Refuting exaggerated reports of “breakthrough infections” and the “Mu” variant

Since mid-July when the CDC published its conclusions from the “Provincetown outbreak,” people have been incredibly concerned about breakthrough infections (infections that occur in fully vaccinated individuals). The CDC made these sound very common and suggested that PCR of viral material from the vaccinated individuals’ noses showed levels of virus similar to those from unvaccinated individuals.

In all the subsequent discussion around the Delta variant, there has been a trend suggesting that the vaccines aren’t working or are providing suboptimal protection. I have written extensively in earlier blog pieces that this is not the case and showed data supporting continued vaccine efficacy. Despite this, unreviewed studies and social media continue to tout breakthrough infections.

In today’s New York Times an excellent article by David Leonhardt looks at actual data surrounding COVID breakthrough infections in the United States. It indicates that the chance of someone who is vaccinated getting infected by COVID on any given day is only 1 in 5000 and is purely dependent upon the number of infected people in the area, which is mainly the result of infections in unvaccinated individuals.

Since the CDC is not actually monitoring breakthrough infections, the estimates are based on statistics from states and municipalities reporting detailed data on Covid infections by vaccination status. This includes Utah, Virginia, and King County Washington (which includes Seattle).

The results are consistent with the idea that only 1 in 5,000 vaccinated Americans have tested positive for Covid each day in recent weeks.
The chances are surely higher in the places with the worst Covid outbreaks, like the Southeast. This is because of the low rates of vaccination in these areas.

Leonhardt also remarks that in places with high vaccination rates where there are many fewer cases — like the Northeast, as well as the Chicago, Los Angeles, and San Francisco areas — the chances appear to be lower, probably less than 1 in 10,000. That’s what the Seattle data shows, for example. (These numbers don’t include undiagnosed cases, which are so mild that they aren’t detected because people do not notice them. Luckily, in those cases most do not pass the virus to anyone else.)

To put this into perspective, given a one-in-10,000 daily chance of being infected, a vaccinated person could wander around his/her community unmasked for more than three months before the combined risk of being infected reaches 1 percent.

Finally, don’t even ask about the “Mu” variant. The latest COVID-19 strain has attracted a lot of attention, again with unsubstantiated reports of “vaccine escape.” These are similar to reports of vaccine escape initially circulated around the Delta variant, which proved not to be true. Time will tell, but for now the vaccines look good!

Published by jbakerjrblog

Immunologist, former Army MD, former head of allergy and clinical immunology at University of Michigan, vaccine developer and opinionated guy.

3 thoughts on “Refuting exaggerated reports of “breakthrough infections” and the “Mu” variant

  1. Thank you for these statistics! Is there any data on breakthrough cases by vaccine type? My husband had the J&J and will be traveling later this month, by plane, and I am concerned. Also, any recommendations on what type of mask to purchase?


  2. I think that as epidemiologists, the folks at the CDC were negligent when they failed to emphasize that multiple demographic factors (not just mostly-male attendees at bars and parties) were likely at play here in the outbreak of COVID-19 cases in Barnstable county. Without a doubt, the population studied in this report is not at all representative of the fully-vaccinated population in the U.S., making this report clearly not at all generalizable. They alluded to this in the Discussion section but could have, and should have, been much more clear and forthcoming about this major factor in the outbreak.

    I personally think that this is probably due to politics entering into the study and management of the pandemic, which is not only a shame, but dangerous as well. I expect this sort of thing from the lay media but not from professional epidemiologists at the CDC.


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