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COVID booster essential, even among individuals previously infected

Joint study challenges assumption that two doses are sufficient for those with hybrid immunity


A long-term, cohort study led by researchers at the Azrieli Faculty of Medicine of Bar-Ilan University and Ziv Medical Center in Safed has produced further insight regarding the interplay between COVID-19 infection and vaccination in providing protection over time.

Seven to nine months after the second dose of the vaccine, antibody levels throughout the cohort dropped and were comparable in all groups including among young people and those infected before vaccination. The booster, however, led to antibody levels ten times higher than after the second dose in all groups within the cohort.

The study, recently published in the journal Clinical Infectious Diseases further showed that all individuals, including those with hybrid immunity (infected and then vaccinated) require subsequent boosters beyond the two initial COVID-19 vaccine doses.

"The fact that antibody levels also decline in individuals with hybrid immunity -- albeit not as rapidly than among those who were never infected -- challenges the previously held assumption that these people don't need further boosters. Our study suggests that they do," said Prof. Michael Edelstein, of Bar-Ilan University's Azrieli Faculty of Medicine, who co-authored the paper, Dr. Kamal Abu Jabal, of Ziv Medical Center and the Azrieli Faculty of Medicine, oversaw the running of the study.

More than 500 healthcare workers at Ziv Medical Center have been participating in the cohort study, which began when COVID vaccines became available, to monitor how the vaccines have protected them over time.

Participants gave blood every two months. Each time the quantity of COVID-19 antibodies in the workers and how it changed over time according, to age, ethnicity, gender and previous infection status was measured. Each group was then compared to understand how antibody levels changed over time in each category.

"Cohort studies are important because COVID-19 is a constantly evolving situation and we still don't know who needs to be vaccinated and how frequently. This question becomes especially complicated in people who've already been infected when considering whether they require vaccination and, if so, how many doses," says Edelstein. "Infection in itself is not enough to protect individuals over the long term."

Two previous studies on the same cohort were published last year. In February 2021 in the journal Eurosurveillance, the researchers reported evidence that those previously infected with the virus responded very strongly to one dose of the Pfizer vaccine, regardless of when they were infected and whether or not they had detectable antibodies against COVID-19 prior to receiving the vaccine. Later, in a study published in Epidemiology & Infection, they found that those infected shortly after receiving one dose needed a second one, contrary to policy at the time.

The research team continues to measure antibody levels in the cohort to understand the effect of the third and fourth doses, how they interact with infection, and how long they last.