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Effectiveness of mRNA BNT162b2 Vaccine 6 Months After Vaccination Among Patients in Large Health Maintenance Organization, Israel

Abstract and Introduction

Abstract

Israel experienced a new wave of coronavirus disease during June 2021, six months after implementing a national vaccination campaign. We conducted 3 discrete analyses using data from a large health maintenance organization in Israel to determine whether IgG levels of fully vaccinated persons decrease over time, describe the relationship between IgG titer and subsequent PCR-confirmed infection, and compare PCR-confirmed infection rates by period of vaccination. Mean IgG levels steadily decreased over the 6-month period in the total tested population and in all age groups. An inverse relationship was found between IgG titer and subsequent PCR-positive infection. Persons vaccinated during the first 2 months of the campaign were more likely to become infected than those subsequently vaccinated. The vaccinated group ≥60 years of age had lower initial IgG levels and were at greater risk for infection. The findings support the decision to add a booster vaccine for persons ≥60 years of age.

Introduction

Coronavirus disease (COVID-19) was identified in Israel at the end of February 2019.[1] As in other countries, Israel has experienced several infection waves. The third wave, largely attributed to entry of the Alpha virus variant into Israel, began during in September 2020; at its peak, >8,000 new cases were being identified daily.[2] Israel was among the first countries to introduce a national vaccination campaign using the mRNA BNT162b2 vaccine (Pfizer-BioNTech, https://www.pfizer.com). The BNT162b2 vaccine received emergency approval for use by the US Food and Drug Administration after the vaccine showed 95% efficacy over an average 2-month follow-up period.[3,4] The vaccine was initially approved for any person ≥16 years of age, with a recommended 21-day interval, 2-dose administration.

The vaccine campaign began on December 20, 2020 (concurrent with a 2-month nationwide lockdown), first targeting all healthcare workers and the population ≥60 years of age and quickly extending to all persons ≥16 years of age. Initially, those persons who had a previous infection were not eligible for vaccination, but within 3 months, policy was changed to offer a single dose to all persons who had a previous infection. By April 2021, >50% of persons ≥16 years of age and 88% of persons ≥50 years of age countrywide had been fully vaccinated.[2] The number of new cases decreased to 140 cases/day by April 2021.[2] Initial population-based studies in Israel comparing vaccinated and unvaccinated groups reported vaccine effectiveness rates of 95%.[5,6]

One of the biggest questions regarding the vaccine is the length of protection provided. In publishing third-phase research results, Pfizer-BioNTech reported a 91% efficacy rate over a 6-month follow-up period and an estimated 6% decrease in efficacy every 2 months.[7] Population-based observational studies in Israel are no longer a feasible method of evaluating long-term effectiveness of the vaccine, given that most persons have now been fully vaccinated. Infection rates in Israel increased again during June–September 2021 (fourth wave), and most (97%) positive cases were infected with the Delta variant (B.1.617.2) (G. Rahib, Israel Ministry of Health Laboratories, pers. comm., 2021 Aug 8). Initial serologic studies of the Delta variant suggest that the BNT162b2 vaccine provides protection against Delta variant infection, but at lower rates than for the Alpha variant (88% vs. 93.7%).[8] Given the increase in infection rates, the dilemma arose whether this increase was attributable to reduced effectiveness of the vaccine against the Delta variant or a waning of protection provided by the vaccine over time.

The objective of this study was to determine if the BNT162b2 vaccine had become less effective in preventing infection, and if so, in which population groups and to what degree. To meet this objective, we conducted 3 discrete analyses to answer the following questions. First, do antibody levels (IgG) of those fully vaccinated decrease over time and if so, for who and how quickly? Second, what is the relationship between antibody level (IgG) and subsequent PCR-confirmed infection? Third, is there a difference in PCR-confirmed infection incidence rates between persons vaccinated in the initial months of the vaccination campaign and persons vaccinated later?

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