The Washington PostDemocracy Dies in Darkness

How close are states to herd immunity?

A coronavirus vaccination is administered at San Diego's Viejas Arena on Thursday. (Bing Guan/Bloomberg News)

The United States will at some point achieve herd immunity, the point at which enough people are immune to the coronavirus that it can’t easily spread from person to person.

That eventuality will happen in one of two ways: Enough people will be vaccinated against the virus that it won’t be able to find a new host when traveling around with an infected person, or enough people will be immune to that particular iteration of the virus after having already been infected with it that the virus is similarly stymied.

The “that particular iteration” qualifier is important, of course: The more the virus spreads, the more it might mutate into a form against which previously infected individuals don’t have any protection. Allowing the virus to spread without containment increases the likelihood of such a mutation, which is itself a reason to push for herd immunity sooner rather than later.

But we’re not yet terribly close. On Thursday, the government announced that 100 million Americans had received a dose of one of the available vaccines. It’s a remarkable achievement — but one that still reflects less than a third of the country’s population. It’s not clear what percentage of the population needs to be immune to achieve herd immunity, but experts tend to agree it’s north of 70 percent.

Of course there are also tens of millions of people who have some form of immunity from having contracted the virus. Again, that’s the race: As cases surge in the Northeast, can enough people be vaccinated to slow the virus’s spread and achieve herd immunity without the health repercussions of rampant infections? It’s like a pincer, with health experts trying to immunize everyone with vaccines and the virus trying to immunize them through infections.

Data from states and the Centers for Disease Control and Prevention collected by The Washington Post shows how each state is doing in that race. Since December (at the left of each state graph below), the number of people receiving full or partial vaccination against the virus has increased rapidly in every place — more rapidly than the number of new infections, happily. But there’s still a lot of ground to cover.

As of Wednesday, the most recent day for which we have full data, nearly every state had more fully vaccinated residents than residents who had contracted the virus at some point. (The exception is Utah.) In states where the virus never had a significant foothold, such as Vermont or Hawaii, the number of people who have been vaccinated is several times larger than the number who have contracted the virus.

On average, 2.2 people have been fully vaccinated in a state for every person confirmed to have been infected with the virus. (It’s important to note that this data may include vaccinations of people who had previously contracted the virus.) That word “confirmed” is important, of course. It’s likely that hundreds of thousands of people have had the virus without it being confirmed through a test, many of whom contracted it at the outset of the pandemic a year ago. Our numbers here, then, are necessarily low. It’s just not clear how low.

In most states, the density of the population (including kids, who aren’t yet cleared for vaccinations) that we know has contracted the virus or been fully vaccinated is around 26 percent. In other words, at least a quarter of the population should have immunity, unless existing natural immunity (from an infection) doesn’t protect against the new variants that are helping power the surge in the Northeast. A lot of people in each state have been partially vaccinated, awaiting the second of two shots.

In other words, we’re a long way from herd immunity, though the gap is closing rapidly. The question, as ever, is how many more people will gain immunity through illness — risking long-term health effects or death — before that goal is achieved.

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