Dear Editor,

Our study aims to compare the short-term adverse effects and their consequences between the RNAm vaccines Commirnaty® (Pfizer) and Spikevax® (Moderna), as well as these short-term adverse effects and their consequences between the first and second doses of these vaccines with each other. Possible long-term adverse effects were not the subject of our study¹.

From the results obtained, it is clear that second doses of both types of mRNA vaccines generate a higher rate of adverse effects than first doses of the same vaccines. Specifically, second doses have an adverse effect for Commirnaty® in 75.64% (95%CI: 73.18-78.10) of those vaccinated and for Spikevax® in 87.85% (95%CI: 81.66-94.04), while first doses for Commirnaty® represented a rate of 67.83% (95%CI: 65.74-69.91) and for Spikevax® 76.10% (95%CI: 71.97-80.23) of those vaccinated.

On the other hand, if one vaccine is compared with the other, the rate of adverse effects of Spikevax® vaccine was higher than that observed for Comirnaty®, both for the first dose OR 1.51 (95%CI: 1.18-1.93) and for the second dose OR 2.32 (95%CI: 1.28-4.22).

The differences in the rate of adverse effects between the two vaccines could be explained by a larger volume of drug injected (0.5 versus 0.3 mL), a higher amount and concentration of mRNA contained in each vaccine (200 μgr/mL versus 100 μgr/mL) (1) or by the different concentration of adjuvants and excipients contained in each vaccine. However, in our study we did not consider the possibility of having had COVID-19 prior to vaccination, since we did not have serology or other laboratory parameters of vaccine recipents that would allow us to identify whether or not they had had COVID-19 prior to vaccination. Considering that having had COVID-19 prior to vaccination may induce greater reactogenicity against the vaccine^2-6, not including this variable in the analysis could lead to biased results. However, the distribution of subjects to receive one or the other vaccine was random and close in time (from January to August 2021) and the overall rate of COVID-19 at the time of administering either vaccine was most probably similar. Therefore, it seems unlikely that the probability of being infected by the SARS-CoV-2 prior to receiving the vaccine was higher for the Spikevax® group compared to the Comirnaty® group, and that could explain the observed differences. Finally, other authors also found a higher reactogenicity after the Spikevax® than with the Comirnaty® vaccine^{(2, 7-9)}.