A new study has added to a growing body of evidence that COVID-19 vaccines and boosters protect against severe illness and death.

Published in the Journal of the American Medical Association, the followed more than 1.6 million U.S. patients over 11 months, and found that among those who were vaccinated and boosted, the incidence of death or hospitalization for COVID-19 pneumonia was only 8.9 per 10,000 persons. According to the study, severe outcomes like these occurred almost exclusively among high-risk patients like older adults, the immunocompromised and those with conditions like diabetes and high blood pressure.

"This is remarkable, good news about the power and effectiveness of receiving COVID-19 boosting for all groups," co-author , a research scientist at the Regenstrief Institute and Roudebush Veterans' Administration Medical Center in Indiana, said in a . "These results, from a period of Delta and Omicron predominance, should encourage people to get vaccinated and boosted."

The patients were tracked between July 1, 2021 and May 30. All received care at facilities belonging to the Veterans Health Administration, which services military veterans and is the largest integrated health care system in the U.S.

The study also found that vaccinated and boosted older adults with immunocompromising conditions or additional diseases, which are known as comorbidities, were ten times more likely to experience severe outcomes like hospitalization or death than those in the study who were considered to be at average risk of severe COVID-19 outcomes.

"With the power of (Veterans Affairs) data, we had such complete information on a large number of patients including many who are older and those who have comorbidities or are immunocompromised, that we could examine this issue thoroughly," Bravata, who is also a professor of medicine and neurology at Indiana University.

The study, however, did not look at unboosted or unvaccinated patients. Nearly 70 per cent of the study's participants, moreover, were 65 or older, while only just over eight per cent were female.