Study population and setting
This study was a retrospective analysis using data on SARS-CoV-2 infections and vaccinations with the Pfizer BNT162b2 mRNA vaccine in Israel to investigate the impact of vaccination on transmission. Data from the Israeli Ministry of Health included age, sex, date of positive SARS-CoV-2 PCR test, date of hospitalization, clinical state during hospitalization, and date of death or recovery for all Israeli citizens from August 28, 2020 to February 24, 2021. Publicly available data from the national vaccination campaign included the number of daily vaccine doses broken down by which dose was received (first or second), city of residence, and age of vaccinees by 10-year age categories. Early vaccinated cities were defined as the top ten cities with the highest percentage of the population above 60 years who were vaccinated or recovered from COVID-19 by January 10, 2021, while late vaccinated cities were the ten bottom cities with the lowest percentage of these individuals. The study also compared the number of new positive tests, positive test percentage, new hospitalized cases and new severe cases between the second lockdown (imposed on September 18, 2020) and the third lockdown (imposed on January 8, 2021), which was after the national vaccination campaign began on December 20, 2020.
Summary of Main Findings
There was an approximate 77% drop in total cases, 45% drop in overall positive test percentage, and 68% drop in total hospitalizations two months following the initiation of the vaccination campaign compared to peak values. This was at a time that 85% of people age 60 years and older had already been vaccinated with two doses of the Pfizer vaccine. These drops were observed in each consecutive group that became eligible for the vaccine. There was approximately 10% less of a decrease in the number of COVID-19 cases and severe hospitalizations in late vaccinated cities compared to early vaccinated cities. When comparing these metrics between the second and third lockdowns, the same trends were not observed during the second lockdown, which was prior to initiation of the vaccination campaign. This lends support to the vaccine being the cause of the drops in cases and hospitalizations.
The study used a large dataset from a universal healthcare system in Israel that enabled the authors to track all diagnosed cases and vaccinated individuals over time to compare before and after the vaccine campaign began.
This was a single study, and therefore it is difficult to truly infer causality. All variables were not exactly the same between the second and third lockdown (i.e. circulating strains of COVID-19, behaviors, transmission prevention measures etc.), so these periods may not be comparable. Behavioral and social differences between cities were not accounted for when comparing different geographic regions. Finally, this study examined the effects of the Pfizer vaccine in the Israeli population, and therefore these results may not be generalizable to other COVID-19 vaccines.
This is the first analysis of the effect of a vaccination campaign on SARS-CoV-2 transmission dynamics at the population level.
This review was posted on: 7 May 2021