Classroom Study Highlights Delta Variant's Threat to Schools

Brenda Goodman, MA

August 27, 2021

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An unvaccinated teacher who periodically removed her mask to read to students started a COVID-19 outbreak at an elementary school in Marin County, California, that eventually infected 22 students and four of their parents. Genome sequencing revealed the strain behind the outbreak was the Delta variant.

On the whole, the elementary school seems to have been doing a good job following recommended mitigation strategies. The windows and doors were open to promote good ventilation, there was a portable high-efficiency air filter in each classroom, and the students and teachers regularly wore masks. 

Only two teachers in the school were unvaccinated, and one came to work while experiencing symptoms she thought were due to allergies.

The study, which was released today in the Morbidity and Mortality Weekly Report , highlights how aggressive the Delta variant is, and how easily it can spread when layers of protection — including vaccination and masking — are haphazardly applied.

"In Northern California, we saw that the absence of optimal, multi-layer protection can result in the spread of COVID in this classroom and beyond," said CDC Director Rochelle Walensky, MD, in a White House press briefing today.

The study comes as cases in children have increased more than fourfold in the US over about 4 weeks, rising from 38,000 cases per week near the end of July to 180,000 cases reported in the week ending August 19, according to data collected by the American Academy of Pediatrics and the Children's Hospital Association. 

The rise in cases coincides with increased travel over the summer months and schools returning to in-person classes. Some governors have banned schools from requiring masks. Many districts are defying their governors to mandate masks anyway.

On Thursday, the Children's Hospital Association wrote a letter to President Joe Biden warning of a "perfect storm" of staffing shortages, surging respiratory syncytial virus cases, children's mental health issues, and rising COVID-19 infections in children that may threaten bed capacity. 

A second study released in MMWR reported that as of the end of July 2021, less than 1 in 3 adolescents ages 12 to 17 had been fully vaccinated against COVID; about 42% had received a single dose.

In the Northern California elementary school outbreak, half of the students — 12 out of 24 — in her classroom tested positive, including all five who were sitting in the front row of the classroom, which was closest to her desk. Three out of five students sitting in the second row tested positive. There was one positive in the third and fourth rows, and two positive cases in the last row.

In addition to cases in this class, there were six cases in a different grade at the school. Gene testing revealed the strains infecting the kids in these two grades were identical, suggesting they were part of the same outbreak. The study authors say that they aren't sure how the virus passed from one grade to the other, but believe it was passed through kids interacting with each other at school. 

The study also revealed that some of the children, who were all too young to be vaccinated, brought their infections home. In addition to the cases split between the two grades, four other students who were siblings of kids in the original classroom tested positive, along with four parents. All the adults who were infected experienced symptoms, despite three of them being vaccinated. None of the people infected in the outbreak was hospitalized.

A third study published today in MMWR covered 80 public school districts in Los Angeles County. It found that school-associated cases in children were 3.4 times lower during the winter surge than in cases in children attributed to community spread, suggesting that prevention measures like masking, physical distancing, cohorting, and contact tracing help to reduce transmission within schools. 

Only 463 school-associated cases were reported among students attending public K-12 schools in Los Angeles County during the last school year, but this was a period when many students were given the choice to remain at home with online classes. The period of the study covers September 2020- March 2021, and doesn't reflect the impact of the Delta variant, the study authors write.

"We know how to protect our kids in school," Walensky said at the press briefing. "We have the tools."

Walensky said the CDC had recognized and was closely following the rise in cases in schools.

"In our outbreak investigations, large scale quarantines or large number of cases are generally occurring in schools because schools are not following our guidance," she said.  The guidance, she says, stresses vaccination for all eligible teachers and students and universal masking.

But others have argued that the rise in cases is happening despite schools relying on and following the CDC's guidelines, particularly guidance that children can safely be spaced at least 3 feet apart to reduce transmission. 

"The CDC recommendations, frankly, there are some valid and good points in them, but they are built on a house of cards," said Michael Osterholm, PhD, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, in his weekly podcast.

"When we talk about a distance of 3 feet is acceptable in school when you have a face cloth covering on defies gravity," he said. 

Osterholm said the CDC should provide more information about better masks for children.

"Who in their right mind believes that an aerosol is only going to be transmitted 3 feet or less or that Plexiglas will make a difference? That's just wrong to continue to promote that, and the CDC guidelines do that, and thus educators have become convinced that they can reopen their schools safely," he said. "We have misled them."

Osterholm said he shared the sentiment of wanting children to be back in schools, but "I don't think that's possible today," he said. "I know that's going to be hard for people to hear."

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