The study, published in the Journal of the American Medical Association (JAMA), showed the effectiveness of the primary series of Covid-19 vaccines a year and a half after administration.
"Our study provides the most comprehensive characterisations of the effectiveness of primary and booster vaccination and prior infection in the US," said researcher Danyu Lin from the University of North Carolina.
According to the researchers, omicron infection can protect against re-infection with omicron, although the level of protection wanes over time.
For the study, the team involved more than 10 million people infected with several Covid-19 variants between March 2020 and June 2022.
While the effectiveness of both vaccines and prior infection waned over time, especially against infection with the omicron variant, the protection against hospitalization and death remained strong.
The researchers found that those who received the primary series of vaccines were less likely to become infected or experience severe outcomes from infection than those who were unvaccinated, even after a year and a half.
Those who received a booster dose had higher protection than those who received only the primary series.