As COVID-19 vaccine distribution begins, many people are asking how likely it is that the vaccines will protect them from coronavirus infection. The Pfizer/BioNTech COVID-19 vaccine is being described as “95 percent effective,” and the Moderna vaccine is reported to be “94-95 percent effective." In order to fully interpret these figures, it is first important to understand the difference between a vaccine’s “effectiveness” versus “efficacy.” Anthony Fauci, the most famous and respected infectious disease specialist in the US, describes it in this way: “efficacy is the result of a trial under controlled conditions; effectiveness is how the vaccine impacts the population that you ultimately want to impact. So we have a highly, highly efficacious, vaccines, plural. How effective they will be on society will be determined by a number of factors including how many people decide they want to get vaccinated.” At this stage, it is more accurate to describe the vaccines as “95 percent efficacious.”

The difference in terminology often goes unrecognized, but it is more than semantics. Differentiating trial results from real-world clinical outcomes serves as an important reminder that there are many potential unknown variables that can impact the outcome of a vaccination plan. “There are several steps that must occur for an efficacious intervention to be effective in clinical practice; therefore, an efficacy trial can often overestimate an intervention's effect when implemented in clinical practice,” said one review. 

In order for a vaccine to be effective at the population level, human behavior, health policy, and medical systems must come together to ensure that individuals actually get the vaccine, including all required doses, that the vaccine is distributed widely and to diverse populations, and that economic and social barriers do not prevent people from receiving it. There are also unknowns related to populations that were not tested in clinical trials; for the Pfizer vaccine, this includes younger adolescents, children, and pregnant people.

The clinical trial results for the Pfizer vaccine published in the New England Journal of Medicine explain that 43,548 participants (people at least 16 years of age in multiple countries) were randomized to receive either the vaccine or placebo.  The vaccine showed similar efficacy results across subgroups defined by “age, sex, race, ethnicity, baseline body-mass index, and the presence of coexisting conditions.” 

The efficacy varied depending on whether the full two doses were given, since some individuals contracted COVID-19 in the period between doses, indicating a vaccine efficacy of 52 percent during this interval. FDA guidelines from June said that the efficacy estimate for a placebo-controlled trial should be at least 50 percent. In comparison, a result of 95 percent efficacious after both doses is very promising and a much better result than many researchers had expected.