A large new study provides some of the strongest evidence yet that vaccines reduce the risk of developing long Covid.

Scientists looked at people in the United States infected during the first two years of the pandemic and found that the percentage of vaccinated people who developed long Covid was much lower than the percentage of unvaccinated people who did.

Medical experts have previously said that vaccines can lower the risk of long Covid, largely because they help prevent severe illness during the infection period and people with severe infections are more likely to have long-term symptoms.

But many individuals with mild infections also develop long Covid, and the study, published Wednesday in The New England Journal of Medicine, found that vaccination did not eliminate all risk of developing the condition, which continues to affect millions in the United States.

“There was a residual risk of long Covid among vaccinated persons,” Dr. Clifford Rosen, a senior scientist at the MaineHealth Institute for Research, who was not involved in the study, wrote in an accompanying editorial. Because of that, Dr. Rosen added, new cases of long Covid “may continue unabated.”

The study evaluated medical records of millions of patients in the Department of Veterans Affairs health system. It involved nearly 450,000 people who had Covid between March 1, 2020, and Jan. 31, 2022, and about 4.7 million people who were not infected during that time.