A study from the University of Lisbon has shown that vaccination and previous COVID-19 infection grants protection against omicron BA.5.
The study was led by Luís Graça, lead researcher at the Instituto de Medicina Molecular João Lobo Antunes in Lisbon and Carmo Gomes, professor at the Medical School of the University of Lisbon.
The study outlines how protection given by hybrid immunity against the COVID-19 subvariant omicron BA.5, lasts for at least eight months after the first infection.
The full findings have been published in the journal Lancet Infectious Diseases.
Omicron BA.5 is common in many countries
The study follows research published in September by the same team showing that infection by the first omicron subvariants of COVID-19, circulating in January and February 2022, provided considerable protection against the omicron BA.5 subvariant. This subvariant has been circulating in Portugal since June and is the predominant variant in many countries. However, the stability of the protection given hybrid immunity had not yet been established.
“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.
In both studies, the researchers used the national COVID-19 case registry. This study is especially comprehensive due to the legal requirement to register all cases of COVID-19 infection at the time in order to gain access to sick leave during mandatory isolation days.
Hybrid immunity provides stable protection
“We used the national COVID-19 case registry to obtain the information of all cases of COVID-19 infections in the population over 12 years old residing in Portugal. 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,” said Gomes.
Using this data, the researchers calculated the relative risk of reinfection over time in people vaccinated with previous infections by the first omicron subvariants of COVID-19. This allowed the researchers to determine the level of protection against reinfection. They found that protection remained high for eight months after initial contact with the virus.
“The protection afforded by hybrid immunity is initially about 90%, reducing after five months to about 70%, and showing a tendency to stabilise at a value of around 65% after eight months, compared to the protection in vaccinated persons that were never infected by the virus. These results show that hybrid immunity conferred by infection with previous subvariants of SARS-CoV-2 in vaccinated people is quite stable,” concluded Luís Graça.