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Study finds low rate of COVID-19 "breakthrough" infections, fewer symptoms in vaccinated people

CDC lacks data on breakthrough COVID infections
CDC lacking data on breakthrough COVID-19 infections 10:31

London — A study conducted in the U.K. offers some of the first large-scale, real-world data on how well vaccination protects people against catching a "breakthrough" COVID-19 infection, and how well it protects breakthrough patients from becoming seriously ill. The results are encouraging.

The peer-reviewed study published Wednesday in The Lancet medical journal will help policy makers and epidemiologists fill in a significant gap in the understanding of the true efficacy of three of the major vaccines being used worldwide.

The U.S. Centers for Disease Control and Prevention, for instance, doesn't have good data on how many people catch COVID-19 after being vaccinated, as it decided in the spring to track only serious, symptomatic breakthrough cases. The British study, on the other hand, used mass-testing data to determine how many breakthrough cases there actually are and how sick those people get.

The vaccines were never intended to prevent infections completely, but to reduce the rates of infection within a population and, most importantly, to reduce the severity of illness in people who do catch it. The study found that people who contracted the coronavirus despite being fully vaccinated were almost twice as likely to have no symptoms at all, compared to the wider population.

Crucially, the odds of a fully-vaccinated person who does catch COIVD-19 ending up hospitalized with severe symptoms were reduced by more than two-thirds compared to an unvaccinated coronavirus patient. The survey also found that the risk of breakthrough patients suffering from long-COVID, with symptoms lasting more than a month, were cut in half by full vaccination.

It's the latest dataset to offer convincing evidence that the vaccines work as intended.

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Researchers from King's College, London, and Harvard in the U.S. carried out the study using self-reported data from more than a million people in the U.K. who had received either the Moderna, Pfizer or AstraZeneca COVID-19 vaccines. The research showed that the risk of severe disease or hospitalization was significantly reduced after a first vaccine dose, but that protection against serious illness — and against catching a breakthrough infection to begin with — increased with the full dosage.

The data, gathered from December 8, 2020, through July 4, 2021, show that of more than 1.2 million adults who received a first dose, fewer than 0.5% reported contracting breakthrough infections two weeks or more after getting the jab. Among those who got both shots, fewer than 0.2% experienced a breakthrough infection a week or more after getting their second shot.

"Among those who did experience a breakthrough infection, the odds of that infection being asymptomatic increased by 63% after one vaccine dose and by 94% after the second dose," the study's authors wrote in The Lancet.

"We are at a critical point in the pandemic as we see cases rising worldwide due to the delta variant. Breakthrough infections are expected and don't diminish the fact that these vaccines are doing exactly what they were designed to do — save lives and prevent serious illness," said study co-lead author Dr. Claire Steves of King's College. "Other research has shown a mortality rate as high as 27% for hospitalized COVID-19 patients. We can greatly reduce that number by keeping people out of the hospital in the first place through vaccination. Our findings highlight the crucial role vaccines play in larger efforts to prevent COVID-19 infections, which should still include other personal protective measures such as mask-wearing, frequent testing, and social distancing."

The data also show that the risk of breakthrough infection is higher for people living in lower-income areas, likely due, the authors said, to closer living quarters and lower overall vaccination rates in these communities. These risks were "most significantly associated with a post-vaccination infection after receiving the first vaccine dose and before receiving a second dose," according to The Lancet.

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As has been found consistently since the coronavirus first emerged, age and underlying conditions, including heart, lung and kidney disease, all seriously increase the risk of severe COVID-19 infection, including in those who have been vaccinated.

 "The increased risks of breakthrough infections for frail, older adults — especially those living in care homes or who require frequent visits to health care facilities — and for other people living in deprived conditions reflect what we've seen throughout the pandemic. These groups are at a greater risk of exposure and are therefore more vulnerable to infection," study co-author Dr. Rose Penfold, also of King's College, said in The Lancet. "Health policies designed to prevent infections, including policies around timing between the first and second dose and potential booster shots, should prioritize these groups."

While the data used for the study did not discern between infections with the Delta variant and other strains of the coronavirus, the vast majority of U.K. cases have been Delta infections since at least early June, and it started spreading rapidly in March. In the U.S., Delta became the dominant strain in July and it now makes up nearly all new cases.

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