Updated: Apr 4
By Dena Bunis, AARP, March 29, 2021
New CDC data confirms effectiveness of mRNA shots at preventing coronavirus infections.
The Pfizer-BioNTech and Moderna COVID-19 vaccines are working in the real world, according to a new federal study of 3,950 health care and essential workers that confirms both vaccines provide 90 percent protection against coronavirus infections two weeks after the second dose.
The Centers for Disease Control and Prevention (CDC) findings mirror how effective the vaccine manufacturers said their products were based on extensive clinical trials. The U.S. Food and Drug Administration (FDA) used the data from the clinical trials when it granted emergency use authorizations (EUA) to both vaccines, which were developed using mRNA technology, in December 2020.
CDC conducted the study by following health care workers, first responders and other essential workers in parts of six states (Arizona, Florida, Minnesota, Oregon, Texas and Utah). The participants in the study tested themselves weekly from December 14, 2020, though March 13, 2021. CDC officials said they were more likely than the general population to be exposed to the virus because of the work they do.
The study also showed that even after one dose, these vaccines offer substantial protection. Both were 80 percent effective two weeks after the first shot. The study was able to detect evidence of any coronavirus infection, whether or not the participants had any symptoms.
Vaccines protect against spread
Bruce Hirsch, an infectious disease specialist and attending physician with Northwell Health, the largest health care system in New York, said this study looked at an important question that was not considered by the clinical trials: whether the vaccines would not only prevent someone getting the shot from becoming infected but also keep them from spreading the infection to others.
"This study says that that is extremely unusual,” Hirsch said, “and that the vaccines seem to prevent not only symptomatic infection but also are very effective at preventing asymptomatic infection.” Hirsch said the self-administered, weekly nasal swabs were sent for PCR testing, which is the most sensitive test for COVID-19. The study importantly shows, he said, that the vaccines do a good job of preventing people from spreading COVID-19 without being aware they are even infected.
The results of the CDC real world study reflect similar findings from studies in Israel, the United Kingdom and of health care workers followed at the University of Texas Southwestern Medical Center, UCLA and UC San Diego in California.
Hirsch, who called the study “gratifying,” cautioned that the CDC findings shouldn't be seen as a license to stop maintaining such precautions as wearing masks or hand-washing and social distancing. “Turning off this epidemic is not like turning off a light switch,” Hirsch said. “The vaccines are very, very effective and very, very helpful. But they are not perfect.” And he pointed out that while the health care workers and other study participants were tested after variants of the virus began to surface in the United States, there continue to be new strains that are still concerning.
"What I hope is that this study is a confirmation that taking a vaccine is the right thing to do and will be very helpful,” Hirsch said, adding that the vaccines still need to be used in combination with all the other strategies that will help shut the pandemic down.
Dena Bunis covers Medicare, health care, health policy and Congress. She also writes the Medicare Made Easy column for the AARP Bulletin. An award-winning journalist, Bunis spent decades working for metropolitan daily newspapers, including as Washington bureau chief for the Orange County Register and as a health policy and workplace writer for Newsday.