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.
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.
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.
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