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Masks helped limit COVID transmission in unvaccinated students — US study

Masking, along with fewer exposure encounters, helped significantly to cut the incidence of COVID-19 infection among largely unvaccinated US university students.

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, reports Medpage Today, multivariate analyses found that any additional exposure was linked with a 40% increase in the chances of contracting SARS-CoV-2, 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 co-authors 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 about 2m 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 comprised 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 with 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 five-seven 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.

Study details
CDC Morbidity & Mortality Weekly Report – SARS-CoV-2 Transmission to Masked and Unmasked Close Contacts of University Students with COVID-19 — St Louis, Missouri, January–May 2021

Terri Rebmann, Travis M. Loux, Lauren D. Arnold, Rachel Charney,
Deborah Horton, Ashley Gomel.

Published in MMWR on 10 September 2021

What is already known about this topic?
During January–May 2021, Saint Louis University implemented a modified quarantine protocol that considered mask use when determining which close contacts required quarantine among an almost entirely unvaccinated student population.

What is added by the report?
Compared with only masked exposure, close contacts with any unmasked exposure had higher adjusted odds of a positive test result. Each additional exposure was associated with a 40% increase in odds of a positive test.

What are the implications for public health practice?
Universal masking and fewer encounters in close proximity to persons with COVID-19 can limit the spread of SARS-CoV-2 in university settings.

The findings in this report are subject to at least five limitations. First, contact tracing data were self-reported, which could introduce social desirability or recall bias or inaccurate data regarding mask use.

Second, the two students who received positive SARS-CoV-2 test results and reported only masked exposure might have had unmasked exposure to COVID-19 cases other than those under investigation, which could lead to underestimating the association between mask use and the percentage of positive test results.

Third, most students were not vaccine-eligible until late spring, so this analysis included very few fully vaccinated students; therefore, vaccination status could not be included in the regression analysis because of low cell counts.

Fourth, indoor versus outdoor exposure information and exposure time were collected but could not be included in the analysis because of a large amount of missing data.

Finally, this study was conducted before the Delta variant became the dominant strain of SARS-CoV-2 in the United States, which could have an impact on these findings given that the Delta variant has increased transmissibility compared to previous variants. Wearing masks and having fewer encounters with persons with COVID-19 reduced the odds of transmission in a university setting. In addition, there was no evidence of secondary transmission from either of the two students with only masked exposure who received positive SARS-CoV-2 test results, and who, because of the modified protocol in place, were allowed to forego quarantine. 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.

CDC recommends that universal masking be adopted in indoor spaces for vaccinated and unvaccinated persons in areas with substantial or high transmission rates and that masks should be worn in indoor spaces in areas without substantial or high transmission rates if you are not fully vaccinated (9). In addition, CDC recommends COVID-19 vaccination for individuals aged ≥12 years.


Medpage Today article – Masks Limited COVID Transmission in Unvaxxed College Kids (Open access)


CDC Morbidity & Mortality Weekly Report – SARS-CoV-2 Transmission to Masked and Unmasked Close Contacts of University Students with COVID-19 — St. Louis, Missouri, January–May 2021 (Open access)


See more from MedicalBrief archives:


Schools guidance to reflect new evidence on distancing in classrooms — CDC


Updated CDC guidelines: Cloth masks protect wearers as well as others


Humidity from masks may lessen severity of COVID-19 — NIH study



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