They said their findings should help settle debates over whether people who have been infected should bother getting vaccinated. They should, the researchers said.
People who had not been vaccinated and who ended up in the hospital were five times more likely to have Covid-19 than people who had been vaccinated within the past three to six months, they found.
“All eligible persons should be vaccinated against COVID-19 as soon as possible, including unvaccinated persons previously infected with SARS-CoV-2,” the researchers wrote in the CDC’s weekly report, the MMWR.
“We now have additional evidence that reaffirms the importance of COVID-19 vaccines, even if you have had prior infection,” CDC Director Dr. Rochelle Walensky said in a statement.
“This study adds more to the body of knowledge demonstrating the protection of vaccines against severe disease from COVID-19. The best way to stop COVID-19, including the emergence of variants, is with widespread COVID-19 vaccination and with disease prevention actions such as mask wearing, washing hands often, physical distancing, and staying home when sick.”
Researchers across the country collected data on 7,000 people treated at 187 hospitals in nine states for Covid-like illnesses between January and September. All were tested for coronavirus. Those who were not vaccinated were 5.49 times as likely to test positive for coronavirus as those who had been vaccinated within the past three to six months — even if they had a recent Covid-19 infection.
The CDC noted that a Israeli study posted online in August found just the opposite — but also noted that the Israeli study looked at people who had been vaccinated six month before or longer. “Understanding infection-induced and vaccine-induced immunity over time is important, particularly for future studies to consider,” they wrote.
“In this study, the benefit of vaccination compared with infection without vaccination appeared to be higher for recipients of Moderna than Pfizer-BioNTech vaccine, which is consistent with a recent study that found higher vaccine effectiveness against COVID-19 hospitalizations for Moderna vaccine recipients than for Pfizer-BioNTech vaccine recipients,” they added.
The researchers did not include people who received Johnson & Johnson’s Janssen vaccine in their study.
They also note that while the study was designed to compare two groups with two different types of immunity– immunity from natural infection versus immunity from vaccination — it’s possible there were some mixups. Plus, they only included hospitalized patients in the study, so the findings may not apply to everyone.
Nonetheless, the findings fit in with other evidence that shows people who have been infected also have a very strong immune response to vaccination, and benefit from getting coronavirus vaccines.