One in 5,000: The real chances of a breakthrough infection

By David Leonhardt of The NY Times. Excerpts:

"The C.D.C. reported a terrifying fact in July: Vaccinated people with the Delta variant of the Covid virus carried roughly the same viral load in their noses and throats as unvaccinated people.

The news seemed to suggest that even the vaccinated were highly vulnerable to getting infected and passing the virus to others. Sure enough, stories about vaccinated people getting Covid — so-called breakthrough infections — were all around this summer: at a party in Provincetown, Mass.; among the Chicago Cubs; on Capitol Hill. Delta seemed as if it might be changing everything.

In recent weeks, however, more data has become available, and it suggests that the true picture is less alarming. Yes, Delta has increased the chances of getting Covid for almost everyone. But if you’re vaccinated, a Covid infection is still uncommon, and those high viral loads are not as worrisome as they initially sounded.

How small are the chances of the average vaccinated American contracting Covid? Probably about one in 5,000 per day, and even lower for people who take precautions or live in a highly vaccinated community.

The estimates here are based on statistics from three places that have reported detailed data on Covid infections by vaccination status: Utah; Virginia; and King County, which includes Seattle, in Washington state. All three are consistent with the idea that about one 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. And in places with many fewer cases — like the Northeast, as well as the Chicago, Los Angeles and San Francisco areas — the chances are 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 often so mild that people do not notice them and do not pass the virus to anyone else.)

Here’s one way to think about a one-in-10,000 daily chance: It would take more than three months for the combined risk to reach just 1 percent.

“There’s been a lot of miscommunication about what the risks really are to vaccinated people, and how vaccinated people should be thinking about their lives,” as Dr. Ashish Jha of Brown University told my colleague Tara Parker-Pope. (I recommend Tara’s recent Q. and A. on breakthrough infections.)"

"The infection rates in the least vaccinated states are about four times as high as in the most vaccinated states"

"These numbers help show why the talking point about viral loads was problematic. It was one of those statements that managed to be both true and misleading. Even when the size of the viral loads are similar, the virus behaves differently in the noses and throats of the vaccinated and the unvaccinated.

In an unvaccinated person, a viral load is akin to an enemy army facing little resistance. In a vaccinated person, the human immune system launches a powerful response and tends to prevail quickly — often before the host body gets sick or infects others. That the viral loads were initially similar in size can end up being irrelevant."

"For the vaccinated, Covid resembles the flu and usually a mild one."

"the risks to young children are so low that Britain may never recommend that most receive the vaccine."

"the risks of getting any version of the virus remain small for the vaccinated, and the risks of getting badly sick remain minuscule.In Seattle on an average recent day, about one out of every one million vaccinated residents have been admitted to a hospital with Covid symptoms."

"the Covid risks for most vaccinated people are of the same order of magnitude as risks that people unthinkingly accept every day, like riding in a vehicle."