Vaccination reduces risk of getting Delta symptoms by 60%, study finds

Vaccines in England were 60 percent effective at preventing symptomatic cases of COVID-19 caused by the Delta variant, researchers reported Wednesday, a figure lower than most previous estimates that scientists nevertheless said ought to be interpreted cautiously.

The results, drawn from testing a random sample of nearly 100,000 volunteers, offered some of the most extensive evidence to date of vaccines’ performance against the Delta variant, which has driven surges of cases in Britain, the United States and elsewhere. They indicated that vaccines were 50 percent effective at preventing people from becoming infected, with or without symptoms.

But scientists cautioned that the results had an exceedingly high degree of statistical uncertainty, and relied largely on volunteers self-reporting their vaccination status.

The study sent tests to a random sample of volunteers, rather than relying on people to seek coronavirus tests themselves, so its figures include people who may not otherwise have thought much of their symptoms or known they had the virus at all. Estimates of vaccine effectiveness tend to be lower in studies that include mild or asymptomatic cases.

The results also did not distinguish between the different vaccines being used in Britain. By mid-July, roughly twice as many people had been fully vaccinated with the AstraZeneca vaccine as with Pfizer’s, the main two shots in use in Britain. In previous studies, the Pfizer vaccine has appeared more effective against the delta variant than AstraZeneca’s.

The researchers, led by a team from Imperial College London, said that their estimates of vaccine effectiveness were lower than those reported previously in England but consistent with data from Israel.

Other studies in England have suggested that vaccines are more than 90 percent effective in preventing people from being hospitalized with a COVID-19 case caused by the Delta variant.

“Participants who reported being vaccinated were at substantially reduced risk of testing positive compared with those who reported not being vaccinated,” the latest study said.

It has not been published in a peer-reviewed journal.

The study also compared the viral loads in vaccinated and unvaccinated people who tested positive for the virus, finding that vaccinated people carried less virus. That suggested, the researchers wrote, that vaccinated people who tested positive were less infectious. Other studies, including from the United States, have found similar viral loads in infected people, whether or not they were vaccinated.

Scientists said that it was difficult to tell from the study precisely how much the vaccines reduced the risk of infection or transmission. The findings were drawn from a random sampling of volunteers over late June and early July, part of a routine infection survey known as React-1.

“The React-1 findings, when coupled with other studies demonstrating the impact of coronavirus vaccines on reducing hospitalisation and death from COVID-19, are encouraging,” said Dr Tom Wingfield, a senior clinical lecturer at the Liverpool School of Tropical Medicine.

But, he said, the results were also “a reminder that, even with extremely high vaccine coverage, we are highly likely to have a further wave of SARS-CoV-infections in the autumn.”

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