Well, yes. Even to a layman, that was clear. The problem with the Provincetown study and later the studies from Wisconsin and Singapore is that the samples were self-selecting. Instead of taking a random sample of the vaccinated population and assessing who was infected, who was symptomatic, and who had a high viral load capable of infecting others, they looked at vaccinated people who sought testing on their own initiative. (Or, in the Singapore case, actually needed hospital care.) Vaxxed people who feel the need to get tested are likely to do so because they have symptoms, but people with symptoms may have higher viral loads than the average infected vaccinated person does.

That’s the number we really need, and which would be hard to find. How many vaxxed people are getting infected but clearing the virus before they become infectious to others? Whether the vaxxed should be wearing masks routinely in daily life instead of for abundance-of-caution reasons around vulnerable people depends on the answer. And we don’t have it.

There are other questions about the data circulating among the pros, though. Some of whom work at the CDC.

The meaning of some of the viral load data has been disputed. Inside the CDC, some officials disagree with the agency’s conclusion that vaccinated people who become infected may spread the virus as readily as the unvaccinated, and argue that more testing needs to be done, including tests that measure how infectious virus particles are, according to a person familiar with the matter…

Some scientists say that the Provincetown study isn’t reliable enough to be the primary driver of a public health policy change. The data is too recent to be independently reviewed by outside experts, and it is too small of a sample and the circumstances of the outbreak are too unique for it to be applied to other parts of the country.

The connection between the Provincetown data and the CDC’s new mask guidelines has struck some scientists as obscure. The guidance applied to areas with high transmission of the virus, which tend to be parts of the U.S. with lower vaccination rates. Vaccination rates in Provincetown were high, around 69% for eligible Massachusetts residents.

Another complaint is that the agency stopped tracking mild breakthrough infections several months ago, believing that it lacked the resources to stay on top of all of them. That contributes to the “denominator problem” I described above. Not only do we have no idea how many infected vaxxed people with low viral loads were walking around Provincetown, we have no idea how many are walking around across the broader U.S. It could be that vaxxed people are infectious to others only rarely, when they have a severe enough case to actually generate symptoms.

Why didn’t the CDC recommend that the vaxxed mask up *around people whom they know are vulnerable* for now and say they’ll get back to us on universal masking as soon as they do some higher quality studies?

I fear the answer is because the new mask guidance is primarily aimed at the unvaccinated, not the vaccinated. They want the unvaxxed to mask up because they’re the ones at risk, but they know that so long as the vaccinated get to go maskless, the unvaccinated will too. The vaxxed had to be punished with a new round of precautions to force the unvaxxed to take precautions as well. The only alternative is a vaccine-passport system, which might be in the offing for blue jurisdictions in NYC but won’t be happening in the red states where it’s needed most.

Nate Silver chimed in with his own complaints about the studies the CDC used for the new guidance this morning:

Also as in the Provincetown study it's a convenience sample, meaning people who chose to be tested, and not a random sample of all infections. That likely biases the sample toward more severe infections since people with more severe symptoms are more likely to seek out testing.

— Nate Silver (@NateSilver538) August 3, 2021

If one way that breakthrough infections manifest themselves is thru very low amounts of virus in some people, but then you eliminate people with very low viral loads from your comparison, you're sort of begging the question. Need to be careful in cases like these.

— Nate Silver (@NateSilver538) August 3, 2021

One lingering question is whether the metric used in the studies as a proxy for viral load, “cycle threshold value,” really is a reliable measure of that in vaccinated people. Yesterday I linked to a thread from virologist Angela Rasmussen challenging that assumption; Rasmussen argued that “Ct values” measure how much viral RNA someone has in their nose and throat, not the amount of infectious virus, which is what we really care about. Former CDC chief Tom Frieden made the same point to the WSJ today: “There’s no one-to-one relationship between high viral load and infectivity, but we’re always making decisions based on imperfect data.”

That’s the best (only?) defense of the CDC guidance at this point. They had information that may have enormous consequences for the trajectory of the pandemic. If vaccinated people were suddenly infectious to the unvaccinated due to Delta’s extreme contagiousness, the public needed to know that right away so that Americans could take care. It will be … awkward if future studies eventually find that, in fact, the vaccinated rarely infect others even with Delta. But better to err on the side of caution, the CDC evidently concluded, by rushing out the guidance and then having to say “never mind” than to keep it quiet until better info was available and risk the vaccinated infecting the unvaccinated en masse.

If they’re worried about potentially vulnerable people, one productive thing they could do is try selling the vaccines to reluctant parents by reminding them that the best way to protect their kids until the kids can be vaccinated is to surround them with vaccinated adults. That’s a two-for-one sales pitch if it works, getting parents immunized and reducing the (already low) risk to their children.

Adjusted for population

Florida had 12X as many kids infected as Massachusetts last week

And Louisiana? About 10X

So what's going on?

None of these kids are vaccinated!

So why are infection rates so much lower in MA?

Right, because kids are protected by adults

4/5

— Ashish K. Jha, MD, MPH (@ashishkjha) August 3, 2021

So when you hear we have a pandemic of the unvaccinated

Yes that includes kids

But largely only in unvaccinated communities

Very few kids in highly vaccinated places are getting sick

So if you want to protect unvaccinated kids

Make sure everyone around them has the shot

Fin

— Ashish K. Jha, MD, MPH (@ashishkjha) August 3, 2021

Exit question: If the U.S. suddenly experiences a sharp decline in COVID cases the same way the UK just has, the CDC’s going to attribute it to the new mask guidance, aren’t they? Whether that’s true or not. On the other hand, the likelihood that we’ll get through our Delta wave as unscathed as Britain is small and dropping by the day.

The US Covid surge continues, but hospitalizations are increasing far faster here than they ever did during the UK's surge. With so many unvaccinated Americans, our surge will be MUCH deadlier. pic.twitter.com/JNncv5GL25

— Dr. Tom Frieden (@DrTomFrieden) August 3, 2021