Medically reviewed in April 2021
Updated on March 31, 2021
Additional reporting by Veronica Hackethal, MD.
Under real-world conditions, the mRNA COVID-19 vaccines developed by Pfizer and Moderna are 90 percent effective in preventing the disease, according to data released on March 29 by the Centers for Disease Control and Prevention (CDC).
The CDC study followed 3,950 health care workers, first responders and other frontline employees in eight locations across the United States as the initial doses of the COVID-19 vaccines were rolled out in December 2020.
The participants were tested on a weekly basis for COVID-19—even if they didn’t have any symptoms of the infection.
The study found that at least 14 days after the first dose, the vaccines were 80 percent effective. And at least two weeks after the second dose, they were 90 percent effective.
Why have the numbers changed?
Pfizer announced the first set of complete results from their ongoing coronavirus vaccine trial in November 2020. The results suggested their two-dose mRNA vaccine, which was developed in partnership with BioNTech, is 95 percent effective against COVID-19, starting 28 days after the first dose.
The late-stage clinical trial results were based on an evaluation of 170 confirmed cases of COVID-19 with 162 observed in the placebo group compared to 8 in the vaccine group.
Just days later, Moderna released its Phase 3 trial results, which found that its mRNA vaccine is 94.1 percent effective against COVID-19.
For Moderna's Phase 3 trial, about 15,000 volunteers were given a placebo, or a shot of saline. Another roughly 15,000 volunteers received the experimental vaccine.
In short, the proportion of symptomatic coronavirus cases fell by 95 percent among those who received the Pfizer vaccine compared to the placebo group. For the Moderna vaccine, the proportion of new, symptomatic COVID-19 cases was decreased by 94.1 percent for vaccinated people compared to the placebo group.
So, what’s the difference between these Phase 3 results and the real-world data?
Trial efficacy vs. real-world effectiveness
Researchers define efficacy as how well a new medical treatment performs under ideal or controlled conditions, like a clinical trial. It’s a rate that’s calculated using a formula. In human trials, it’s the percent reduction in new, symptomatic COVID-19 cases among vaccinated people, compared to people who got the placebo.
“These numbers only refer to the percent reduction in COVID cases between placebo and vaccinated. They do not translate into your chances of getting COVID if you’ve been vaccinated,” explains Namandje Bumpus, PhD, professor of pharmacology and molecular sciences at the Johns Hopkins University School of Medicine. “They really refer to the clinical trial specifically, not the real world.”
On the other hand, the effectiveness of a new medical treatment describes how well it actually performs in the real world.
Why the CDC study is important
The CDC study included a total of 2,961 people who received at least one shot (including five people who got the Johnson & Johnson vaccine) and 989 unvaccinated people. Overall, researched confirmed a total of 205 cases of COVID-19. Of these, 161 involved unvaccinated people, eight infections were reported in partially immunized people and three fully vaccinated people were diagnosed with the infection. The researchers noted that 33 cases were excluded from the findings because their immune status couldn’t be verified.
Among the confirmed COVID-19 cases, only two people were hospitalized, and no deaths were reported.
Overall, these real-world results are not very different from the clinical trials. But this data is needed to show how well and safely these vaccines work outside of tightly controlled conditions.
As more people are vaccinated, this data will continue to be collected, providing a better idea of how well these vaccines protect people over time.
