CDC Study Confirms That COVID-19 Vaccines Block Transmission In the Real World

Receiving a vaccine wearing a face mask.

Xavier Lorenzo / Getty Images

Key Takeaways

  • A new CDC study found that the Pfizer and Moderna vaccines prevented COVID-19 transmission among healthcare personnel, essential workers, and first responders. 
  • The results revealed that two weeks after the second dose of the vaccine, participants’ risk of infection was reduced by 90%. 
  • The study helps to quell fears that the efficacy of the vaccines wouldn't hold up outside of clinical trials.

One in four U.S. adults is now fully vaccinated against COVID-19, inching the nation closer to reaching herd immunity. Now, experts are looking to see how the vaccines are holding up in real-world settings outside of clinical trials.

Last week, the Centers for Disease Control and Prevention (CDC) released data from a real-world study confirming that the Moderna and Pfizer-BioNTech vaccines worked at preventing infections in real-world settings (outside of the lab) where healthcare personnel, essential workers, and first responders work.

This data provides a more holistic picture of the vaccines' effectiveness in practice, despite real-world variables and circling variants.

Cutting Transmission

From December 14, 2020, to March 13, 2021, researchers observed the efficacy of the Pfizer and Moderna vaccines among 3,950 participants by having each individual self-collect nasal swabs for COVID-19 testing each week. They aimed to examine vaccine efficacy against infection, including asymptomatic infections.

The study revealed that two or more weeks after the second dose, participants’ risk of infection dropped by a whopping 90%. The CDC researchers also found that 80% of participants in the U.S were protected against COVID-19 after just one dose. Similar studies conducted in the United Kingdom and Israel—the world’s leading vaccinated country—showed that one dose of two-dose vaccines yielded 60 to 70% effectiveness against COVID-19.

"To have two vaccines that prevented 90% of infections by two weeks after the second shot and to have data that indicates a single dose prevented 80% of infections by two weeks after vaccination is truly remarkable,” Javeed Siddiqui, MD, MPH, infectious disease physician practicing in Northern California, tells Verywell. “This data provides additional information to further support the efficacy of these vaccines and trust for the public that science has developed a highly effective vaccine.” 

The researchers also found that only a small number—10.7%—of infections in the study were asymptomatic, demonstrating, according to the CDC, that "these two mRNA vaccines can reduce the risk of all SARS-CoV-2 infections, not just symptomatic infections."

“[The study] paints a picture of safer communities in the very near future, so while persons are eager to get back to unshuttered lives now, we can do so much more safely in the coming weeks to months with higher percentages of us vaccinated,” Kathleen Jordan, MD, infectious disease specialist in California and senior vice president at Tia, tells Verywell. 

The study's findings will eventually inform policies and guidelines on what is considered safe among those fully vaccinated, according to Jordan. “The CDC study confirms what we have been seeing and provides us scientific evidence to make decisions about what is safe or not in our communities,” she says. “While I advise persons to get vaccinated for their own best health, this study also confirms that another compelling reason to get vaccinated is to protect those around you,” Jordan says. 

What This Means For You

The COVID-19 vaccines offer the best protection against COVID-19 infection and transmission. To search for a vaccine, visit VaccineFinder or visit your local public health department’s website for instructions on how to register in your state. Everyone 18 and older will be eligible to receive the COVID-19 vaccine by April 19, and in some states everyone 16 and older will be eligible by that date.

The Vaccines Are Holding Up in Real-World Settings

Jordan notes that one of the best advantages of this data is the study's large sample size. “This study was large enough to show a significant difference and supports the expectation that we should see a decline in COVID-19 cases as vaccinations arise,” Jordan explains. 

She continues to say that time will only tell whether there will be a decline in infections as the percentage of people vaccinated rises. “We should be able to see that in the coming weeks to months,” Jordan says. 

When data from the phase three COVID-19 vaccine clinical trials were released, Siddiqui shared that individuals expressed concern on how the vaccines would hold up in real-world settings. “Now we have an additional study, a real-world study that demonstrates similar efficacy,” Siddiqui says.

Jordan hopes this study will help break through some of the concerns people are expressing about getting vaccinated. “The CDC study confirms what we have been seeing in our vaccinated people: those that are vaccinated are both staying healthy and now they are also not carriers of the virus or causing outbreaks—even with high-risk positions with exposures to SARS Cov-2,” she says.

“We are in the fight for our lives. We need to use every tool at our disposal. Vaccination is a critically important tool that needs to be utilized,” Siddiqui adds. “This is not a wait-and-see moment, this is a get vaccinated as soon as possible moment. Please get vaccinated!” 

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. COVID-19 vaccinations in the United States.

  2. Centers for Disease Control and Prevention. CDC real-world study confirms protective benefits of mRNA COVID-19 vaccines.

  3. Associated Press. Biden makes all adults eligible for a vaccine on April 19.

By Kayla Hui, MPH
Kayla Hui, MPH is the health and wellness ecommerce writer at Verywell Health.She earned her master's degree in public health from the Boston University School of Public Health and BA from the University of Wisconsin-Madison.