In a recent study published in the latest issue of New England Journal of Medicineresearchers conducted a retrospective cohort study in Israel to assess the incidence of re-infection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in patients who had recovered from coronavirus disease 2019 (COVID-19).
Vaccine hesitation may have stemmed from personal safety concerns in patients who wanted to be assured that the COVID-19 vaccine is safe and beneficial. In addition, data for vaccine prove efficiency in patients who had recovered from COVID-19 is still limited. Although current guidelines recommend vaccination to all, including those who have been infected in the past, it is crucial to assess how long the protective immunity lasts.
In Israel, although it is a personal choice to take a COVID-19 vaccine, the Israeli Ministry of Health issues a Green Pass, an immunity passport, from October 2021 onwards, only to those who are vaccinated within six months of arriving from COVID-19.
About the study
The researchers reviewed data from electronic medical records from members of Clalit Health Services, Israel, which has health data for ~ 52% of the population to assess re-infection rates in patients who had recovered from SARS-CoV-2 infection no less than 100 days before receiving any COVID-19 vaccine.
They compared the incidence of recurrent infection between patients vaccinated with the BNT162b2 vaccine and unvaccinated patients; identified re-infection on the date of the SARS-CoV-2-positive quantitative reverse transcriptase-polymerase chain reaction (RT-qPCR) test at least 100 days after the primary infection. According to the Israeli Ministry of Health, the 100-day cut-off allows cured patients to receive their COVID-19 vaccination after the recommended time delay of three months after primary infection. In addition, they estimated the association between vaccination and re-infection after adjustment for demographic factors and other time-varying covariates using the Cox proportional hazard regression model.
In a secondary analysis, they assessed the efficacy of the vaccine, estimated at one minus hazard ratio (HR), among patients who had received one and two doses of vaccine. All the qualified subjects were between 16 and 110 years old, had contracted primary SARS-CoV-2 infection between 23 August 2020 and 31 May 2021, identified by the date of the first SARS-CoV-2-positive RT-qPCR.
During the study period, the population of vaccinated and unvaccinated groups was dynamic. Up to seven days after receiving the first dose of vaccine, participants remained in the unvaccinated group; later they moved into the vaccinated group. Two previous studies, one large observational study on BNT162b2 booster dose and another a randomized, controlled trial of the BNT162b2 vaccine, have validated that 7-day time lag is ideal for analyzing vaccine efficacy.
Among the vaccinated patients, those in the age group of 16 and 64 years and over 65 years had the adjusted HR for reinfection of 0.18 and 0.40, respectively; subsequently, the observed vaccine efficacy among patients in these two age groups was 82% and 60%, respectively.
Although the effectiveness of the vaccine was lower among patients over 65 years of age than the younger patients, vaccination still provided significant immune protection. In contrast, the reinfection rate among the elderly patients was much lower, with only 3.02 cases per 100,000 people per. day against 10.79 cases per. 100,000 people per. day among the younger patients.
The adjusted HR for reinfection among the patients who had received one vaccine dose and two doses was 0.98, suggesting that two vaccine doses did not provide additional protection than one vaccine dose to previously infected SARS-CoV-2 patients. It is worth noting here that only 19% of the vaccinated patients received more than one vaccine dose during the study period; however, previous SARS-CoV-2 exposure combined with one dose of vaccine helped them achieve a more robust and sustained immunogenic response.
The study observations showed that among patients who had recovered from COVID-19, one dose of the BNT162b2 vaccine reduced the risk of recurrent infection by 60% -82% across all age groups. In addition, the second vaccine dose provided no additional benefit against re-infection compared to one vaccine dose.
More importantly, the data evidence gathered in this study supports a public health policy mandating vaccination of patients who have recovered from COVID-19 in Israel.