By Nina Cosdón
Antibody levels decline in people with hybrid immunity similarly to vaccinated people, meaning everyone needs a booster shot to protect against future potentially serious infections.
A second, additional "booster" shot of the Pfizer-BioNTech COVID-19 vaccine is now officially recommended for all Americans over the age of 5. However, less than 50% of eligible individuals have received a booster shot.
A common reason for not receiving a booster is the belief that "natural immunity," or the antibody response generated by a previous COVID-19 infection, provides sufficient protection.
How effective is a previous COVID-19 infection at preventing future infections? Is this “natural immunity” as protective as COVID-19 vaccination? A long-term cohort study found that everyone, even those with a previous COVID-19 diagnosis, required a booster shot.
“The fact that antibody levels also decline in people with hybrid immunity, although not as rapidly as in those who were never infected, challenges the previous assumption that these people don’t need further boosters. Our study suggests that they do,” said Michael Edelstein, a professor at Bar-Ilan University’s Azrieli School of Medicine and co-author of the paper.
The study, published in Clinical Infectious Diseases , examined antibody titers in Israeli healthcare workers over 11 months. The participating healthcare workers were from Ziv Medical Center, a regional government referral hospital in Safed, northern Israel. Participants' IgG antibody levels against SARS-CoV-2 were measured before their primary vaccination and then at six subsequent time points. Healthcare workers with detectable anti-N IgG antibodies and/or a documented positive SARS-CoV-2 PCR test were considered previously infected.
Participants donated blood every two months, and researchers measured changes in their antibodies over time and according to age, ethnicity, race, sex, and prior infection. All healthcare workers with suspected COVID-19 infection underwent PCR testing.
The cohort included 985 healthcare workers who received at least one vaccine dose and one serological test after vaccination. Of these, 86 received one vaccine dose, 141 received two doses, and 758 received three doses. 118 healthcare workers had contracted COVID-19 before vaccination and were therefore considered to have “hybrid immunity” from both vaccination and prior infection.
Los participantes que eran más jóvenes o tenían una infección previa por COVID-19 inicialmente tenían títulos de anticuerpos IgG más altos (p<0,001 en ambas poblaciones). Sin embargo, a los 6-7 meses después de la vacunación primaria, las diferencias en el nivel de IgG disminuyeron significativamente o desaparecieron. «La infección en sí misma no es suficiente para proteger a las personas a largo plazo», dijo Edelstein.
The proportion of healthcare workers infected before primary vaccination and reinfected after their first dose (4.2%) was similar to the proportion of healthcare workers without prior COVID-19 infection who became infected after the second dose (4.7%). In the 40 participants who developed advanced infection after the second dose, their pre-infection IgG levels were similar to the IgG levels of vaccinated participants who did not become infected.
Across all age groups, researchers found a widespread decline in antibodies over time and high IgG levels after receiving a booster vaccine. “Our data suggest that immune decline occurs in vaccinated, naturally infected, and infected-then-vaccinated groups, regardless of age and ethnicity,” the study authors wrote. “Large, detailed, ongoing observational cohorts that measure antibody function and have sufficient incidence of clinical outcomes will help clarify the extent to which, after how long, and in terms of which variants, these individuals become at risk again.”