What to Know About COVID Breakthrough Infections

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Reports of vaccinated people being infected and sickened with the Delta variant of the coronavirus have multiplied in recent weeks, prompting anxiety and confusion over whether vaccines are as effective as they were advertised as well as whether fully vaccinated people need to take measures — like masking up again — to protect themselves. But while it’s unclear exactly how many vaccinated people are now catching COVID, available data indicate that these infections remain very uncommon and, even more important, that the coronavirus vaccines in use in the U.S. continue to provide extremely effective protection against serious illness and death from COVID-19. Below is everything we do and don’t know about breakthrough infections and how worried vaccinated people should be.

What is a breakthrough infection?

A breakthrough infection occurs when someone who is fully vaccinated contracts the virus. Such infections are to be expected with any vaccines since they are never 100 percent effective at blocking infection. Thus far, the vast majority of COVID breakthrough infections have not led to breakthrough disease, however. That’s one of the main reasons many coronavirus and public-health experts have highlighted how breakthrough infection can be a misleading term.

Breakthrough disease — fully vaccinated people who develop significant symptoms from a breakthrough infection — is far more worrisome to scientists but is also far more rare in COVID cases, at least so far.

But for the most part, as the Atlantic’s Kathryn J. Wu explains, a vaccinated immune system is up to the task:

Post-vaccination infections, or breakthroughs, might occasionally turn symptomatic, but they aren’t shameful or aberrant. They also aren’t proof that the shots are failing. These cases are, on average, gentler and less symptomatic; faster-resolving, with less virus lingering—and, it appears, less likely to pass the pathogen on. The immunity offered by vaccines works in iterations and gradations, not absolutes. It does not make a person completely impervious to infection. It also does not evaporate when a few microbes breach a body’s barriers. A breakthrough, despite what it might seem, does not cause our defenses to crumble or even break; it does not erase the protection that’s already been built. Rather than setting up fragile and penetrable shields, vaccines reinforce the defenses we already have, so that we can encounter the virus safely and potentially build further upon that protection.

How many breakthrough infections are happening?

First off, from the available data, the vast majority of new COVID cases currently being detected nationwide — and nearly all cases of serious illness and death due to COVID — are happening among unvaccinated people.

That being said, there is no official national count of breakthrough infections and no complete picture of just how often they are occurring. On the other hand, because breakthrough infections have so often made the news due to their novelty, that can create a perception of more cases than are actually happening — particularly without more robust tracking of the actual cases to provide context.

The CDC tracks breakthrough infections only among people in the U.S. who have tested positive for COVID and been hospitalized or died — i.e., fully vaccinated people who have developed severe illness from COVID. According to the CDC, as of July 19, fewer than just over 5,900 of the country’s more than 195 million people who have been fully vaccinated against the coronavirus have been hospitalized or died in addition to testing positive for COVID-19.

It’s also important to remember that as more and more people get vaccinated, the number of breakthrough infections will also rise, since they will naturally make up a larger share of the overall cases.

Why is it important to track breakthrough infections?

Some scientists have criticized the CDC for not doing more to track breakthrough cases. In a recent interview with STAT News, Dr. Céline Gounder, an infectious disease expert at NYU’s Grossman School of Medicine, offered a succinct explanation of why having more data about these cases would be useful:

Those people who are still getting infected despite being vaccinated, they may not get sick, but it is possible that they could transmit the infection on to others. And so that’s something we still don’t really have a handle on. There is some evidence from the sports leagues, where they do a lot of testing, that some of these people may, in fact, be contagious. And so that is concerning.

The second reason that we really want to be tracking breakthrough infections is for what we call genomic surveillance, which is where we look at new variants that are starting to emerge and what do those look like? You’re more likely to find new emerging variants among people who have breakthrough infections. We’re sort of flying blind with respect to that, because we’re not assessing those breakthrough infections.

What effect is the Delta variant having on breakthrough infections?

Although there is some preliminary data indicating that the vaccines are slightly less effective against the Delta variant, the overwhelming evidence thus far indicates that the COVID vaccines administered in the U.S. continue to provide strong protection against illness and death from Delta and every other variant of concern.

As to how much Delta is driving breakthrough infections, more data is needed, but the variant’s significantly higher transmissibility is undoubtedly having an impact. A variant that leads to more cases is inevitably going to mean more cases for the unvaccinated and vaccinated alike — though far more for the former.

Can people with breakthrough infections spread COVID?

Likely yes, but to what extent that is actually happening remains unclear at this point; there just isn’t enough conclusive data to determine that yet. Regardless, it is also more likely that fully vaccinated people with symptomatic cases are more contagious than people with asymptomatic cases.

The bottom line: If anyone develops COVID symptoms — whether they are fully vaccinated or not — they should take precautions to make sure they don’t spread COVID to others, like isolating themselves until they feel better.

If you’re vaccinated, how can you protect yourself from getting a breakthrough infection?

As always, wearing a high-quality, well-fitting face mask in situations of higher risk of COVID exposure — as in crowded indoor public spaces, particularly in parts of the country where case numbers are high or rising — still offers meaningful protection against transmission. Good ventilation is also important, as when opting to socialize outdoors instead of indoors, or opening windows and/or having some kind of air-filtration system in indoor spaces.

Do breakthrough infections mean the vaccines don’t work?

No. Vaccines are never 100 percent effective at blocking every infection or illness. The COVID vaccines work just as they are supposed to.

This post has been updated to include more information.

What to Know About COVID Breakthrough Infections