"This is interesting information. It's not earth shattering. We have moved beyond the time when we could really rely on these results because Omicron is here and this study ended before Omicron became prevalent," Dr. Nancy Nielsen, the Senior Associate Dean of Health Policy at the University at Buffalo, said.
This study looked at more than a million COVID-19 cases. It was the largest study of its kind.
"Basically, the study looked at how much protection, and how long the protection lasts based on whether you were vaccinated if you previously were infected. That has been a debate for a long time," Dr. Nielsen said.
The study found prior infection with the Delta variant provided strong protection against the Delta variant.
"This paper tells us, that prior infection, particularly with the Delta variant, affords significant protection against severe disease and hospitalizations at least in the pre-Omicron period," Dr. Thomas Russo, Professor and Chief of Infectious Disease at the University at Buffalo, said.
It also said people who never had COVID-19 and are unvaccinated are the most risk.
"This paper also tells us that if you've never been infected, and never vaccinated, you're at significantly increased risk for getting infected and developing severe disease and bad outcomes," Dr. Russo said.
The study does not factor in the Omicron variant or booster shots, but experts said the study points to the need for a third dose.
"Lastly, this paper tells us that vaccine induced immunity likely wanes over time therefore the importance of a booster dose particularly if you're a senior, immunocompromised, or more vulnerable due to underlying disease," Dr. Russo said.