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Who had COVID-19 vaccine breakthrough cases?


After a summer of reports of breakthrough coronavirus infections, when it seemed that everyone knew someone who tested positive after vaccination, recently released federal data sheds light on how common these cases really were, how severe they became and who was most at risk.

By Aliza Aufrichtig and Amy Schoenfeld Walker


After a summer of reports of breakthrough coronavirus infections, when it seemed that everyone knew someone who tested positive after vaccination, recently released federal data sheds light on how common these cases really were, how severe they became and who was most at risk.


Compared with the unvaccinated, fully vaccinated people overall had a much lower chance of testing positive for the virus or dying from it, even through the summer’s delta surge and the relaxation of pandemic restrictions in many parts of the country. But the data indicates that immunity against infection may be slowly waning for vaccinated people, even as the vaccines continue to be strongly protective against severe illness and death.


“The No. 1 take-home message is that these vaccines are still working,” said Dr. David Dowdy, a public health researcher at the Johns Hopkins Bloomberg School of Public Health. “If you saw these data for any disease other than COVID, what everyone’s eyes would be drawn to is the difference between the unvaccinated and fully vaccinated lines.”


The data shows notable differences in breakthrough death rates by age and slight differences in both case and death rates by vaccine brand, trends that experts say are important to consider as tens of millions of Americans weigh whether to get a booster shot.


The data, from the Centers for Disease Control and Prevention, is based on health department records from 14 states and two cities. A second dashboard reveals similar trends for hospitalized patients with and without vaccination.


All vaccinated age groups saw similar rates of breakthrough infection, and they all had much lower rates of infection and death compared with their unvaccinated peers.


While every age group had similar rates of breakthrough cases, death rates varied more drastically by age. Unvaccinated seniors were the most likely to die from COVID of any group. Still, vaccinated people 80 and older had higher death rates than unvaccinated people under 50.


“Age is our top risk factor for vaccine breakthrough deaths,” said Theresa Sokol, the state epidemiologist in Louisiana, one of the jurisdictions that contributed to the CDC data.


Breakthrough deaths among seniors may be because of immunosenescence, or the weakening of the immune system in older people, said Heather Scobie, a public health researcher at the CDC who helps lead the team that produced the new data.


“They don’t usually form as robust a response to vaccination,” added Scobie. “Hopefully the booster dose for ages 65 years and older will address that issue.”


The federal data also makes it clear that all three brands of vaccine administered in the United States substantially reduced rates of cases and deaths. But among those vaccinated, Johnson & Johnson recipients had slightly higher rates of breakthrough cases and related deaths. And Pfizer-BioNTech recipients had slightly higher rates than those who got Moderna.


Similar data from scientific studies helped shape the new federal recommendation that all Johnson & Johnson recipients, 18 and older, receive a booster dose at least two months after getting the first shot, Scobie said.


In contrast, booster shot recommendations for those who got the Pfizer or Moderna vaccine are focused on high-risk groups, including those over 65, and younger adults at greater risk of severe COVID-19 because of medical conditions or where they work.


The CDC data, which will be updated monthly, is the closest yet to a detailed, nationally representative view of breakthrough cases and deaths. States are not required to report this information — though many do in myriad formats — and the CDC had previously only provided estimates of total breakthrough hospitalizations and deaths.


The data can also give scientists a crude understanding of the effectiveness of the vaccine over time. If the ratio of cases or deaths among the unvaccinated to those among the vaccinated holds steady, the vaccines are thought to be maintaining their protection.


For example, the ratio of case rates declined somewhat in the summer, to six times as high for the unvaccinated in August from about 11 times as high in mid-June, giving scientists reason to believe that the vaccine’s protection against infection might be waning slightly. The ratio for deaths has been flatter over time for all but the oldest age groups, an indication that vaccine protection against death is holding strong.


The CDC data so far runs through early September and captures only the crest of the delta wave. But data from states like New York and California shows similar patterns through September and October. That suggests that the vaccines, despite some slight differences among the brands, are still working to protect against the most severe outcomes.


Note: The data excludes partially vaccinated people. Overall rates are age-adjusted to make the fully vaccinated and unvaccinated populations more comparable. Data includes reports from Alabama, Arizona, Arkansas, Colorado, Florida, Georgia, Idaho, Louisiana, Massachusetts, Michigan, Nebraska, New Mexico, New York City, Seattle, Utah and Wisconsin.


Sources: Centers for Disease Control and Prevention; Outbreak.info (date when delta variant became dominant)

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