Virginia updates its estimates of breakthrough and non-breakthrough COVID-19 infections on Friday. I thought it was worth posting the data for the most recent week.
I’ll get to why I thought this was worthwhile below.
Lack of cooperation in COVID-19 contact tracing
The nice thing about Virginia’s estimate is that it’s “clean”, in the sense of being based on a match between two lists: Persons vaccinated, and persons with a positive test. It doesn’t require any cooperation from those who were infected, or any contact tracing, or any of that.
And that’s a good thing, because, as it turns out, a lot of people refuse to cooperate with COVID-19 contact tracing. For example, three-quarters of persons in New Jersey routinely refused to cooperate with COVID-19 contact tracing. Roughly half of Maryland residents ignore COVID-19 contact tracing requests Lack of cooperation with contact tracers has been publicly noted in North Dakota. Only about half of persons contacted in Texas would cooperate. Similar results hold in Pennsylvania, North Carolina, and New York.
And in Massachusetts, more than half of residents won’t even pick up the phone when a contact tracer calls. In fact, the Massachusetts contract tracing efforts have become so dysfunctional that the Commonwealth of Massachusetts recently cancelled its main contract for that service. (By contrast, their track record early in the pandemic was pretty good.)
In fact, one might say that refusing to cooperate with COVID-19 contact tracing is more-or-less a nation-wide phenomenon.
I wonder who the non-cooperators are?
Now, before I go further, I want to ask a question.
Consider two individuals.
Individual A got vaccinated in a timely fashion, and follows CDC guidance regarding COVID-19 hygiene.
Individual B refuses vaccination and refuses to wear a mask or otherwise engage in COVID-19 hygiene.
Which person do you think is more likely to to ignore a request for COVID-19 contact tracing? Is it the vaccinated, mask-using Individual A? Or the anti-vaccine, no-mask individual B?
My guess is that that it’s Individual B. And while I can’t find any study that says that directly, that’s consistent with national survey-based results on that subject. Less than half of those surveyed said they would be comfortable with all the required aspects of COVID-19 related contact tracing. And — where have we seen this before — willingness to cooperate splits strongly along party lines, with Democrats being much more willing to cooperate with contact tracing than Republicans. Much the same as COVID-19 vaccination does.
And so, is there a practical lesson here? I sure think so.
If you identify a cohort of individuals through contact tracing, you’ll end up with a cohort that differentially skips the un-vaccinated. Not by design, but simply because those who won’t cooperate with a request to get vaccinated are largely the same people who won’t cooperate with a request to trace their contacts once they’ve gotten a COVID-19 infection.
Relevance to this week’s Morbidity and Mortality Weekly.
You have no doubt been seeing headlines about a July 4th outbreak in Provincetown, Massachusetts where it is claimed that three-quarter of those who were infected were fully vaccinated. That’s based on research by CDC published in this weeks Morbidity and Mortality Weekly Report.
(Of all the people blathering about this week”s issue of the MMWR, I’d bet that I’m just about the only one who routinely read the MMWR as part of my professional life.)
The nut-o-sphere has been having a field day with this, and it inevitably morphs into the bogus claim that three-quarters of all COVID-19 cases in Massachusetts are breakthrough infections. This, despite the fact that the Massachusetts Department of Public Health went out of its way to say otherwise, just prior to publication.
But let’s be boring and do the math, using current data for all of Massachusetts. Based on the most recent week of reporting, reporting from Massachusetts says that 38% of new COVID-19 cases are “breakthrough” cases, that is, cases among the vaccinated. This particular period includes the bulk of the reporting for that outbreak. It is unclear whether they include partially-vaccinated individuals in that total. Currently, 84% of the adult population in Massachusetts has received at least one dose, 75% of the adult population is fully vaccinated. I’ll assume they only include the fully-vaccinated.
So, doing the math, based on the data as-reported:
Relative risk of infection, for the vaccinated, in Massachusetts, during this period = (38%/75%) / (62%/25%) = 20%.
That’s high enough to be interesting. That’s above the 10% that keeps popping up every time I do this calculation with observational data. It’s vastly higher than the rate observed in Virginia.
But it’s a far cry from 74%.
And so, how on earth did the CDC manage to arrive at a figure of 74% for that one outbreak? When you get right down to it, how did the CDC know those people attended those parties, so they would know the set of people from which to calculate that 74%?
It’s not like they’ve implanted microchips in people, to track them. As far as we know.
The only way the CDC could identify those people is if those individuals voluntarily told them, one way or the other. And I think the key phrase for understanding this extreme estimate of breakthrough infections is this pair of seemingly-innocuous lines in the methodology section:
“During July 10–26, using travel history data from the state COVID-19 surveillance system, MA DPH identified a cluster of cases among Massachusetts residents. Additional cases were identified by local health jurisdictions through case investigation.”
In other words, what you’re looking at in this study isn’t the universe of people who attended those parties. It’s the people who attended those parties and then cooperated fully with the subsequent contact tracing.
And, best guess, that’s how you ended up with that eye-popping 74% figure. The sample is restricted to individuals who cooperated with contact tracing in some form. And so the sample probably skips most of those who have refused to get vaccinated.
There is no doubt that there was quite an outbreak from what was described as a series of packed parties in bars. Seems like every time I read about outbreaks these days, it’s from parties of some sort.
It’s absolutely true that the breakthrough infection rate currently being reported in Massachusetts is higher than I would expect, even accounting for the fact that this is observational data.
And, apparently, this is the research that convinced the CDC that fully-vaccinated individuals are perfectly capable of spreading COVID-19.
But it’s not 74% of all new infections. Breakthrough infections accounted for 74% of the individuals that the CDC identified, via contact tracing, as having attended those parties. In effect, based on those who would voluntarily report the whereabouts of themselves and their friends.
What the rate for the actual universe of party-goers is is not known. Short of locking up the entire town and interrogating then under torture, that’s simply not knowable.