Among persons who had been previously infected with SARS-CoV-2, protection against reinfection decreased as the time increased since the last immunity-conferring event, according to a recent research published in the NEJM.
However, this protection was higher than that conferred after the same time had elapsed since receipt of the second dose of the vaccine among previously uninfected persons. A single dose of vaccine after infection reinforced protection against reinfection.Infection with SARS-CoV-2 provides natural immunity against reinfection. Recent studies have shown a waning of the immunity provided by the BNT162b2 vaccine. The time course of natural and hybrid immunity is unknown.
Using the Israeli Ministry of Health database, the researchers extracted data for August and September 2021, when the delta variant was predominant, on all persons who had been previously infected with SARS-CoV-2 or who had received coronavirus 2019 vaccine. They used Poisson regression with adjustment for confounding factors to compare the rates of infection as a function of time since the last immunity-conferring event.
The number of cases of SARS-CoV-2 infection per 100,000 person-days at risk increased with the time that had elapsed since vaccination with BNT162b2 or since the previous infection. Among unvaccinated persons who had recovered from infection, this rate increased from 10.5 among those who had been infected 4 to less than 6 months previously to 30.2 among those who had been infected 1 year or more previously.
Among persons who had received a single dose of vaccine after a previous infection, the adjusted rate was low (3.7) among those who had been vaccinated less than 2 months previously but increased to 11.6 among those who had been vaccinated at least 6 months previously. Among previously uninfected persons who had received two doses of vaccine, the adjusted rate increased from 21.1 among those who had been vaccinated less than 2 months previously to 88.9 among those who had been vaccinated at least 6 months previously.
An understanding of the rates of waning immunity after immunity-conferring events is important for policy making regarding the need for and the timing of additional vaccine doses. We found that protection against the delta variant waned over time in both vaccinated and previously infected persons and that an additional vaccine dose restored protection.