California Classroom Outbreak Explained

COVID Variants Infection and Spread School

A recent CDC outbreak investigation found that an unvaccinated teacher infected twelve elementary school students out of a class of 24 students in California.

All students were too young to be eligible for the vaccine.

Key take-aways:


With school starting up we know many parents are apprehensive, and news of this MMWR report last week didn’t help. The infections *were* linked to the more transmissible Delta variant, making us all wonder if this was a harbinger for things to come this school year.

❓What happened?

The “index” or first case in the cluster, the teacher, was unvaccinated and tested positive for SARS-CoV-2 on May 23rd after attending social events May 13-16th.

Key facts:

*The classroom had portable HEPA filters and doors and windows were left open (well done!).
*The teacher was one of only two teachers in the school unvaccinated.
*The teacher had worked for three days AFTER becoming symptomatic (May 19-21) but prior to testing.
*The school required masking indoors, but the teacher was reportedly UNMASKED when reading to the class.

Students from the teacher’s class began testing positive May 22nd. Students were seated in five rows with desks 6 feet apart. Mask compliance by students overall was reported to be good.


80% (8 out of 10) students in the two rows closest to the teacher’s desk were infected, and 28% (4 out of 14) in the back three rows.

This pattern suggests that those closer to the teacher’s desk were more at risk and that the unmasked reading may have been an important exposure. We don’t know the quality or fit of the masks worn by the students.

Several parents and other children in the grade were later infected from infected children, but no other school staff were infected. Fortunately, no infections from this cluster were hospitalized.

❓What can we learn from this?

➡️ Delta is *very* transmissible, especially in unvaccinated populations, which includes children under age 12.
➡️ Even with good ventilation measures in place, loud, projected, unmasked talking by an infected person is a high risk for transmission.

❓Does this mean the school year is doomed? We’ll be honest, it’s going to be a bumpy ride for sure.

For us, this outbreak highlights the importance of VACCINATION for those eligible and keeping your MASK ON while talking. Also-STAY HOME with any symptoms, even if they seem like allergies or a cold. While we’ve all been there with dosing kids with ibuprofen and sending them to school in the Before Times, now is the time to be community-minded and err on the side of caution.

If this teacher had done ANY of the three preventive actions above, this outbreak may not have happened. But we’ll never know.

This brings us to a final point—outbreak investigations are just that—they investigate clusters of infections that DO happen and describe the circumstances looking for clues to transmission. This means that by definition the cluster is somewhat unusual- unusual enough to merit a thorough investigation and CDC write-up.

This was NOT a study of a random sample of schools comparing how different prevention measures relate to infection risk. For those panicking that the open windows and HEPA filter didn’t prevent these infections, take heart that we’re likely missing all the other classrooms where transmission WAS prevented. We don’t do CDC/MMWR reports on outbreaks that never happen. It could have been that the air currents in the room were particularly unlucky or the teacher was especially contagious.

Like a car accident investigation, these reports can give us clues about what were factors in this particular accident/outbreak, but they don’t tell us scientifically what factors are associated with more accidents or transmission in general. This requires a control group of similar classrooms where the teacher did *not* infect their students, so we can compare what’s different. Don’t worry, these types studies are underway too, but with all the caveats that things are muddy and we don’t randomly assign prevention measures in schools.


➡️ Delta is very contagious.
➡️ Even schools with excellent mitigation measures will face challenges this year.

This particular case emphasizes the importance of:


Those Nerdy Girls

Read the full CDC MMWR report here.

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