PORTLAND, Ore. (KOIN) — The human body’s immunity to COVID-19 gets stronger as more time passes between vaccinations and infections, according to a new study conducted by Oregon Health & Science University researchers. 

The laboratory study measured the antibody response in blood samples taken from 96 “generally healthy” OHSU employees who have gained “hybrid immunity” — meaning they have been vaccinated for COVID-19 and also contracted and fought the infection naturally. The immune response measured in these subjects uniformly showed that their immune systems became stronger as more time passed between vaccinations and infections.

The longest period between vaccination and infection measured in the study was 404 days.

Co-senior author Fikadu Tafesse, a Ph.D., associate professor of molecular microbiology and immunology at the OHSU School of Medicine, said that, based on the study’s findings, healthy people don’t need more than one COVID-19 booster per year.

“Longer intervals between natural infection and vaccination appear to strengthen immune response for otherwise healthy people,” Tafesse said. 

The study was published weeks before a Jan. 26 advisory panel meeting where the FDA will discuss possible changes to the nation’s COVID-19 vaccine strategy. The research is OHSU’s latest laboratory discovery to show strengthened immune responses through hybrid immunity.

Study co-author and doctor of medicine Marcel Curlin said that this measurable change in immunity is likely related to the body’s immune response maturing over time.

“The immune system is learning,” Curlin said. “If you’re going to amplify a response, what this study tells us is that you might want to boost that response after a longer period of learning rather than early after exposure.”

The study’s findings also suggest a long-lasting effect for “memory cells”: B cells that recognize invading viruses and generate antibodies to fight the viruses and their variants. Based on this information, the authors said that unvaccinated people who have contracted COVID-19 can still greatly benefit from receiving the vaccine.

“Relying on natural infection alone is a bad idea, ‘given the risks of severe illness, long-term complications, and death,’” OHSU said, quoting the study’s authors.

Researchers say that the findings are the latest to show that the virus is evolving to an endemic state and warned that more-frequent boosters may be needed for older or immunocompromised people, writing: “Our results point to a future where inevitable vaccine breakthrough infections would be expected to help build a reservoir of population-level immunity that can help blunt future waves and reduce the opportunity for further viral evolution.”