USA study clearly shows mRNA vaccines prevent infections: 90%/80% efficacy after 2nd/1st dose

An interim report was published today of a study following essential workers at 6 centers in the USA to assess efficacy of the Moderna or Pfizer vaccines as distributed in the USA population.

The study involved 3980 participants of which 2479 received both doses, 477 received a single dose, and 1024 had not been vaccinated yet. All participants were tested weekly with SARS-CoV-2 PCR tests, so unlike the phase 3 studies, this study can truly study efficacy of the vaccine in preventing infection, not just symptomatic infection.

Here are the key results:

this study showed 90% efficacy after both doses, and >80% efficacy after first dose of vaccination of Moderna or Pfizer vaccine.

The 90% efficacy is slightly lower than the 94-95% seen in the trials for preventing symptomatic disease, but considering this includes preventing asymptotic disease and also including a time in which immune escape variants are in the population, this is very impressive. Also, the 80% efficacy after first dose is excellent, and comforting to those whose second dose is delayed (or to countries purposefully delaying second dose to get more of the population a first dose first).

The fact that these vaccines prevented infection was obvious upon careful thought to anyone looking at the data from the phase 3 trials, but many were confused/thrown off by the disclaimer that "the vaccines haven't been proven to prevent infection" that was mentioned simply because the phase 3 trials did not do weekly testing, but only after reported symptoms.

Just another brick in the wall of validation of clear efficacy of these remarkable vaccines.