Having two doses of a COVID-19 vaccine has been linked to negative protection against symptomatic infection with the disease, scientists say, while prior infection without vaccination provides about 50% immunity, according to a study analyzing the Omicron wave in Qatar.
The study, published in the New England Journal of Medicine on June 15, looked at the Omicron wave in Qatar that occurred between December 2021 and approximately February 2022, comparing vaccination rates and immunity in more than 100,000 people infected and uninfected with Omicron.
The study authors found that those with prior infection but no vaccination had 46.1 and 50% immunity against both subvariants of the Omicron variant, even at an interval of more than 300 days since previous infection.
However, people who received two doses of the Pfizer and Moderna vaccine but had no previous infections were found to have negative immunity against the BA.1 and BA.2 Omicron subvariants, indicating an increased risk of contracting the COVID-19 compared to an average person.
More than six months after receiving two doses of the Pfizer vaccine, immunity to any Omicron infection has fallen to -3.4%.
But for two doses of the Moderna vaccine, immunity against any Omicron infection fell to -10.3% after more than six months since the last injection.
Although the authors reported that three doses of the Pfizer vaccine increased immunity by more than 50%, this was measured just over 40 days after the third vaccination, which is a very short interval. By comparison, natural immunity persisted at around 50% when measured more than 300 days after the previous infection, while immunity levels fell to negative numbers 270 days after the second dose of vaccine. .
These figures indicate a risk of waning immunity for the third dose of vaccine over time.
The findings are supported by another recent study conducted in Israel which also found that natural immunity wanes much more slowly than artificial or vaccinated immunity.
The study found that both natural and artificial immunity waned over time.
People previously infected but not vaccinated had half the risk of reinfection compared to those who had been vaccinated with two doses but not infected.
“Natural immunity wins again,” Dr. Martin Adel Makary, a public policy researcher at Johns Hopkins University, wrote on Twitter, referring to the Israeli study.
“Among people who had previously been infected with SARS-CoV-2, protection against reinfection decreased as time increased,” the authors concluded, “however, this protection was higher” than the protection conferred in the same time interval by two doses of the vaccine.
Enrico Trigoso contributed to this report.