Vaccinated people who were infected by the first Omicron subvariants have four times greater protection than jabbed people who did not catch the COVID-19 infection, according to a study.

The research, published recently in the New England Journal of Medicine, analysed the probability of vaccinated people becoming infected with the subvariant BA.5 currently in circulation.

The researchers in Portugal estimated the degree of protection conferred by infections with previous variants and used real-world data.

"Vaccinated people who were infected by Omicron subvariants BA.1 and BA.2 have a protection against infection with subvariant BA.5, in circulation since June, about four times greater than vaccinated people who were not infected at any time," said Luis Graca, a professor at the University of Lisbon.

"Infections in 2020 and 2021 that occurred through infection with earlier variants of the SARS-CoV-2 virus also confer protection against infection for the more recent Omicron variant, although this protection is not as high as that of individuals infected with the BA.1 and BA.2 variants, at the beginning of 2022," said Grace, co-leader of the study.

"These results are very important because the adapted vaccines that are in clinical development and evaluation are based on the BA.1 subvariant of the virus, which was a dominant variant in infections in January and February 2022," the researchers said.

Until now, it was not known what degree of protection this subvariant provides against the strain that is currently in circulation, they said.

The researchers had access to the registry of COVID-19 cases at Portugal's national level.

"We used the Portuguese national registry of COVID-19 cases to obtain information on all cases of SARS-CoV-2 infections in the population over 12 years of age residing in Portugal," said Manuel Carmo Gomes, associate professor at the University of Lisbon.

The virus variant of each infection was determined considering the date of infection and the dominant variant at that time. We considered the infections caused by the first variants of Omicron BA.1 and BA.2 together," Gomes said.

The researchers then analysed the probability of a person that was previously infected to be reinfected with the current variant, which allowed them to calculate the percentage of protection provided by previous infections.

The study demonstrates that previous infection in vaccinated people continues to confer for the variants that are known for their ability to evade the immune response, such as the subvariant currently dominant, they added.