1. mRNA booster vaccines for Covid-19 were highly effective against delta infection but less effective against omicron infection.
2. mRNA booster vaccines demonstrated strong protection against Covid-19 related hospitalization and death due to both variants.
Evidence Rating Level: 2 (Good)
Study Rundown: Qatar launched a booster vaccination program in mid-September 2021 to combat the immune-evasive omicron variant of Covid-19. Both the Pfizer-BioNTech and the Moderna mRNA vaccines were used. However, there is a gap in knowledge as to understanding the effectiveness of booster vaccination against symptomatic Covid-19-related hospitalization and death as compared to the two-dose primary series alone. This study found that based on the national cohort of vaccinated persons in Qatar, mRNA boosters were highly effective against delta infection but less effective against the omicron variant, and led to strong protection against Covid-19 related hospitalization and death due to both variants. This study was limited by factors such as the lower severity of omicron as compared to previous variants, the young population of Qatar, and the time lag between infection and severe forms of Covid-19, as well as being unable to match cohorts for factors such as coexisting conditions, occupation, or geographic location. Nevertheless, these study’s findings are significant, as they demonstrate those mRNA boosters are effective against the delta variant and less so for the omicron variant, but still, demonstrate effectiveness against serious adverse outcomes such as hospitalization and death amongst vaccinated people in Qatar.
Click to read the study in NEJM
Relevant Reading: BNT162b2 Vaccine Booster and Covid-19 Mortality
In-Depth [retrospective cohort study]: This retrospective cohort study analyzed a population of 2,239,193 persons in Qatar who had received either two doses of the Pfizer-BioNTech or Moderna vaccine between December 2021 and January 2022. Those who also received a booster were matched with persons who did not receive a booster. Patients who had received at least 2 doses of the Pfizer-BioNTech or Moderna vaccine were eligible for the study. Patients who had previously documented infection before the start of follow-up were excluded from the study. The primary outcome was a symptomatic infection, defined as a positive PCR test of a nasopharyngeal swab based on clinical symptomology compatible with respiratory tract infection. Outcomes in the primary analysis were conducted via matched cohorts and Cox proportional-hazards regression models. Based on the analysis, the cumulative incidence of symptomatic omicron infection was 2.5% (95% confidence interval [CI], 2.3 to 2.5) amongst Pfizer-BioNTech vaccinated persons who received a booster, and 4.5% (95% CI, 4.3 to 4.6) in the non-booster cohort after 35 days of follow-up. Booster effectiveness against symptomatic omicron infection compared to patients who only received a 2-dose primary series was 49.4% (95% CI, 47.1 to 51.6). Booster effectiveness against symptomatic delta infection compared to the primary 2-dose series was 86.1% (95% CI, 67.3 to 94.1). And for Covid-19-related hospitalization and death due to omicron, booster effectiveness as compared to the primary series was 76.5% (95% CI, 67.3 to 94.1). Overall, this study demonstrated that mRNA boosters were highly effective against the delta variant and less effective against the omicron variant, but offered strong protection against both strains for related hospitalization and death.
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