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‘Breakthough infections’ occur when someone gets sick even though they have been vaccinated

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An Israeli health worker prepares to administer a dose of the Pfizer-BioNTech vaccine. AFP VIA GETTY IMAGES

A small Israeli study indicates that some of the new coronavirus variants may put people who have been vaccinated at higher risk of breakthrough infections, though U.S. health officials questioned some of the wording used in the preliminary research.

These types of cases are called “breakthrough infections,” which occur when someone who has completed their COVID-19 vaccination later gets sick from the virus.

The preprint, which published Friday and has not been peer reviewed, gained attention over the weekend after it said that the B.1.351 variant was more likely to infect people in Israel who had been vaccinated with Pfizer Inc.’s PFE, +1.01% COVID-19 vaccine, compared with other strains of the virus.Advertisement

There are several documented variants of concern, including the B.1.351, first identified in South Africa; the B.1.1.7, first detected in the U.K.; and the P.1 out of Brazil, that are thought to be easily transmissible.

The researchers identified eight cases of breakthrough infections caused by the B.1.351 variant and 134 cases of the B.1.1.7 variant out of the roughly 400 people included in the study who had tested positive for the virus during or after the vaccination period. The control group of unvaccinated people oddly had only one case caused by B.1.351.

“Our results show reduced effectiveness against the [South African] variant only in a short window of time (7-13 days post-second dose) since all the breakthrough cases we saw were in this time frame,” Adi Stern, a professor at Tel Aviv University and one of the co-authors, said in an email. “However, once again the caveat is that our sample size is small and this requires further research.”


When asked about the study, federal health officials on Monday downplayed the accuracy of the findings.

“That preprint, as it were, was about as confusing as you possibly could be,” Dr. Anthony Fauci, President Joe Biden’s chief medical advisor, told reporters during a White House briefing. “It made it seem like you were more likely to get the [B.1.]351 if, in fact, you were vaccinated against the mRNA. That wasn’t the case. If you were going to get infected with anything, you would get infected with the more difficult variant, which was 351. That doesn’t mean you have a greater chance of getting it.”


Pfizer directed questions about the study to the authors.

What we know so far about vaccines and variants

Fauci said that clinical data so far indicates the mRNA vaccines developed by Pfizer and BioNTech SE BNTX, -0.51%, as well as Moderna Inc. MRNA, -1.08%, still provide protection against B.1.1.7, but vaccine efficacy is thought to drop against the B.1.351 variant, which is a less common strain of the virus at this time, at least in the U.S.


Both mRNA vaccines had very high efficacy rates in Phase 3 clinical trials, with each reporting rates around 95%, though the trials were conducted last fall, before two of these new variants had been detected. The real-world effectiveness rate for both of these vaccines is thought to be closer to 89%, according to at least one preprint study published by the Mayo Clinic in February, but that is still considered a very strong rate of protection against the virus.

Here’s what we know — and don’t know — about ‘breakthrough infections’

While getting COVID-19 shots may dramatically lessen the possibility that someone will become infected with the virus, breakthrough infections for the vaccinated are still possible, especially with these new, more infectious variants of the coronavirus circulating throughout the world.Advertisement

Moderna last month said it will begin clinical trials, in partnership with the National Institutes of Health, on a vaccine that specifically aims to protect against the B.1.351 variant. Pfizer is likewise running trials for a booster shot and a new vaccine specifically targeting the South African variant.

“What we do know when these breakthrough infections do occur is they tend to occur with fewer symptoms, less virus, [and] less transmissible virus,” Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said during the same briefing on Monday. “We’re still learning about the transmissibility of this virus in the context of these breakthrough infections.”


Breakthrough infections can also occur if immunity wanes, the shot is damaged because of something like a storage issue, or someone’s age or medications limit their immune response, Fauci added.

Here’s why Americans are paying close attention to Israel’s vaccination campaign

Of all the countries in the world with vaccination campaigns, Israel has vaccinated the largest percentage of its residents. About 53% of people in Israel have been fully vaccinated as of Sunday, according to a government dashboard. In comparison, the U.S. has vaccinated about 22% of its albeit much larger population as of Monday, according to the CDC.


Pfizer’s vaccine is the predominant shot used in Israel’s campaign, and so countries that are also relying on that drug maker’s vaccine are curious to see what real-world data is coming out of Israel right now, even though there are significant differences between immunization protocols across nations.

The Israeli study also found that the B.1.1.7 (U.K.) variant is the most dominant form of the virus in Israel (as it is in the U.S.) though the B.1.351 (South Africa) strain made up less than 1% of the cases included in the research.

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“There may be higher rates of vaccine breakthrough with B.1.351, but it is possible that (a) vaccine effectiveness coupled with enacted non-pharmaceutical interventions remain sufficient to prevent its spread, and/or (b) B.1.1.7
outcompetes B.1.351, possibly due to its high transmission rate,” the researchers concluded.

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