Nearly 4,000 health care workers, police, firefighters and other essential workers participated in a 13-week study run by the Centers for Disease Control and Prevention to measure the effectiveness of Covid-19 vaccines under real-world conditions.
The study, published on Monday in the Morbidity and Mortality Weekly Report, shows that Moderna and Pfizer’s mRNA-based vaccines offer 90 percent effectiveness at preventing coronavirus infections—not just symptoms—two weeks after the second dose. In other words, the vaccinated group of participants saw 90 percent fewer cases than if they had not been vaccinated, according to the CDC.
The new study differs from clinical trials because the participants who received the vaccine knew that they were vaccinated. The participants were tested for Covid-19 each week, which allowed the researchers to spot infections even if they were asymptomatic or mild. The participants also held jobs with the highest risk of exposure to the coronavirus that causes Covid-19, and the 13-week study from mid-December to March included the winter surge.
Overall, experts found the results encouraging.
“It’s not surprising, but it’s incredibly reassuring,” says vaccine expert Paul A. Offit of the Children’s Hospital of Philadelphia, who was not involved in the CDC study, to Lena H. Sun at the Washington Post. “It’s yet another reason to get vaccinated.”
The study worked with 3,950 participants, most of whom were white and between 18 and 49 years old, from across six states. Half of the participants worked in health care, while about 20 percent worked as first responders, and the remainder worked in other frontline jobs like teaching, delivery and retail.
Out of 2,479 people who were at least two weeks out from their final dose of the vaccine, just three tested positive for the virus.
During the 13-week study, nearly 75 percent of participants received at least one dose of the Moderna or Pfizer vaccines, writes Helen Branswell for STAT News. About 63 percent received the Pfizer vaccine, 30 percent received the Moderna vaccine and five people received the Johnson & Johnson vaccine, which requires one dose. The researchers are now trying to verify what vaccine the remaining participants received, per the Washington Post.
To measure the effectiveness of the vaccine—which is different from efficacy, a measure of how the vaccine works in clinical trials—the participants swabbed their noses each week and sent the sample to a lab in Marshfield, Wisconsin.
Out of 994 people who were not vaccinated during the study, 161 developed coronavirus infections. In eight cases, people who had received one dose of the vaccine became infected.
“The evidence base for (currently available) COVID-19 vaccines is already strong, and continues to mount ever higher with studies like this one,” writes David Holtgrave, the dean of the University at Albany’s School of Public Health, to the Associated Press’ Mike Stobbe in an email.
The three post-vaccination infections are called breakthrough cases. While they are extremely rare, they are a reminder that people who are vaccinated are not entirely invincible. Two other studies, conducted at the University of Texas Southwestern Medical Center and at health centers run by the University of California, also found fewer than ten breakthrough cases for over 8,000 and over 14,000 vaccinated workers, respectively, Denise Grady reported for the New York Times last week. Many of the breakthrough cases were mild or didn’t involve symptoms of Covid-19, and were identified serendipitously through routine testing, which suggests that the vaccine is effective at preventing severe disease.
University of California San Diego Health infectious disease specialist Francesca J. Torriani, who led the UC study, told the New York Times that the results show the importance of keeping mask-wearing and social distancing guidelines in place. Torriani said, “These measures have to continue until a larger segment of the population is vaccinated.”