A new peer-reviewed study shows that two doses of an mRNA COVID-19 vaccine yield negative protection against symptomatic SARS-CoV-2 infection, while previous infection without vaccination offers about 50% immunity.
The findings, published on June 15, 2022 in the New England Journal of Medicine (NEJM), analyzed data from over 100,000 Omicron-infected and non-infected residents in Qatar from Dec. 23, 2021, through Feb. 21, 2022.
The authors compared the effectiveness of the Pfizer and Moderna mRNA vaccines, natural immunity from previous infection with other SARS-CoV-2 variants and hybrid immunity (a combination of infection and vaccination) against symptomatic Omicron infection and severe, critical and fatal COVID-19 disease.
Researchers found those who had a prior SARS-CoV-2 infection but had not been vaccinated had 46.1% and 50% immunity against the BA.1 and BA.2 Omicron subvariants more than 300 days after the previous infection.
However, individuals who received two doses of the Pfizer and Moderna mRNA vaccines but had not been previously infected, ended up with negative immunity against the BA.1 and BA.2 Omicron subvariants—indicating an increased risk of infection compared to someone without prior infection and COVID-19 vaccination.
Six months after the second dose of Pfizer, immunity against any Omicron infection dropped to -3.4% below an average person without infection and vaccination, which as a control, was set at 0.
For two doses of Moderna, immunity against any Omicron infection dropped to -10.3% about six months after the last dose.