Comparison of two highly-effective mRNA vaccines for COVID-19 during periods of Alpha and Delta variant prevalence.
Level 3 - non-randomized controlled study
Non-randomized matched retrospective cohort study
PubMed 34401884 · doi:10.1101/2021.08.06.21261707
What was done
Researchers conducted a retrospective matched cohort study in the Mayo Clinic Health System from January to July 2021 comparing the effectiveness of Moderna (mRNA-1273) and Pfizer/BioNTech (BNT162b2) COVID-19 vaccines during periods dominated by the Alpha and Delta variants. In Minnesota, 25,589 vaccinated individuals were matched 1:1 with 25,589 unvaccinated individuals on age, sex, race, prior SARS-CoV-2 PCR testing history, and vaccination date. Direct breakthrough infection rates between matched mRNA-1273 and BNT162b2 recipients were also evaluated across sites in Minnesota, Wisconsin, Arizona, Florida, and Iowa.
What was found
Across the full study period, effectiveness against infection was 86% (95% CI: 81–90.6%) for mRNA-1273 and 76% (95% CI: 69–81%) for BNT162b2; effectiveness against hospitalization was 91.6% (95% CI: 81–97%) and 85% (95% CI: 73–93%), respectively. In July 2021, when Delta prevalence exceeded 70% in Minnesota, protection against hospitalization remained 81% (95% CI: 33–96.3%) for mRNA-1273 and 75% (95% CI: 24–93.9%) for BNT162b2, but protection against infection declined to 76% (95% CI: 58–87%) for mRNA-1273 and 42% (95% CI: 13–62%) for BNT162b2. In multi-state direct comparisons, mRNA-1273 recipients had a 50% lower risk of breakthrough infection than BNT162b2 recipients (IRR = 0.50, 95% CI: 0.39–0.64), and in Florida during July, a 61% lower risk (IRR = 0.39, 95% CI: 0.24–0.62).
Why it matters
This study provides head-to-head real-world evidence that although both mRNA vaccines reliably prevent severe disease leading to hospitalization, protection against infection attenuated more rapidly for BNT162b2 than mRNA-1273 as the Delta variant emerged.
Limits
The study is observational and relies on electronic health record data, leaving potential for residual confounding from unmeasured exposure risks, health-seeking behaviors, or differing testing frequencies. Subgroup estimates for July hospitalization exhibited wide confidence intervals due to small event counts, and genomic sequencing was not available for every individual case.
Cited by
- supports The Mayo Clinic study found that the Moderna vaccine was approximately 77% effective against SARS-CoV-2 infection during the Delta surge, roughly twice as protective as the Pfizer vaccine.
- supports The Mayo Clinic study found that both the Pfizer and Moderna mRNA vaccines remained 80% to 97% effective at preventing COVID-19 hospitalizations during the Delta surge.