Vaccines vs. Delta


    I don’t usually read the EducationWeek, but their recent article1 that, for some reason, appeared in my Google News feed grabbed my attention (a small win for the Google AI, I suppose). Sometime in late May of 2021, a teacher at a K-8 elementary school in Marin County, CA, got infected with the Delta variant2 of the SARS-CoV-2 virus.

    The article provides a summary of a much more detailed CDC report3. The teacher came to work, took the mask off to read a book to the class of twenty-four students, and infected half of them. The kids came home and infected their parents: four adults in total – three of them vaccinated. That’s what got my attention.

    Let’s suppose that the 12 infected students translate to 24 parents in their late 20s – very early 30s, on average, and 85% of them are white. This group of adults became eligible for vaccination around mid-April of 2021.  The CDC report states:

    …approximately 72% of eligible persons in the city where the school is located were fully vaccinated.

    The county data4 shows that by May 23, about 41% of the county’s residents were vaccinated. So, let’s assume that all 24 parents in question were eligible, and 72% – or 17 people – were fully vaccinated. Three of them got sick – that’s close to an 18% infection rate among the vaccinated. One of the seven unvaccinated parents also caught the virus – that’s just over 14% of the unvaccinated group.

    There’s probably more to this story, but, at least based on the information in the original CDC paper, this breakdown of the Delta variant cases seems odd to me: an awful lot of vaccinated adults got sick, and – even stranger – there were fewer infections  – both in absolute and relative numbers – among the unvaccinated.

    1. Rong-Gong Lin Ii, Los Angeles Times. “CDC: An Unvaccinated Teacher Took Off Their Mask to Read Aloud. Half the Class Got COVID.” Education Week, 30 Aug. 2021,
    2. “Coronavirus Disease 2019 (COVID-19).” Centers for Disease Control and Prevention, 26 Aug. 2021,
    3. Lam-Hine, Tracy. “Outbreak Associated with SARS-CoV-2 B.1.617.2 (Delta) Variant in an Elementary School — Marin County, California, May–June 2021.” MMWR Morb. Mortal. Wkly. Rep., vol. 70, 2021, doi:10.15585/mmwr.mm7035e2.
    4. “Marin County COVID-19 Vaccinations | Marin County Coronavirus Information.” 30 Aug. 2021,