Vaccinated people who were infected by the earlier Omicron subvariants have four times greater protection than vaccinated people who were not infected against the new variants like BA.5, finds a new study.

The study indicates that previous infection in vaccinated people (the so-called hybrid immunity) continues to confer immunity for the variants that are known for their ability to evade the immune response.

"Vaccinated people who were infected by Omicron sub-variants 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 lead author Luis Graca from the University of Lisbon.

"Infections in 2020 and 2021 that occurred through infection with earlier variants of the SARS-CoV-2 virus (ancestral lineage, Alpha and Delta variants) 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," Graca added.

For the study, published in the New England Journal of Medicine journal, the team accessed 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," the researchers said.

"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," they added.