COVID VACCINESCOVID Vaccines Offer Lasting Protection against Reinfection: Studies

By Mary Van Beusekom

Published 18 February 2022

Two new studies suggest good, durable protection of COVID-19 vaccines against recurrent infection. NEJM editor-in-chief said that COVID-19 survivors can still benefit from subsequent vaccination, although the ideal time to vaccinate is yet unknown: “There is an advantage, and although the absolute risk difference may be small, it’s real. Also, there doesn’t appear to be a safety issue with getting boosted.”

A pair of studies yesterday in the New England Journal of Medicine (NEJM) suggest good, durable protection of COVID-19 vaccines against recurrent infection.

Fewer Reinfections among Older Participants
Researchers from Clalit Health Services in Tel Aviv, Israel, retrospectively analyzed the electronic health records of 83,356 recipients of at least one dose of the Pfizer/BioNTech COVID-19 vaccine after recovery from infection and 65,676 unvaccinated survivors.

The observational study began on Mar 1, 2021, after the Israeli Ministry of Health approved COVID-19 vaccination for all patients who had recovered from COVID-19 3 or more months before. All participants had recovered from a primary SARS-CoV-2 infection from Aug 23, 2020 (190 days before the study period), to May 31, 2021 (90 days after study initiation). Average patient age was 39.3 years (range, 16 to 110).

COVID-19 reinfection occurred in 354 of 83,356 vaccinated participants (0.4%; 2.5 cases per 100,000 person-days) and in 2,168 of 65,676 of their unvaccinated peers (3.3%; 10.2 per 100,000).

In the 16- to 64-year age-group, 326 of 73,972 vaccinated participants (0.4%; 2.6 cases per 100,000 person-days) were reinfected, compared with 2,120 of 60,877 of their unvaccinated counterparts (3.5%; 10.8 per 100,000). 

Among patients 65 years and older, reinfection occurred in 28 of 9,384 vaccinated (0.3%; 1.5 cases per 100,000 person-days) and 48 of 4,799 unvaccinated participants (1.0%; 3.0 per 100,000). The study authors said that the difference between the two age-groups may be explained by an assumption that older COVID-19 survivors would have taken more precautions against reinfection than younger people.

According to a Cox proportional-hazards regression model analysis, the adjusted hazard ratio (aHR) for reinfection in the vaccinated group, relative to the unvaccinated, among 16- to 64-year-olds was 0.18. Among those 65 and older, the aHR was 0.40. Estimated vaccine effectiveness in the younger age-group was 82%, compared with 60% in the older group.

A secondary analysis showed that the aHR for reinfection among the 67,560 participants who received one COVID-19 vaccine dose, compared with 15,251 who received two doses, was 0.98. “Given the previous exposure to the virus, it seems that the primary vaccine dose in recovered patients provided a more robust and longer immunogenic response than the first dose alone in patients without previous Covid-19,” the researchers wrote.