The CDC Says This One Thing Is Most Likely to Cause COVID After Vaccination

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Since the first two COVID vaccines were first approved six months ago, we've been hearing that they're about 95 percent effective. But what you may not realize is that the mRNA vaccines from Pfizer and Moderna are 95 percent effective at preventing symptomatic COVID cases, which still leaves a chance of asymptomatic infection and a small margin of vaccine recipients who could come down with a symptomatic case. The point of the vaccine, however, is to stop severe cases and based on the latest data from the Centers for Disease Control and Prevention (CDC), it's doing its job. According to their findings as of April 30, only 1,155 of the 101 million fully vaccinated people in the U.S. have come down with a severe or fatal case of COVID, meaning 0.001 percent. However, there is one clear commonality among those who develop COVID after vaccination.

Based on the CDC's research, which was released on May 25, 64 percent of breakthrough infections—the medical term for vaccinated people who get infected—are caused by variants of concern. But really, it's one variant in particular: B.1.1.7. which originated in the U.K. and caused a disastrous wave of illness there. According to the CDC, 64 percent of breakthrough cases were caused by variants of concern, but more than half of those were due to B.1.1.7—56 percent, to be exact. Behind that was the California variant B.1.429 (25 percent); the other California variant of concern, B.1.427 (8 percent); the Brazil variant, P.1 (8 percent); and the South Africa variant, B.1.351 (4 percent).

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Another new study out of the University of Washington (UW) Medicine, also released on May 25, similarly found that the B.1.1.7. variant is behind the majority of breakthrough cases. The research—which has not yet been peer-reviewed but was posted on medRxiv—involved 20 healthcare workers within the UW Medicine hospital system who ended up getting infected with COVID after vaccination between February and April 2021.

All 20 of the breakthrough infections were also caused by variants of concern. Of these cases, 40 percent were caused by the U.K. variant, B.1.1.7, but another variant was right up there with it—another 40 percent of cases were caused by the California variant B.1.429. The higher ratio in this study versus the CDC's data could be because of the spread of this particular variant on the West Coast, where it originated and where UW Medicine is based. The rest of the cases were caused by the other California variant B.1.427 (10 percent); the South Africa variant (5 percent); and the Brazil variant (5 percent).

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"Variants of concern (VOCs) are those strains that show evidence of increased transmissibility, more severe disease, reduced neutralization by antibodies elicited by past infection or vaccination, reduced efficacy of treatments, or failures in diagnostic detection," the UW Medicine researchers wrote in their study. "Overall, variants of concern were proportionally over-represented in breakthrough cases."

Study co-author Pavitra Roychoudhury, PhD, an acting instructor at UW Medicine, told Reuters: "These infections could lead to the continued spread of variants of concern, particularly in areas with low vaccination rates."

At the start of April, the B.1.1.7 variant became the dominant coronavirus strain in the U.S. "These trends are pointing to two clear truths," CDC Director Rochelle Walensky, MD, said at a White House press briefing at the time. "One, the virus still has hold on us—infecting people and putting them in harm's way—and we need to remain vigilant. And, two, we need to continue to accelerate our vaccination efforts and to take the individual responsibility to get vaccinated when we can."

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