Outbreaks aren’t supposed to happen among the vaccinated, right? Last week the CDC told us that it had found just 5,800 confirmed cases of COVID out of 66 million people who’ve been vaccinated, a rate of .008 percent. If that rate holds universally, there shouldn’t be any infections in a nursing home of a few dozen people. At most, there should be one purely fluky one.

Viewed through that lens then yeah, an outbreak of any sort among any vaccinated cohort is of concern inasmuch as it strongly suggests that there are a lot more undetected “breakthrough infections” among vaccinated people than the CDC knows about. In the case of the Kentucky nursing home, they had a group of unvaccinated residents and staff alongside a group of vaccinated ones so they went ahead and tested everyone. Lo and behold, they found a bunch of asymptomatic infections among those who’d been inoculated.

So maybe the “true” rate of infection among the vaccinated is more like 0.8 percent instead of .008. Maybe it’s even higher and the CDC is still in the dark.

On the other hand, virologists are celebrating the data from the nursing home because it proves that the vaccines provide strong protection even when the conditions for a truly devastating outbreak are all present. The case involved an elderly population living in close quarters amid an outbreak that infected dozens — and, to top it off, the virus that infected them turned out to have several mutations known to make it more dangerous than standard coronavirus. A year ago, a situation like that would have cut a swath through the home. As it is, just one vaccinated resident died in this case. Most didn’t even have symptoms.

The outbreak involved a variant of the virus that has multiple mutations in the spike protein, of the kind that make the vaccines less effective. Vaccinated residents and health care workers at the Kentucky facility were less likely to be infected than those who had not been vaccinated, and they were far less likely to develop symptoms. The study estimated that the vaccine, identified as Pfizer-BioNTech, showed effectiveness of 66 percent for residents and 75.9 percent for employees, and were 86 percent to 87 percent effective at protecting against symptomatic disease.

In the Kentucky outbreak, the virus variant is not on the C.D.C.’s list of those considered variants of concern or interest. But, the study authors note, the variant does have several mutations of importance: D614G, which demonstrates evidence of increased transmissibility; E484K in the receptor-binding domain of the spike protein, which is also seen in B.1.351, the variant first recognized in South Africa, and P.1. of Brazil; and W152L, which might reduce effectiveness of neutralizing antibodies.

Raw numbers: 75 out of 83 total residents had been vaccinated. All eight of the unvaccinated were infected during the outbreak versus just 18 of the vaccinated. So, with a nasty variant at work on a group that included older and infirm people, the vaccines still prevented infection in more than two-thirds of the elderly and prevented symptoms in more than 85 percent of the whole group. Of the eight unvaccinated residents who were infected, two died, but among the 18 who got infected despite being vaccinated, just one did. One public health professor marveled that the vaccines stood up as well as they did among a group with weaker immune systems battling a variant with a threatening mutation:

An outbreak involving a variant with 484K in a vulnerable vaccinated population still showed *very* strong (86.5%) protection against symptomatic illness. This is *very* good news, and should build further confidence in vaccination https://t.co/LpToXsBrXU

— Bill Hanage (@BillHanage) April 21, 2021

More data from nursing homes came out of Chicago today, per the Times. Among 78 facilities, there were 627 infections among residents and staff — but only 22 of them were among vaccinated people and two-thirds of those were asymptomatic. Yet another study of “breakthrough infections” at Rockefeller University looked at 417 employees of the school who had been fully vaccinated to see how many had been infected. Answer: Two, for a rate of 0.5 percent. That’s 60 times higher than the .008 percent rate that the CDC revealed last week but still very small. And it should go without saying that the rate of “breakthrough infections” may be higher or lower within different age groups. Among a group of vaccinated elderly people whose immune systems ain’t what they used to be even after being primed by Pfizer and Moderna, maybe the rate is one percent or higher. Among a group of healthy young adults who’ve been vaxxed, maybe it’s even lower than .008 percent.

Either way, the rate within both groups that experiences symptoms will be much smaller than the rate of infection, and the rate that needs hospital care will be smaller still.

On the other hand, Patterico has a point here. If more vaccinated people are getting infected than we know of, more vaccinated people may be transmitting the virus than we suspect:

I don't have time to read this, I'm too busy bitching at Fauci for saying immunized people should still be cautious https://t.co/czJpGwn8LR

— Patterico (@Patterico) April 21, 2021

I’ve spent the last week throwing the .008 percent figure at experts like Fauci when they recommend that vaccinated people stay out of restaurants until community spread is lower. I still think that advice is too cautious since the rate of “breakthrough infections” remains exceedingly low by any standard (as Fauci himself acknowledges). But those infections do seem to be happening more frequently than the one-in-11,000 ratio implied by the CDC’s data given the results from Rockefeller U and the Kentucky nursing home. How much more frequently? *Shrug* But you can understand why vaccinated older people especially might continue to take basic precautions when they’re indoors and around large groups of strangers, at least until we get a little closer to herd immunity. A one-in-100 chance of infection, say, is meaningfully different from one-in-11,000.

I’ll leave you with this, a reminder that herd immunity is not just real, it’s spectacular.

You know vaccines are working when your country is opening, no lockdown, mobility is increasing and (today):
—9.3 million people, 1 death, 135 new cases (Israel)
—67.9 million people, 22 deaths, lowest number of hospitalized patients in 8 months, and falling (UK) pic.twitter.com/oM5c6yXlZc

— Eric Topol (@EricTopol) April 21, 2021