The efficacy of the BNT162b2 vaccine following recovery from Covid-19
The efficacy of the BNT162b2 vaccine following recovery from Covid-19

The efficacy of the BNT162b2 vaccine following recovery from Covid-19

Our study showed that receiving the BNT162b2 vaccine in patients who had recovered from Covid-19 was associated with significantly lower re-infection rates. These results are consistent with data from studies that have shown strong immunological responses to vaccination in previously infected individuals.11.19

Although vaccine efficacy was lower among patients aged 65 years or older than among younger patients, the vaccine still offered significant protection among elderly patients. Among the unvaccinated patients, however, the re-infection rate among the older patients was much lower than among the younger patients (3.02 cases per 100,000 persons per day against 10.79 cases per 100,000 persons per day). This observation can be explained if we assume that elderly patients who had already been infected with SARS-CoV-2 would have observed increased social distancing and other necessary precautions, especially during the increase of the delta variant, even if they had decided to to be vaccinated. Therefore, the differences in reinfection rates between vaccinated and unvaccinated elderly patients were lower than those in the younger population.

In the secondary analysis, we found that receiving more than one vaccine dose was not associated with greater efficacy. However, it should be noted that only 19% of the vaccinated patients received more than one dose of vaccine during the study period. In view of the previous exposure to the virus, it appears that the primary vaccine dose in recovered patients produced a more robust and longer immunogenic response than the first dose alone in patients without previous Covid-19. These results are in line with the results of a previous study conducted in Italy.11

Since March 2021, the Israeli Ministry of Health has recommended the administration of a single dose of vaccine to patients who have recovered from Covid-19, with a dose to be administered 3 months after recovery from the primary infection. However, not all patients eligible for this dose rushed to receive a post-recovery vaccine. Shortly after Covid-19 vaccines became available, Israel established an immunity passport policy, also known as the Green Pass, with the primary purpose of allowing safe easing of Covid-19 restrictions.20 Initially, the Ministry of Health issued a green passport to all patients who had recovered from Covid-19 without any restriction. But in October 2021, the Ministry of Health decided, due to the increase in the delta variant, that patients who had not been vaccinated within 6 months of recovery would not be entitled to a Green Pass.21

In Israel, receiving a Covid-19 vaccine is a personal choice. Vaccine retention after recovery from Covid-19 may be due to personal safety concerns in patients who wanted to ensure that the vaccine was safe and beneficial to them. On the other hand, some patients who had a history of severe symptoms during their illness might have been willing to do anything that would avoid re-infection, and therefore had a greater incentive to get the vaccine.

Our study has several strengths. First, the results are based on the integrated medical record system from Clalit Health Services with detailed demographic, clinical and laboratory test data, including all dates and results of RT-qPCR tests, updated daily with information from the Ministry of Health’s data warehouse. . Second, the large cohort is available for analysis with a relatively long-term follow-up. Third, the study period covered the entire increase in the delta variant in Israel, where the incidence of Covid-19 was one of the highest in the world.22 Thus, the number of reinfections was sufficient to demonstrate the efficacy of the vaccine.

Our study also has several limitations. As in any real-world observational study, patients were not randomly assigned to receive or did not receive the vaccine. Very confusing is expected to occur due to lack of randomization due to significant differences in the clinical backgrounds and sociodemographic characteristics of the two groups. This limitation is inherent in any real-world, population-based study of vaccine efficacy, since the patients who received a vaccine may differ from those who did not.8.23 We tried to overcome such a bias by adjusting for variables known to affect the number of Covid-19 complications. However, measurement or correction may not have been performed sufficiently for unobserved or unmeasured sources of bias.

Another possible source of skew is the variation in exposure to SARS-CoV-2 during the study period. To minimize this potential bias, we only included patients in the study until 31 May 2021, before the start of the increase in the delta variant. Therefore, we assumed that after adjustment for all covariates, the possible exposure variation had a similar effect in the vaccinated and unvaccinated groups.

A further limitation of this study is that re-infections were identified on the basis of a positive result on RT-qPCR assay, a procedure that would miss patients who were re-infected but were not aware of their infection, or those who decided to avoid RT-qPCR testing, which would be more likely in mild cases. Assuming that infection was more likely to be asymptomatic or only mildly symptomatic in those who had been vaccinated, testing might have been less likely in this group. Thus, many registrations of infection may have been missed – a factor that could have significantly distorted the rate of re-infection in the vaccinated group and resulted in an overestimation of the vaccine’s effectiveness. Therefore, we compared the overall test rate in the two groups and found that testing was more frequent in the vaccinated group (Table S4).

A further limitation is that we did not assess data on the severity of infection or on hospitalization or death in the re-infected patients, as these results were outside the scope of the study. In a recent study involving a large national cohort in Qatar, the risk that re-infection would result in hospitalization or death was 90% lower than the risk associated with primary infection.24

Finally, our results were limited to the BNT162b2 vaccine. Although a recently published study provided evidence that the mRNA-1273 vaccine is slightly more effective than the BNT162b2 vaccine in participants who had received two doses of vaccine,25.26 We can not deduce whether this observation is relevant to avert reinfection with respect to patients who have recovered from Covid-19. Despite these limitations, we believe that our results can provide meaningful answers to a crucial question regarding vaccination policy with respect to patients following recovery from Covid-19.

Our study showed that among patients who had recovered from Covid-19, receiving one dose of the BNT162b2 vaccine was associated with an 82% lower risk of recurrent SARS-CoV-2 infection among those between 16 and 64 years and a 60% lower risk among those aged 65 or older. No significant difference in reinfection risk was found for two doses of vaccine compared to one dose. The evidence gathered in this study during an increase in the delta variant in Israel supports a public health policy of vaccinating patients who have recovered from Covid-19, especially in places where the delta variant still gives cause for concern.

Leave a Reply

Your email address will not be published.