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Hybrid immunity against omicron BA.5 lasts about eight months after a breakthrough COVID-19 infection, according to a small study in Portugal. While it was established that infection after vaccination provides an immune boost, the length of that boost was not known until now.

“The protection afforded by hybrid immunity is initially about 90%, reducing after five months to about 70%, and showing a tendency to stabilize at a value of around 65% after eight months, compared to the protection in vaccinated persons that were never infected by the virus,” said co-lead author Luís Graça.

He added that, “These results show that hybrid immunity conferred by infection with previous subvariants of SARS-CoV-2 in vaccinated people is quite stable.” Graça is group leader at the Instituto de Medicina Molecular João Lobo Antunes (iMM, Lisbon) and full professor at the Medical School of the University of Lisbon.

The study was published this week in Lancet Infectious Diseases. It was led by Graça and Manuel Carmo Gomes, associate professor at the Faculty of Sciences of the University of Lisbon.

The FDA approved the first COVID-19 vaccine, Pfizer and BioNTech’s COMIRNATY (COVID-19 Vaccine, mRNA), in August 2021 for individuals 16 years of age and older. The agency recently expanded that approval to include adolescents and some children. The protection afforded by BA.1 against infection by the now surging BA.5 subvariant is important because current vaccines are based on BA.1.

This study follows results from this team published in September’s New England Journal of Medicine where they showed, by studying the widely vaccinated Portuguese population, that infection by the first omicron subvariants of SARS-CoV-2, circulating in January and February 2022, conferred considerable protection against the new omicron BA.5 subvariant. Portugal was one of the first countries to see a rapid rise in BA.5, which remains the predominant variant in many countries.

“In September, we had observed that infection by the first omicron subvariants conferred protection for the BA.5 subvariant about four times higher than vaccinated people who were not infected on any occasion, showing the importance of hybrid immunity for protection against new infections. Now, we show that this protection conferred by vaccination together with previous infections is stable and maintained until at least eight months after the first infection,” explained Graça.

As in the previous study, the researchers used Portugal’s national COVID-19 case registry until September 2022, which is especially comprehensive due to the legal requirement that all cases of SARS-CoV-2 infection must be registered to gain access to sick leave.

“These data from the Portuguese population allows us to conclude about hybrid immunity because vaccination had already covered 98% of this population by the end of 2021. The virus variant of each infection was determined considering the date of infection and the dominant variant at that time,” explained Manuel Carmo Gomes, co-leader of the study.

About the calculations involved, João Malato, the study’s first author said, “With these data, we calculated the relative risk of reinfection over time in people vaccinated with previous infections by the first omicron subvariants of SARS-CoV-2, allowing us to conclude on the level of protection against reinfection. We found that protection remains high eight months after contact with the virus.”

This study shows that infection by previous subvariants of the SARS-CoV-2 virus, which causes COVID-19, has the ability to confer additional protection after vaccination, and that this protection is stable.

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