School closures early in the COVID-19 pandemic cut the projected number of coronavirus infections in the San Francisco Bay Area in half, but schools can reopen safely if they follow a series of safety precautions, according to computer modeling work by University of California, Berkeley researchers.

If elementary, middle and high schools had not shut down in March 2020, nine Bay Area counties examined by the researchers would have experienced 27,000 coronavirus infections and 4,600 deaths during the initial wave, instead of the 14,000 cases and 4,000 deaths actually recorded, the researchers found using an individual-based epidemiological transmission model. Their peer-reviewed results were published April 13 in the Journal of the Royal Society Interface. 

The Bay Area has seen some of the most restrictive school closure policies in the U.S., with some public schools being closed for upward of a year. While teachers and politicians who support school closures have pointed to the high risk of coronavirus infections within classrooms, other education experts say closures are widening the divide between poor and wealthy students. 

"The Bay Area was one of the first regions to adopt physical distancing measures on March 17, and these historic long-term school closures in addition," said Kristin Andrejko, a second-year epidemiology Ph.D. student at UC Berkeley. 

Andrejko and Jennifer Head, a third-year UC Berkeley Ph.D. student, were both lead authors of the study. 

The unprecedented nature of widespread school closures meant the researchers could not rely on historical data to create an epidemiological model — data about what American children and teenagers do when their schools are closed for long periods of time simply did not exist. 

"We didn't really know where kids went during school closures," Head said.

To address this gap, Andrejko led a team that surveyed 612 Bay Area households with at least one child. They collected data on social contacts — defined as interactions within six feet lasting more than 5 seconds — as well as information about race, income and household size, among other factors. 

The researchers found that young children had far more contacts than teenagers during the March through June period examined by the study. Children who were 5 to 12 years old had an average of 1.58 contacts per day, compared to just 0.78 contacts per day for teenagers aged 13 to 17. 

"Children had about twice as many contacts as teenagers, which was kind of a unique finding," said Andrejko, attributing the difference to the fact that children of essential workers were forced to accompany their parents to work or go to day care while teenagers could be more self-sufficient at home. 

They also found differences based on race and ethnicity, finding that Hispanic households had 2.32 more contacts than households of other racial and ethnic backgrounds. Andrejko said the researchers hypothesized that this occurred because essential workers are disproportionately Hispanic.

While the Berkeley researchers' survey only covered the first wave of the pandemic, this trend appears to have continued throughout 2020 and into 2021. Hispanic and Latino residents of the Bay Area have been infected with COVID-19 at a rate nearly four times higher than white residents, The Mercury News reported last month. 

Using the contact survey data compiled by Andrejko's team, Head's group created an individual-based epidemiological transmission model to estimate the effects of school closures, finding that school closures halved the number of confirmed COVID-19 cases and saved 663 lives. 

The researchers also estimated that workplace closures averted 828 deaths and restrictions on social gatherings saved 503 lives. 

They also created models incorporating school reopening precautions such as social distancing, mask wearing and dividing students into cohorts. The simulation was created before coronavirus vaccines became available. 

But even without vaccinated teachers or students, Head said schools can implement precautions allowing them to open with relative safety. Elementary schools can open using precautions such as masks, social distancing and cohorts, while high schools require more stringent measures, including staggering the days certain groups of students attend school in person. 

Head noted that these findings "really align" with what the Centers for Disease Control and Prevention has recommended recently, and added that she agrees with current vaccine distribution plans that prioritize teachers.

While the Bay Area is demographically different than the U.S. as a whole, with higher average levels of education and income, the researchers said their work can help inform epidemiological conversations around the country.

The counties examined in the study were Alameda, Contra Costa, Marin, Napa, San Francisco, San Mateo, Santa Clara, Solano and Sonoma.

The paper, "School closures reduced social mixing of children during COVID-19 with implications for transmission risk and school reopening policies," published April 13 in the Journal of the Royal Society Interface, was authored by Jennifer R. Head, Kristin L. Andrejko, Qu Cheng, Philip A. Collender, Sophie Phillips, Anna Boser, Alexandra K. Heaney, Christopher M. Hoover, Sean L. Wu, Graham R. Northrup, Karen Click, Joseph A. Lewnard and Justin V. Re, University of California, Berkeley; and Naomi S. Bardach, University of California, San Francisco.