What do we know about each vaccine’s efficacy?

 

Paul E. Sax, M.D.

Both vaccines are remarkably effective. In large clinical trials (Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine. opens in new tab by L.R. Baden et al., and Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine. opens in new tab by F.P. Polack et al.) that enrolled tens of thousands of people, the vaccines lowered the chance of developing Covid-19 by around 95% as compared with placebo injections. As summarized quite accurately in this XKCD cartoon. opens in new tab, sometimes data are so strong we don’t even have to do a statistical analysis — that’s what happened with both of these clinical trials. The results were that good.

Although we consider data from randomized, placebo-controlled trials to be the strongest form of clinical evidence, additional details make the results even more compelling. First, the vaccines prevented not only any disease due to SARS-CoV-2, but — quite importantly — severe disease. Prevention of severe disease could convert Covid-19 from the global threat it is now into more of a nuisance, like the common cold. Second, the studies enrolled participants who were quite representative of the U.S. population — age, sex, race, and ethnicity all broadly included. Third, while both vaccines are given as two doses, some protection became apparent just 10 to 14 days after the first dose.

The efficacy noted after the first dose has raised questions about whether we should be vaccinating twice as many people with one dose rather than giving people the full two-dose schedule. opens in new tab. However, the 95% vaccine efficacy results come after the second dose, which boosts the immune response and is likely to make it more durable. For now, in the United States, the Centers for Disease Control and Prevention (CDC) and Food and Drug Administration (FDA) recommend proceeding with the two-dose schedule whenever possible.

Overall, these impressive results put the two mRNA vaccines up there with our most effective vaccines to date — more like measles than influenza vaccines, at least in the short term. The protection is far better than anyone anticipated, which is why many of us specialists in infectious diseases, virology, immunology, and public health become downright giddy when asked to discuss the vaccine’s efficacy.

An important caveat to the favorable efficacy results in the clinical trials is that the performance of the vaccines in clinical practice — the effectiveness — may be different. Although the study population in both studies includes people at high risk for Covid-19 complications, participants had to be clinically stable and sufficiently healthy to come in for their study visits. We do not yet know how effective the vaccines will be in a sicker population, such as the frail older people of advanced age living in nursing homes, or the severely immunocompromised. Population-based studies that include hundreds of thousands of people ultimately will give us a better sense of these “real world” data. (Last reviewed/updated on 27 Jan 2021)

https://www.nejm.org/covid-vaccine/faq?fbclid=IwAR2qXCfc_JtouAxbMAHEMAzjVe--mMQ3zaX1GAJvszeroZyWDse8NfPnzKw