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Protection afforded by the BNT162b2 and mRNA-1273 COVID-19 vaccines in fully vaccinated cohorts with and without prior infection

Our take —

This preprint, which has not yet been peer-reviewed, assessed whether individuals vaccinated after a prior SARS-CoV-2 infection had better protection against future infection compared to vaccinated individuals without a previous SARS-CoV-2 infection. The study included more than 700,000 vaccinated individuals in Qatar and used PCR testing data from the country’s main public healthcare provider between December 21, 2020 and June 6, 2021. Prior SARS-CoV-2 infection among people vaccinated with BNT162b2 (Pfizer-BioNTech) reduced documented cases by 85%, offering evidence that prior infection further strengthened the already robust protection of Pfizer’s vaccine. Prior infection also bolstered the already strong protection of the mRNA-1273 (Moderna) vaccine, reducing cases by 15%, though this reduction was not statistically significant. In previous studies from Qatar’s general population, the Pfizer vaccine offered 89.5% and 75% protection against infection with the Alpha and Beta variants respectively (vs. 100% and 96.4% protection with the Moderna vaccine), which may explain why prior immunity offered additional benefits for the former but not the latter.

Study design

Retrospective Cohort

Study population and setting

The aim of this study was to investigate whether individuals vaccinated after a prior COVID-19 infection had better protection against future infection compared to vaccinated individuals who had never previously been infected. Data from the Hamad Medical Corporation, Qatar’s main public healthcare provider and the national provider for all COVID-19 healthcare, was used to study the effect of prior infection in Qatar’s population. Data was pulled between December 21, 2020 through June 6, 2021. December 21, 2020 was the start of Qatar’s national immunization campaign, which began with the Pfizer vaccine and later added the Moderna vaccine. Consequently, the data came from two national, retrospective, matched-cohort studies; one for each vaccine. Cohorts were matched in a 1:1 ratio by sex, five-year age group, nationality, and calendar week of first vaccine dose in order to control for differences in risk of exposure, as well as differences in circulating variants at that time. During this time period, the Alpha and Beta SARS-CoV-2 variants were dominant in Qatar. The analysis compared the incidence of documented SARS-CoV-2 infection in people who were at least 14 days past their second dose of the Pfizer vaccine and who had experienced a prior PCR-confirmed infection, to the incidence among people at least 14 days after second dose of Pfizer vaccine and who had not experienced a prior infection. The same analysis was performed for the Moderna vaccine.

Summary of Main Findings

Both vaccines proved to be highly effective against the Alpha and Beta variants circulating at the time the data was collected, as seen by very low incidence of infection rates in these cohorts. However, prior infection enhanced protection of people vaccinated with the Pfizer vaccine, but not those vaccinated with the Moderna vaccine. Prior infection of individuals vaccinated with the Pfizer vaccine reduced their incidence rate by 85%, and this effect was not seen for people who received the Moderna vaccine. One potential explanation for this could be the fact that Moderna was slightly more effective in Qatar against the Alpha and Beta variants overall, so the existence of prior immunity may have had no additional benefit.

Study Strengths

This retrospective study had a large sample size, with over 500,000 individuals with Pfizer vaccine and over 200,000 individuals with Moderna vaccine. Another strength is the fact that two different vaccines were analyzed.

Limitations

Pfizer and Moderna were administered at different doses and in different intervals between doses, so it is difficult to make a direct comparison between the two vaccines. Prior infection was only identified via a positive PCR result, and therefore there could have been more infections undetected in the population due to mild symptoms or asymptomatic cases. The cohort studied was mainly composed of working-age adults, and therefore the results will not necessarily be generalizable to other age groups. Co-morbidities and other socio-demographic factors were not included in the analysis. Finally, the study was performed with retrospective data under the conditions of the Alpha and Beta variants, and right now the primary circulating virus is the Delta variant.

Value added

First study investigating the effect of prior SARS-CoV-2 infection on vaccine efficacy, for two different vaccines.

This review was posted on: 30 July 2021