In the early days of the COVID-19 mRNA vaccine rollout in the USA this year, vaccines were in short supply and people had no choice as to which vaccine to try.  Both two-dose vaccines from Pfizer and Moderna seems to have similar 95% efficacy against serious disease.  Recent studies are revealing significant differences between the Pfizer and Moderna vaccines. 

Moderna has nearly 3X the amount of active ingredients in Pfizer: 100mg vs 30mg.  Moderna created 2.6X more antibodies in patients than Pfizer – with both being negatively correlated with patient age and both declining exponentially over time.  The higher amount of initial antibodies allow Moderna to be more effective and durable against the Delta variant than Pfizer: the risk of a breakthrough case was 2X lower.  While Pfizer is recommending a booster 6 months after the second shot, Moderna is recommending a booster before this winter.  Moderna’s proposed booster has only 50 mg of active ingredients, but it is still proving to be highly effective in phase 2 clinical trials to date.   

As for side effects, a greater percentage of participants who received the Moderna vaccine reported reactogenicity: 82% vs 69% for Pfizer.  Many people (13%) taking Pfizer reported side effects with both doses that sidelined them for 1-2 days.  A small percentage (0.6%) reported symptoms severe enough to require a visit to the ER, and only 0.25% required hospitalization.  It is believed that serious side effects with Moderna are also <1%

If you are immunocompromised and probably did not get full protection from two shots – get your booster shot now unless you had severe side effects.  If you’ve had severe side effects after the second Moderna wait a few weeks for the 50 mg Moderna booster.  Otherwise, on September 20th, all Americans should be eligible for a booster shot.  Get it to improve your protection against severe COVID-19 illness.  Always mask up and social distance for additional layers of protection.