Wastewater study finds masks prevent COVID-19 in schools

A study that used wastewater signals to identify cases of COVID-19 found that even a small increase in the proportion of people wearing masks at elementary schools in one California county significantly reduced the chance of a case.

The findings were reported Wednesday at the Conference on Retroviruses and Opportunistic Infections by Rebecca Fielding-Miller, PhD, MSPH, an assistant professor of infectious diseases and global public health at the University of California, San Diego.

“There have been some really nice studies looking at the impact of ... masking policy on COVID cases, and there have been some randomized control trials looking at the effect of telling people to mask on COVID-19 cases,” Fielding-Miller said during a press conference. “But it turns out that telling people to mask is a little bit different than people actually masking .”

From March 2 to May 27, 2022, Fielding-Miller and colleagues partnered with 26 elementary schools in four school districts in San Diego County to implement wastewater monitoring, which included pulling wastewater samples every 15 minutes from each school.

Four trained observers sent to 10 of the schools counted how many students and adults were fully masked, partially masked or unmasked “on campus during school pickup,” Fielding-Miller said. In 60 of these observations over 6 weeks, 38.6% of individuals were observed fully masked.

The authors found that the odds of a positive wastewater signal in the 5 days after one of these observations decreased by 47% (adjusted OR = 0.53; 95% CI, 0.28-0.99) for each 10% increase in the proportion of people who were fully masked.

Fielding-Miller was asked why they chose elementary schools — in this case, schools serving historically marginalized communities — as the location for their study.

“We don’t know what long COVID looks like in kids,” Fielding-Miller replied. “And also, a kid getting sick is 5 to 10 days of a parent not being able to go to work. In the communities we were working with, about 25% of parents told us pretty repeatedly that that’s an economic hit they couldn’t afford to take. Even if you leave aside the clinical issues, there are a lot of social issues to kids getting sick all the time.”

Fielding-Miller called the findings “significant and striking.”

“This is the first study I know of like this; there's still a lot to be done,” Fielding-Miller continued. “But to me, it's really promising for how we can be effective even with real-world masking strategies, and how it doesn't have to be all or nothing. Even a little nudge can keep kids safer at school.”