Here’s why San Diego Unified’s mask mandate won’t slow COVID-19.

The San Diego Unified School District adopted a mask mandate on July 18 for the last two weeks of summer school, automatically triggered by the prevalence of COVID-19 infections in the county. The intention of the policy, which could be reinstated for the new school year based on community spread, was to slow infection rates.

Such policies have been the subject of intense national media attention. Sadly, this is a reflection of how masking has become a divisive, political issue. However, San Diego Unified’s masking policy is ultimately about science and should be immutable to political fads.

Pre-pandemic, masking was discouraged by experts because the evidence then was negative on the protective effects of masking for the wearer. For instance, a randomized study of health care providers who wore cloth masks for four weeks in 14 hospitals in Hanoi, Vietnam, during 2011 found they experienced higher rates of respiratory illness, laboratory-confirmed viral infection and influenza-like illness than controls — who followed usual practice while working. Guidance by the World Health Organization in January 2020 stated that “cloth (e.g. cotton or gauze) masks are not recommended under any circumstance.”

With an abundance of uncertainty early during the pandemic, community masking using cloth or reusable surgical masks was encouraged in the hope that despite the lack of supporting real world evidence, there may be a community benefit if we all mask.

Observational studies that compared community practices supported this hope. However, this type of evidence is biased. Communities with higher masking rates are different from those with lower masking rates. For instance, communities with higher masking rates may have higher rates of working from home, social distancing and other protective practices. These differences, instead of masking, may be responsible for community infection rates.