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Effectiveness of COVID-19 Vaccines BNT162b2 and mRNA-1273 by Days from Vaccination: A Reanalysis of Clinical Trial Data

The phase 3 clinical trial of mRNA COVID-19 vaccines BNT162b2 reported 52% effectiveness from dose-1 to dose-2, and 95% effectiveness after 7 days from dose-2. The phase-3 report commented that dose-1 effectiveness seems to rise after a delay, which raises a need to measure the timing of this rise among mRNA vaccines. 
 
We extracted daily COVID-19 infection data from the phase-3 clinical trial of BNT162b2 and mRNA-1273, carried from July to December 2020. The BNT162b2 was given to 21,720 participants with a second dose at day 21, while mRNA-1273 was given to 15,181 participants with dose-2 at day 28. We calculated vaccine effectiveness on each day from 0 to 111, with a 7-day moving average. We compared the effectiveness to that of the non-mRNA vaccine, ChAdOx. 
 
The BNT162b2 effectiveness at days 7, 14, and 21 was 12%, 80% and 94% respectively, followed by a mean of 94% effectiveness until day 111. The mRNA-1273 effectiveness at days 7, 14, and 21 was 33%, 62%, and 90% respectively, followed by a mean of 95% effectiveness until day 111. The effectiveness of single-dose ChAdOx reached 90% at day 80, while the double-dose reached 90% at day 110. 
 
Our findings indicate that dose-1 of BNT162b2 and mRNA-1273 reached 90% effectiveness by day 21, before dose-2 took effect. Future studies should examine dose-1 effectiveness after day 21 using real-world data from the United Kingdom, since it delayed dose-2 to 84 days. Our finding of increased dose-1 effectiveness against COVID-19 could be used to encourage people to vaccinate, especially those who wait longer to receive dose-2 due to the vaccine shortage. The estimation of vaccine effectiveness by day might assist in predicting a person's risk of contracting COVID-19 during planned, time-sensitive, medical care.