The real chances of a breakthrough infection.


By David Leonhardt

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.)

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