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The COVID-19 Vaccine’s ‘Efficacy’ Doesn’t Mean What You Think

February 16, 2021

If you think that, with a 95 percent efficacy, the Pfizer-BioNTech vaccine protects 95 of 100 people vaccinated, you’d be 100 percent wrong.

A vaccine’s efficacy actually refers to the vaccine’s performance in clinical trials. In this case, people who received the Pfizer-BioNTech vaccine had a 95 percent lower risk of getting COVID-19 than people in the control group who were not vaccinated. A vaccine’s effectiveness, meanwhile, measures the vaccine’s real-world performance when distributed to people who did not participate in the trials.

Moderna’s vaccine, with a 94.5 percent efficacy, also produced powerful results during clinical trials. Both vaccines, using messenger RNA (mRNA) technology, were issued an Emergency Use Authorization by the Food and Drug Administration within months of the pandemic’s origins in Wuhan, China.

Already, the available COVID-19 vaccines have been particularly effective at reducing hospitalizations and deaths. And Connecticut’s test positivity rate, reported Feb. 16 at 2.83 percent, is among the lowest in several months. Gov. Ned Lamont said if the rate remains low, the state on March 19 would increase indoor event capacity to 100 people, from the current 25, or 50 percent of the venue’s capacity. Outdoor events would increase to 200 people, from the current 50 limit.

“This vaccine is the equivalent of putting a human being on the moon,” says Dr. Patrick Troy, division chief of pulmonary critical care and sleep medicine at Hartford Hospital. “It’s breathtaking. To give some historical perspective, before the COVID vaccine, the most rapidly developed vaccine was the mumps vaccine in the 1960s. It took four years. The average is more like 10 to 15 years.”

The two-dose measles, mumps and rubella vaccine is 88 percent effective against mumps, 97 percent effective against measles. The seasonal flu vaccine, depending on the year’s flu strain, is typically no more than 60 percent effective and sometimes as low as 40 percent.

Statisticians set vaccine efficacy standards more than century ago as researchers vaccinated some people and gave others a placebo. Then the waiting begins: Which study participants get sick? In the Pfizer-BioNTech trials, 170 of 43,661 people tested positive for COVID-19. Of that number, only eight had received the actual vaccine.

Researchers immediately saw the percentage of unvaccinated people who became infected was much higher than that of vaccinated participants. The difference in those numbers reveals a value called efficacy. So, for example, if placebo and vaccine groups become sick at equal rates, the efficacy is zero. If no one who becomes infected has been vaccinated, the efficacy is 100 percent.

The takeaway from the Pfizer-BioNTech trials: No, the 95 percent efficacy does not mean 5 percent of vaccinated participants got COVID-19. In fact, the actual percentage of vaccinated participants in both the Pfizer-BioNTech and Moderna trials who got COVID-19 was 0.04 percent. So whichever vaccine you get is the right one. The efficacy numbers prove it.

“The best vaccine is the one available in front of you,” says Keith Grant, Senior System Director for Infection Prevention at Hartford HealthCare.