Covid-19 mRNA Vaccines — Six of One, Half a Dozen of the Other

Abstract

In many countries, the availability of vaccines has marked a turning point in the Covid-19 pandemic. Although the vaccines are imperfect, breakthrough infections in fully vaccinated people remain quite rare, even with recently emerging variants. Countries with high vaccination rates have largely been able to reopen, and rates of severe illness and death have dropped dramatically. But this has not been a smooth process. Different vaccines have become available at different times, and access to them has varied markedly from country to country. Thus, the choice of which vaccine to use has been driven in great part by availability rather than by science. In fact, it has not been entirely clear how the vaccines we have compare with one another. Comparing the safety and effectiveness of vaccines is not simple in the absence of head-to-head trials. Data on real-world effectiveness can be subject to many limitations because the treated populations may vary in unanticipated ways. In addition, real-world data often provide relatively imprecise estimates of effectiveness that can be of limited value when highly effective agents are being compared.