The Marin County Case
An Infected Teacher Reads to Her Class, and 55% of Students Come Down with COVID. Masking, Ventilation and HEPA Filters Weren’t Enough.
“On May 25, 2021, the Marin County Department of Public Health (MCPH) was notified by an elementary school that on May 23, an unvaccinated teacher had reported receiving a positive test result for SARS-CoV-2, the virus that causes COVID-19.”
So begins one of the most fascinating and important investigations we’ve read during this pandemic. It appeared in the Aug. 27 edition of the CDC’s Morbidity and Mortality Weekly Report (MMWR).
The report – whose lead author was Tracy Lam-Hine of the MCPH and the University of California at Berkeley School of Public Health – focused on the behavior of the elementary school teacher, the high level of student masking and distancing in the classroom, and the frightening rate of infection among those students.
But there is another lesson here that the report does not explore but that deserves serious attention: the inadequacy of CDC-recommended measures to remove SARS-CoV-2 from interior air.
Marin, California, a suburb of San Francisco, is the 14th richest county in the nation. Some 75% of its residents are fully vaccinated – the highest rate in the state and one of the highest in the country.
The unnamed, unvaccinated teacher was reading aloud to students and “reportedly unmasked” even though the school required it. The teacher also “became symptomatic on May 19 with nasal congestion and fatigue” but continued working through May 21. The students, it appears, were masked, and parents of those infected “suggested that students’ adherence to masking and distancing guidelines in line with CDC recommendations was high in class.” Desks were all spaced six feet apart.
Between May 23 and 26, all but two of the 24 students were tested for COVID, and 12 of them came up positive – a 55% rate. Eight of the 12 were symptomatic, including four of the five students in the first row (the fifth tested positive as well). In the second row, three of four students tested received positive results, as did two of four in the fifth row, despite being 30 feet away from the teacher. The case provides a powerful example of how the SARS-CoV-2 virus can linger in the air, drifting far from its original source.
In another classroom, separated from the first by a large outdoor courtyard, 14 students were tested, and six received positive results. Several students in other grades, siblings of those in the original classroom, were later found to be infected, as were four parents (only one of whom was unvaccinated). In all, there were 27 cases, all apparently stemming from the index case involving the teacher.
The rapid spread of the virus throughout the classroom occurred before the Delta variant, which is twice as contagious as previous variants, became established as dominant in the United States. In late May, Delta (B.1.617.2) accounted for only about 5% of cases.
But how exactly did the spread occur in the teacher’s classroom? Below is a diagram from the MMWR report:
In the school, the report notes, “All classrooms had portable high-efficiency particulate air filters and doors and windows were left open.” In fact, as you can see from the diagram, the room was receiving a good deal of cross-ventilation. Ventilation plus high-efficiency particulate air (HEPA) filters is what the Centers for Disease Control and Prevention (CDC) recommends for interior spaces.
A single case is not dispositive, of course, but the CDC should take careful note of the Marin County experience, and a reconsideration of its advice may be in order. The CDC says it bases its recommendations on guidance from ASHRAE, the American Society of Heating, Refrigeration, and Air-Conditioning Engineers, a trade group dominated by the HVAC industry.
The question that CDC, other government agencies, and facilities managers should explore is whether ventilation (either via open windows or HVAC systems) combined with a passive filtration system like HEPA is the best means of mitigating indoor pathogens like SARS-CoV-2. The Marin experience shows that it is not.
The CDC recommends a “layered approach,” and that is certainly correct. Ventilation is necessary but not sufficient. Filters, on the other hand, present problems, the main one being that they are passive. They sit there waiting for pathogens to come to them. But not all pathogens do. Some continue to float in the air, and ventilation, as we saw in Marin, blow them all over an enclosed space. It’s almost certain that some pathogens won’t flow to where the HEPA unit is placed (in this case at the front of the room). In the back two rows, at a minimum of 24 feet away from the teacher, 37.5% of students tested positive.
The alternative to passive filtration is active antiviral elimination – that is, a system that sends particles throughout a space at high speed to deactivate pathogens chemically. Some of these systems also have deficiencies, including production of toxic byproducts like formaldehyde as well as ozone. More advanced versions, including one called Advanced Photocatalysis, developed by the Dallas firm ActivePure, create more efficient reactions, reducing rather than creating byproducts. In trials involving rooms that were already equipped with filter systems, Advanced Photocatalysis was found to reduce the level of pathogens by more than 50%.
CDC is correct to advocate a layered response to COVID, but each of those layers needs to work with maximum effectiveness. Yes, the teacher should have been vaccinated, but it is clear that a large segment of the U.S. population will never be vaccinated. Yes, the teacher should have been wearing a mask, but all of the students evidently were and masking every minute of a workday or school day is nearly impossible. Yes, the teacher should have reported a possible infection right away, but as we enter the cold and flu season, not every runny nose equals COVID. Yes, ventilation is necessary, but large banks of windows were open on a California spring.
The layered element that needs addressing is one that supplements ventilation. In the case of Marin, mitigation was passive and demonstrably ineffective. Mitigation, it seems logical to conclude, needs to be active: seek and deactivate. The CDC and everyone concerned about limiting the damage of COVID would do well to look beyond HVAC industry guidance, as school systems in Philadelphia and throughout the country are currently doing.
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