Prior exposure to SARS-CoV-2, the virus causing COVID-19, neither guarantees a high level of antibodies nor immunity to re-infection as thought earlier, finds a study emphasising the need to get vaccinated against the deadly infectious disease.

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The study, led by researchers from the Northwestern University in the US, shows that two months after the second Pfizer/Moderna vaccination, antibody response decreases 20 per cent in adults with prior cases of COVID-19. It added that prior exposure to SARS-CoV-2 does not guarantee a robust antibody response to the first vaccine dose. This directly contradicts the assumption that contracting COVID will naturally make someone immune to re-infection.

The findings, published in the journal Scientific Reports, further supports vaccination (and two doses), even for people who have contracted the virus previously. "Many people, and many doctors, are assuming that any prior exposure to SARS-CoV-2 will confer immunity to re-infection. Based on this logic, some people with prior exposure don't think they need to get vaccinated. Or if they do get vaccinated, they think that they only need the first dose of the two-dose Pfizer/Moderna vaccines," said Thomas McDade, Professor of Anthropology in the Weinberg College of Arts and Sciences and a faculty fellow with the University's Institute for Policy Research.

"Our study shows that prior exposure to SARS-CoV-2 does not guarantee a high level of antibodies, nor does it guarantee a robust antibody response to the first vaccine dose. For people who had mild or asymptomatic infections, their antibody response to vaccination is essentially the same as it is for people who have not been previously exposed," he added.

For the study, the team tested blood samples from adults who had contracted COVID to measure how long the immunity benefits of Pfizer and Moderna vaccines last and how well they protect from newer variants. When the team tested blood samples from participants collected about three weeks after their second vaccine dose, the average level of inhibition was 98 per cent, indicating a very high level of neutralising antibodies.

The scientists tested emerging variants B.1.1351 (Beta), B.1.1.7 (Alpha) and P.1 (Gamma) and found the level of inhibition to viral variants was significantly lower, ranging from 67 per cent to 92 per cent. They also found that antibody response declined after two months by about 20 per cent and that the antibody response to vaccination varied based on a history of prior infection.

Further, individuals with clinically confirmed cases of COVID-19 and multiple symptoms also showed a higher level of response than those who tested positive but had mild symptoms or were asymptomatic.