Pfizer and Moderna Vaccines Are Safe and Effective

Last week, Moderna and Pfizer released initial results of phase 3 trials for their COVID-19 vaccine candidates in the New England Journal of Medicine. Based on these results, both vaccine candidates are safe to use and effective against COVID-19.


Releasing vaccine trial methods and data is a rare occurrence, and the data shows that both companies have achieved their targets for the vaccine - prevent COVID-19 infection after completing the two doses with no serious adverse effects.


Cumulative incidence of COVID-19 in Pfizer vaccine trial. Source: NEJM

Cumulative incidence of COVID-19 in Moderna vaccine trial. Source: NEJM

Both charts show a dramatic difference between the placebo groups and the vaccinated groups after the second dose (21 days after the first dose for Pfizer and 28 days after the first dose for Moderna) Pfizer reports 95% protection against COVID-19 and Moderna reports 94.1% efficacy in preventing COVID-19 (FDA's minimum efficacy is 50%). The Moderna vaccine data also showed that it can prevent severe cases of COVID-19; all severe cases in the Moderna trial were in the placebo group and none in the vaccinated group.


Reported local and systemic reactions to Pfizer vaccine, by age group. Source: NEJM

Reported local and systemic reactions to Moderna vaccine. Source: NEJM

Local reactions (i.e., at the site of injection) and systemic reactions (e.g., fatigue, headaches, muscle aches) were moderate in severity and resolved within 2 days of injection.


The vaccine trials are ongoing and participants will continue to be monitored but with these promising results, the Pfizer and Moderna vaccines have received emergency use approvals by the FDA. Healthcare workers, long-term care facility residents, and frontline workers have received their first doses of the vaccines.