COVID Vaccines Plus infection can lead to months of immunity

Even people who have had covid-19 get long-term benefits from a complete vaccination regimen, according to three new studies. In addition, one of the studies found that the “hybrid” immunity caused by vaccination and infection is long-lasting, providing very effective protection against symptomatic disease for at least six to eight months after vaccination.

The information was collected before the Omicron variant appeared, which raises some doubts about the relevance of the studies today. But if the results are sustained, they can provide information about vaccination programs and vaccine passes, which some countries require for entry to places such as restaurants. The work also counteracts high-profile claims that people who have had covid-19 do not benefit from vaccination.

Just such a statement helped to start some of the research. Brazilian President Jair Bolsonaro “said he already had covid-19, and for that reason it is not necessary to get vaccinated,” said Julio Croda, an infectious disease doctor and epidemiologist at the Oswaldo Cruz Foundation in Rio de Janeiro, Brazil. Croda and his colleagues used Brazilian vaccination and infection databases to test such claims.

The researchers found that between February 2020 and November 2021, people who had previously been infected with SARS-CoV-2 and then received a vaccine dose – made by either Pfizer – BioNTech, Oxford – AstraZeneca, SinoVac or Johnson & Johnson – avoided as many as 45% of covid-19 cases that the group would have been expected to suffer from without vaccination. Full regimens with two-dose vaccine prevented as many as 65% of the expected infections and more than 80% of the expected cases of severe covid-19. “The big message is this: you need to have a complete covid-19 vaccination program,” Croda said.

“Immunity passport”?

Some authorities take previous infections into account when deciding who should have access to public places such as concerts and restaurants, but others only consider vaccination status. Peter Nordström, epidemiologist at Umeå University in Sweden, says that this dichotomy made him and his colleagues perform another of the studies. Using journals collected by the Swedish Public Health Agency between March 2020 and October 2021, the researchers showed that Swedish residents who had been infected with SARS-CoV-2 had a 95% reduction in their risk of developing covid-19 compared to people who had no immunity and the protection grew during the three months after infection and lasted for at least 20 months after infection. One dose of vaccine reduced the risk of infection by an additional 50%, and a second dose stabilized the additional protection for six months after vaccination.

Although vaccination increases protection, Nordström thinks that the immunity that only infection gives is worth considering. – Maybe we should have immunity passes instead of vaccination passes. So you are considered immune – and less likely to transmit the disease – if you have been fully vaccinated or if you have had a documented previous infection, he says.

Epidemiologist Victoria Hall at the UK Health Security Agency in London and her colleagues conducted the third study by tracking infections in thousands of healthcare professionals from March 2020 to September 2021. Researchers found that previous infections prevented more than 80% of covid-19 cases that would otherwise have been expected during the year after infection, but the protection decreased to around 70% after one year. Study participants who received two doses of the Pfizer-BioNTech or Oxford-AstraZeneca vaccine after one infection had close to 100% protection for at least six to eight months after the second dose. “Protection decreased over time after vaccination, and even after infection, but remained persistently high in those with hybrid immunity,” Hall wrote in an email to Nature.

Miguel Hernan, an epidemiologist at the Harvard TH Chan School of Public Health in Boston, Massachusetts, says the studies show the almost universal benefit of full vaccination. Some nations have issued guidelines encouraging people who have had covid-19 to receive only a single dose of vaccine: a move that “may be justified in a situation of vaccine deficiency, but not otherwise,” Hernan wrote in an email to Nature.

Variation can change the game

Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston, Massachusetts, says the findings are in line with previous research. “Post-infection vaccination, or post-vaccination infection, results in particularly robust antibody responses,” he wrote in an email to Nature. But Barouch notes that all three studies are based on data collected before the Omicron variant appeared. He and others warn that previous infections will provide imperfect protection against new strains.

Dan Kelly, an epidemiologist for infectious diseases at the University of California, San Francisco, underscores this concern. Omicron differs so much from the strains analyzed in the studies that the results may not apply to people who became infected with Omicron after being vaccinated. His advice to people who fall into this category: “Just be really careful.”

This article is reproduced with permission and was not published until April 6, 2022.

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