers from Yale University, the study was published this month in the journal Nature Communications. The authors studied the topic because, while vaccination and prior infection are known to provide some protection against infection, the virus has still been able to sometimes evade immunity. Scientists call this “leaky” protection.
The new findings support a long-held theory that infection is more likely based on how much of the virus a person is exposed to and for how long.
Designing a study to evaluate this is challenging, so the authors decided it was best to do theirs at 13 correctional facilities. The facilities were all in Connecticut, and they all regularly did COVID testing on residents with and without symptoms. Researchers analyzed the risk of whether someone caught the virus based on where they lived in relation to an infected person, such as in the same cell or in the same cellblock.
The likelihood of becoming infected was also higher based on whether a person was vaccinated, had a prior infection, or both. The study took place from June 2021 to May 2022, when the Delta and Omicron virus variants were widespread. During that time, about 15,000 people spent at least 1 night housed in a facility, 48% of people had completed the primary vaccine series, and 27% were boosted.
The researchers found that during the Omicron period, people who shared a cell with an infected person had a nearly five times the risk of also becoming infected, and people who lived in the same cellblock had nearly four times the risk of infection. Having a prior infection, being vaccinated, or both did reduce a person’s risk of getting COVID-19, but people who shared a cell with an infected person still faced a significantly increased risk.
The authors wrote that the findings provide a case for continued contact tracing, particularly in places where people live close together, like prisons or nursing homes. They said such tracing should include not just people who share a room, but also people who were together during recreation times or meals. The findings also suggest the continued benefits of social distancing, quarantine and isolation, masking, and improved ventilation and airflow, they wrote.
Akiko Iwasaki, PhD, an immunobiologist at Yale who was not part of the study, told Nature that the outcome of the study “just makes intuitive sense. But now there’s evidence that these [measures] are probably going to be important to help the vaccine-mediated immunity work for you.”