Having two doses of a COVID-19 vaccine has been linked with negative protection against symptomatic infection with the disease, scientists say, while a previous infection without vaccination offers around 50 percent immunity, according to a study analyzing the Omicron wave in Qatar.

The study, published in the New England Journal of Medicine on June 15, examined the Omicron wave in Qatar that occurred from around December 2021 to February 2022, comparing vaccination rates and immunity among more than 100,000 Omicron infected and non-infected individuals.

The authors of the study found that those who had a prior infection but no vaccination had a 46.1 and 50 percent immunity against the two subvariants of the Omicron variant, even at an interval of more than 300 days since the previous infection.

However, individuals who received two doses of the Pfizer and Moderna vaccine but had no previous infection, were found with negative immunity against both BA.1 and BA.2 Omicron subvariants, indicating an increased risk of contracting COVID-19 than an average person without prior infection and vaccination.

Six months after getting two doses of the Pfizer vaccine, immunity against any Omicron infection gradually dropped to -3.4 percent, below an average person without infection and vaccination (control) which would be set at 0.

For two doses of the Moderna vaccine, immunity against any Omicron infection dropped to -10.3 percent after more than six months since the last injection.

The authors reported that three doses of the Pfizer vaccine increased immunity to over 50 percent. However, considering that immunity was measured at only 42 days after the third vaccination, this is a very rapid immune decline over a short time period.

In comparison, previous infection gave 50 percent immunity, even over 300 days after the infection, which is a far longer period of protection.

Immunity levels for two COVID-19 vaccines fell to negative figures 270 days after the second dose of vaccine. These numbers predict a trend of more rapidly waning immunity for vaccines compared to immunity from infections.

The findings are supported by another recent study from Israel that also found natural immunity waned significantly more slowly compared to artificial, or vaccinated, immunity.

The study found that both natural and artificial immunity waned over time.

Individuals that were previously infected but not vaccinated had half the risks of reinfection as compared to those that were 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 persons who had been previously infected with SARS-CoV-2, protection against reinfection decreased as the time increased,” the authors concluded, “however, this protection was higher” than protection conferred in the same time interval through two doses of the vaccine.

Enrico Trigoso contributed to this report.