Daily Dose: Moderna, Pfizer COVID-19 Vaccines go Head-to-Head in Safety Study

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Last week, we reported on a study published in JAMA Network that compared the risk of adverse events between the Moderna (mRNA-1273) and the Pfizer (BNT162b) mRNA COVID-19 vaccines overall, by frailty level, and by prior history of the adverse events of interest.

The study

Researchers conducted the retrospective cohort study between December 11, 2020, and July 11, 2021. For their primary outcomes, investigators used a list of 12 serious adverse events identified by the US Food and Drug Administration that could potentially occur after mRNA vaccination and a composite of thromboembolic events. The secondary outcome of interest was 28-day risk of diagnosed COVID-19. They analyzed customer data from CVS and Walgreens linked to Medicare claims between December 2020 and July 2021.

The final cohort numbered 6 388 196 adults who received the mRNA-1273 or BNT162b2 vaccine between the study dates and had 28 days of follow-up. Just more than half received BNT62b2. Average participant age was 76.3 years; 59.4% were women; and 86.5% were White. More than one-third of individuals were categorized as prefrail (38.1%) or frail (6.0%).

The findings

Overall, older adults in both vaccine groups had a very low risk of adverse events, nor did the vaccines differ in risk for the majority of outcomes investigated. However, the Moderna vaccine (mRNA-1273 ) was associated with a lower risk of several of the potential events, including a 4% lower risk of pulmonary embolism and a 2% lower risk of thromboembolic events compared to the Pfizer vaccine, BNT162b2. Researchers point out that safety and efficacy are interrelated and that this finding may suggest greater protection by mRNA-1273 vs BNT162b2 against COVID-19.

The risk of being diagnosed with COVID-19 also was lower by 14% (RR 0.86; 95% CI, 0.83-0.87) among those vaccinated with mRNA-1273, a benefit the investigators found was attenuated by level of frailty (frail: RR, 0.94; 95% CI, 0.89-0.99).

Authors' commentary

"Future research should seek to formally disentangle differences in vaccine safety and effectiveness and consider the role of frailty in assessments of COVID-19 vaccine performance.”

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