In a new study from Northwestern University, researchers found that prior COVID-19 infection alone doesn’t guarantee a high level of antibodies against the coronavirus or a strong response after just one dose of two-dose vaccines.
The team examined 27 vaccinated Chicago-area adults; some had previously tested positive with the coronavirus.
They submitted blood samples two to three weeks after their first and second dose of either the Pfizer or Moderna vaccines, and two months after the second dose.
The researchers analyzed the blood samples for antibodies against the coronavirus.
When they tested blood samples from participants collected about three weeks after their second vaccine dose, the average level of inhibition was 98%, indicating a very high level of neutralizing antibodies.
But antibody levels against emerging variants were much lower, ranging from 67% to 92%.
Also, blood samples from previously infected people collected two months after the second vaccine dose showed antibody responses had declined by about 20%.
Another finding was that participants who’d had multiple COVID-19 symptoms had a stronger immune response to the vaccines than those who had mild symptoms or were asymptomatic.
The team says many people, and many doctors, are assuming that any prior exposure to SARS-CoV-2 will confer immunity to reinfection.
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.
But this 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.
The study was conducted before the emergence of the Delta variant, but the findings still apply, the team says.
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The study is published in Scientific Reports. One author of the study is Thomas McDade.
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