A new study published in Scientific Reports has found that prior exposure to SARS-CoV-2 does not guarantee higher levels of antibodies, and that full vaccination offers more protection against the virus than previous infection or a single dose of vaccine.
Study participants were selected from a racially and ethnically diverse pool of adults in the Chicago area and were provided with antibody testing kits devised by scientists at Northwestern University. The individuals submitted blood samples two to three weeks after their first dose of Pfizer or Moderna vaccine, as well as two months after the second dose. Scientists measured whether the blood samples could inhibit the binding between coronavirus’ spike protein and the ACE2 receptor.
“When we tested blood samples from participants collected about three weeks after their second vaccine dose, the average level of inhibition was 98 percent, indicating a very high level of neutralizing antibodies,” said study lead author Thomas McDade, professor of anthropology in the Weinberg College of Arts and Sciences at Northwestern University.
The researchers found that the antibody response to vaccination varied according the participants’ history of prior infection. Individuals who previously contacted SARS-CoV-2 and had multiple symptoms had much higher antibody levels than those who were asymptomatic or had mild cases.
“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,” explained Professor McDade.
“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.”
Moreover, the emergence of new variants decreases protection from both prior infection and vaccination. For the emerging variants B.1.1351 (South Africa), B.1.1.7 (UK), and P.1 (Brazil), scientists found that the levels of inhibition ranged from 67 to 92 percent.
“As far as protection goes after vaccination, the story is the same for all the variants, including delta – the vaccine provides good protection, but not as good protection as for the original version of the virus for which the vaccine was designed. Combine that with the fact that immunity wanes over time, you get increased vulnerability to breakthrough infection,” said Professor McDade.