Study shows child groups spread virus at high rate
It is one of the most pressing questions of the pandemic: When will it be safe for schools to reopen?
A new study from UCSF Benioff Children’s Hospitals could help school districts get closer to an answer.
Using public data and a simple equation, researchers came up with a method of calculating the percentage of children in a local community infected with the coronavirus who do not show any symptoms.
“Knowing what the rate of asymptomatic infections among children is the first step you need to have to understand the risk of COVID transmission in any congregate group setting, whether it’s a classroom or child care or children’s hospital,” said senior author Dylan K. Chan, a pediatric otolaryngologist at UCSF. The big takeaway? There’s a relatively high chance of spreading coronavirus when you gather groups of children together in schools, even if they’re not showing symptoms.
The percentage of asymptomatic individuals under 18 consistently falls in line with the num
ber of confirmed cases in the general population, according to the study published by JAMA Pediatrics on Tuesday.
For example, the asymptomatic pediatric prevalence in San Francisco for the first few weeks of August was around 1.1%, based on data from the Johns Hopkins Coronavirus Research Center.
Based on the equation presented in the study, in a classroom of 22 students — even if all the individuals are screened and asymptomatic — that means there would be about a 24% chance that at least one child in that group is infected and showing no symptoms.
In a region with a higher case count, such as Merced County in California’s Central Valley — where the estimated pediatric prevalence is 4.6% — that number bumps up to 64%.
To come up with the formula, the researchers compiled coronavirus test results from more than 58,000 children in 25 cities across the U.S. between April and June (each patient was tested before an elective medical procedure unrelated to COVID19). They then compared the data with weekly confirmed case counts in each geographic area.
The researchers reversed their findings to come up with the equation that they hope local health officials and school districts can use to weigh risks in their regions: asymptomatic pediatric prevalence (%) = 1.07 x the weekly incidence (per 1,000 general population) + 0.23.
UCSF created an interactive tool to allow users to estimate the likelihood that a child in any county in the U.S. may have asymptomatic COVID19. This tool can approximate the risks associated with classrooms of different sizes.
“We know asymptomatic pediatric prevalence is somewhere in the 0.5% to 2% range,” Chan said. “That is a relevant number to know. It’s not 0.1% and it’s not 10%. If you know it’s around 1% that can give you a really good estimate of what the likelihood is that there will be an infected kid in a random class of 22 kids.”
It is a high rate, Chan said, but it comes with a caveat. The study does not address the viral load of the infected individual.
“A huge black box is not knowing how infectious these asymptomatic kids are,” he said. “The two key things you need to know is: What the risk is of having an asymptomatic infected kid, which is what our study tries to address. The second thing is, what is the chance that asymptomatic kid is going to infect other people? That second part is a much harder question to answer, but that’s so critical.”
Another study published this week by researchers at Duke University attempts to address that. Breaking out a sample of 293 schoolage children infected by SARSCoV2 into three age groups, the report finds there is no difference in the viral load between symptomatic and asymptomatic children.
It also concludes that asymptomatic infections are most common in elementary schoolage children. Out of those in the 6 to 13 age group, 39% were asymptomatic. In comparison, 25% in the 0 to 5 group did not show any symptoms, and 24% in the 14 to 20 group.
The researchers found that because children show milder symptoms of infection, it’s hard to track their role in the spread of the virus.
Nearly 9.3% of the reported coronavirus cases in the U.S. are children. Roughly eight out of 100,000 pediatric infections require hospitalization, according to a study published this month by the Centers for Disease Control and Prevention. Among those, 1 in 3 requires intensive care.
In the most basic terms, the UCSF study confirms what many public health experts already assumed: that schools can reopen safely only after community transmission subsides.
“An asymptomatic prevalence rate of 0.5% sounds low, but once you get a group of kids together, the risk that at least one of them is infected is quite high,” said Chan, who has three children enrolled in the San Francisco public school system.
Even with significant operational upgrades, including rapid test kits and temperature checks, few schools around the world have successfully reopened, based on a study by Cornell University.
In places such as Israel, Japan and Germany, many schools had to close and revert to remote learning after new COVID19 surges.
As schools reopened in the US, there was a 90% increase in the number of COVID19 cases among children, according to an ongoing report by the American Academy of Pediatrics and the Children's Hospital Association.
Florida, Georgia and Mississippi reported new coronavirus clusters within days of starting classes.
Chan said the UCSF report does not offer any recommendations for school leaders. The authors want to give the people in charge of deciding when to bring children back into classrooms another tool that could help them inch closer to an answer.
“It’s an impossible question,” he said. “It’s never going to be completely safe. All we can do as a community or society is decide what our threshold for safety is.”