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Latest data show increase in breakthrough COVID-19 cases


Preliminary data from seven states suggests that breakthrough COVID-19 infections among vaccinated people may be on the rise because of the more contagious Delta variant.

Breakthrough cases accounted for about one in five newly diagnosed cases in six of the states, according to the New York Times. Hospitalizations and deaths among vaccinated people may be higher than previously thought as well.

“Remember when the early vaccine studies came out, it was like nobody gets hospitalized, nobody dies,” Robert Wachter, MD, chairman of the department of medicine at the University of California, San Francisco, said in an interview. “That clearly is not true.”

The New York Times analyzed data in seven states – California, Colorado, Massachusetts, Oregon, Utah, Vermont, and Virginia – that are tracking the most detailed information. The trends in these states may not reflect the numbers throughout the country, the newspaper reported.

Even still, the numbers back up the idea that vaccinated people may need booster shots this fall to support their earlier vaccine doses. Federal health officials are scheduled to approve the extra shots in coming weeks, potentially in September. The first people to receive booster shots will likely be health care workers and nursing home residents who took the first vaccines in December and January.

“If the chances of a breakthrough infection have gone up considerably, and I think the evidence is clear that they have, and the level of protection against severe illness is no longer as robust as it was, I think the case for boosters goes up pretty quickly,” Dr. Wachter said.

Previous analyses of breakthrough cases included data from June and earlier, the newspaper reported. But since July, COVID-19 cases have soared again because of the Delta variant, and the most recent numbers show an uptick among vaccinated people. In Los Angeles County, for instance, fully vaccinated people account for 20% of new COVID-19 cases, which is up from 11% in May, 5% in April, and 2% in March, according to a late July report from the Los Angeles County Department of Public Health.

What’s more, breakthrough infections in the seven states accounted for 12%-24% of COVID-19 hospitalizations in those states. About 8,000 breakthrough hospitalizations have been reported to the CDC. Still, the overall numbers remain low – in California, for instance, about 1,615 people have been hospitalized with breakthrough infections, which accounts for 0.007% of the state’s 22 million vaccinated people, the Times reported.

The breakthrough infections appear to be more severe among vaccinated people who are older or have weakened immune systems. About 74% of breakthrough cases are among adults 65 or older, the CDC reported.

The increase may shift how vaccinated people see their risks for infection and interact with loved ones. Public health officials have suggested that people follow some COVID-19 safety protocols again, such as wearing masks in public indoor spaces regardless of vaccination status.

As the Delta variant continues to circulate this fall, public health researchers will be researching more about breakthrough cases among vaccinated people, including whether they have prolonged symptoms and how easily they may pass the virus to others.

“I think some of us have been challenged by the numbers of clusters that we’ve seen,” Michael Osterholm, PhD, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, told this news organization.

“I think that really needs to be examined more,” he said.

A version of this article first appeared on

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