An unvaccinated teacher in a California elementary school infected half her students and 26 people in total when she contracted the Covid-19 Delta variant, researchers for the US Centers for Disease Control and Prevention (CDC) found.
The researchers said the teacher attended school for two days despite displaying symptoms of Covid-19, and read aloud to her class without a mask during that time. Infections corresponded to the classroom’s seating chart, with the students sitting closest to the teacher the most likely to be infected.
Authorities said the report showed why vaccinations, masks and other prevention measures remain critical to prevent Covid-19 infections as US schools reopen. They also warned that anyone with symptoms of Covid-19 should stay home, to avoid infecting others.
“Evidence has repeatedly demonstrated that multi-layer prevention strategies – such as vaccination for all children and adults who are eligible; masks for all students, teachers, staff, and visitors; ventilation; cohorting; physical distancing; and screening testing – work to prevent the spread of Covid in schools,” said Dr Rochelle Walensky, the CDC director, at a press briefing on Friday.
The report is likely to increase calls for vaccine mandates in schools, which some districts such as New York City have already implemented. Children in the California investigation, in Marin county, were too young to be vaccinated.
Children aged 12 to 17 are eligible to be vaccinated, though coverage levels have lagged behind older age groups more susceptible to complications from Covid-19.
Tom Frieden, a former head of the CDC, said: “Delta is doubly infectious, and we need to step up our game to keep kids in school safely. That means teachers, staff and eligible students need to be vaccinated, and EVERYONE needs to be masked. Multiple layers of protection are essential.”
But Walensky said: “Unfortunately, many schools have opted not to implement these recommended tools. We recognize and are closely following cases and hospitalizations in children at the same time as school reopening.”
There have been approximately 2m Covid-19 infections in the five-to-17 age bracket and 300 associated deaths, another CDC report released on Friday found.
In the outbreak in Marin county, researchers investigated an unnamed elementary school. It is customary for the CDC to withhold identifying information in disease investigations.
The outbreak took place in mid-May to June and began with the infected teacher who was one of only two staff members at the school who were unvaccinated. The teacher had symptoms of Covid-19 in mid-May but continued working until she received a positive coronavirus test.
The teacher did not wear a mask as she read to students, even though the school required face coverings indoors. Half of the children in her class were infected, as were six students in a separate grade and eight family members of the teacher’s students. In total, one teacher infected 26 people.
The high levels of infections also showed, researchers said, how Delta can be especially transmissible in unvaccinated populations, such as children too young for the shot.
However, there was a bright spot. High vaccination levels in Marin county probably prevented more community infections, which would have been expected based on previous research.
The CDC also released a report on how Los Angeles county schools had reduced cases among students below the rate of community transmission.
Researchers found mitigation measures such as masks, physical distancing and tests “might” help reduce Covid-19 contagion in schools. Notably, the study was conducted before the Delta variant came to dominate US cases.
A third study from the CDC found just 32% of teens between 12 and 17 had been fully vaccinated, though as with adult vaccinations, levels varied widely by state. Mississippi had the lowest vaccination rate while Vermont had the highest, 20% versus 70% of adolescents respectively.