Masking worked, along with fewer exposure encounters, to help limit the incidence of COVID-19 infection among largely unvaccinated university students in Missouri, researchers found.

Among nearly 400 close contacts of students who tested positive for SARS-CoV-2, the likelihood of a positive test result was more than five times higher in situations where at least one of the two individuals was unmasked versus situations where both were masked (adjusted odds ratio [aOR] 5.4, 95% CI 1.5-36.5, P=0.008), reported Terri Rebmann, PhD, RN, of St. Louis University in Missouri, and colleagues.

Notably, multivariate analyses found that any additional exposure was linked with a 40% increase in the chances of contracting SARS-CoV-2 (aOR 1.4, 95% CI 1.2-1.6), the authors wrote in the Morbidity and Mortality Weekly Report.

“These findings reinforce that universal masking and having fewer encounters in close contact with persons with COVID-19 prevents the spread of SARS-CoV-2 in a university setting,” Rebmann and coauthors noted. “Universities opening for in-person instruction could consider taking mask use into account when determining which unvaccinated close contacts require quarantine if enforced testing protocols are in place.”

The St. Louis Health Department permitted the implementation of a “modified quarantine” at St. Louis University in January 2021. This protocol was “mask-dependent,” where only the exposed unvaccinated-unmasked would require quarantine, while those exposed while masked were not required to quarantine. Those who came into close contact after masked exposures underwent daily health screenings, rather than quarantine.

The university enforced a mask policy in line with CDC guidelines, but also added outdoor enforcement. Unmasking was permitted exclusively while dining.

An exposure occurred when two people came within six feet of each other for at least 15 minutes over the course of 24 hours. Unmasked exposures occurred when at least one person was without a mask in close contact with others. RT-PCR testing was performed five days to a week post-exposure.

From January to May 2021, researchers performed 9,335 tests for SARS-CoV-2, and noted students may have been tested more than once. This cohort was comprised of 71% women, and 89% of students were unvaccinated.

Overall, 2.8% (n=265) of students tested positive for SARS-CoV-2, with 378 close contacts exposed. Of these, 7% (n=26) of contacts were among masked students and 93% (n=352) were among unmasked students. Overall, 31% of contacts tested positive.

However, 32% of these cases were among those with any unmasked exposure compared to 8% of those who reported masked exposures only.

The number of positive cases after unmasked exposures was also higher for the unvaccinated (33%) and those partially vaccinated (20%), compared to fully vaccinated students (0%; P=0.007). However, only 5% of students were fully vaccinated, with 6% partially vaccinated.

The analysis had several limitations, Rebmann and co-authors acknowledged. It was conducted prior to the emergence of the Delta variant, which recently demonstrated enhanced transmissibility. Due to small sample sizes, vaccination status was excluded from calculations and a single student could have been tested multiple times. Contact tracing was self-reported and most students were ineligible for vaccines during the study.

“For universities considering a similar approach, if masked unvaccinated close contacts are not required to quarantine, testing 5-7 days after exposure will be important because of the small risk for infection that could lead to secondary transmission if isolation is not implemented rapidly, especially in populations with low vaccination coverage,” the authors explained.