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Covid vaccines provide stronger immunity than past infection, CDC study finds

Unvaccinated people who had recovered were five times more likely to test positive again than people who were recently fully vaccinated.
Image: Boston's Fenway Park To Serve As Mass Vaccination Site
A woman receives her first dose of a Covid-19 vaccine at Fenway Park on Jan. 29, 2021, in Boston. Scott Eisen / Getty Images file

Vaccination against Covid-19 provides stronger protection than immunity from a previous infection with the coronavirus, according to a study from the Centers for Disease Control and Prevention published Friday.

The study looked at more than 7,000 people hospitalized with Covid-like illnesses, and found that those who were unvaccinated — but had a previous case of the illness — were five times more likely to have a confirmed case of Covid than people who were fully vaccinated and had not had Covid before.

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A previous infection does provide some degree of immunity and protection against reinfection, but the findings suggest that the protection conferred by vaccination is stronger.

“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.

Deepta Bhattacharya, a professor of immunology at the University of Arizona, cautioned that it can be very difficult to compare vaccine-induced immunity to infection-induced immunity.

“What I would say is that these are maybe not the right terms of the debate,” he said. “The reason to prefer vaccine-induced immunity is that infections can make you really sick, not that they don’t leave you immune.”

In the study, researchers looked at people hospitalized with Covid symptoms in 187 hospitals across nine states from January to September — a period that included both the alpha and the delta variants. Patients were included if they had a previous case of Covid in the past three to six months, or had been fully vaccinated with either the Pfizer-BioNTech or the Moderna vaccines in the past three to six months. Johnson & Johnson vaccine recipients were not included because there was not enough data, the researchers said.

A total of 6,328 fully vaccinated people in the study were hospitalized with Covid-like illnesses, but among them, only 324, or 5.1 percent, tested positive for the virus. In the second group — unvaccinated but previously infected —  there were a total of 1,020 people hospitalized, 89 of whom, or 8.7 percent, tested positive for the virus. 

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Both the Pfizer and the Moderna vaccines provided higher protection than previous infection. Protection from the Moderna vaccine was stronger than protection from the Pfizer vaccine, a finding that’s been observed in an earlier CDC report. The protective effects of the vaccines were also higher in adults 65 and older compared with people ages 18 to 64.

Because the study only included people vaccinated or previously infected within the previous six months, the researchers cautioned that the protective effects may wane over time.

Previous research has shown that so-called hybrid immunity, or natural immunity from a previous infection plus vaccine-induced immunity, leads to particularly robust protection — another reason that people who have been previously infected should get vaccinated.

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