Clinical trials are rigorous studies that are performed before a new vaccine, drug therapy, or non-drug treatment is approved for widespread use, and they’re conducted under controlled conditions.

The Centers for Disease Control and Prevention (CDC) and other experts are continuing to assess how COVID-19 vaccines work in real-world conditions; however, research continues to illustrate that all COVID-19 vaccines authorized for use in the United States provide protection against COVID-19.

The vaccines have been in use since December, and about 165 million Americans have been fully vaccinated. Federal health officials had enough data as of April to announce that the two mRNA vaccines available in the U.S. are effective in the prevention of symptomatic and asymptomatic infections outside of a controlled environment.* 

Early study confirms clinical trial results

The study, conducted by researchers at the Centers for Disease Control and Prevention (CDC), ran from December 14, 2020, through March 13, 2021. The research team enrolled 3,950 health care workers, first responders, and other high-risk frontline workers, none of whom had previously been infected. 

During the study period:

• 62.8% of participants received both shots of either the Moderna or the Pfizer vaccine. 
• 12.1% received one shot of either.

Participants collected their nasal swabs for PCR testing each week, enabling the researchers to identify asymptomatic, as well as symptomatic, infections. The research team also surveyed participants about symptoms associated with the infection, including:

• Fever
• Chills 
• Cough 
• Shortness of breath 
• Sore throat 
• Diarrhea 
• Muscle aches
• Loss of smell or taste 

They also cross-checked participants’ medical records for illnesses, and results showed that:

• Among the fully vaccinated, there were .04 infections per 1,000 people.
• Among those who’d had one vaccine dose, there were 0.19 per 1,000 people.
• Among those who were unvaccinated, there were 1.38 infections per 1,000 people.
• Of those who developed infections, 58% of the infections were detected before people had symptoms, and 10.2 % of infected people never developed symptoms

These results are consistent with the clinical trial findings that:

• Two doses prevented 90% of infections by two weeks after the second shot.
• One dose prevented 80% of infections by two weeks after vaccination.

Concerns about variants and a fourth surge

New surges of COVID-19 infections, due in large part to the highly transmissible Delta variant, have been occurring in areas of the country with large pockets of unvaccinated populations. The CDC is still urging all eligible Americans to get vaccinated, as symptoms will likely be less severe, even in the event of breakthrough cases. The agency is also once again recommending masks indoors, even for those who have been fully inoculated. 

Even before the Delta variant became the dominant strain,  Dr. Rochelle Walensky, director of the CDC, warned Americans not to be cavalier about the seriousness of COVID-19. She continues to urge people to continue taking precautions and to get vaccinated as soon as they are eligible.

*Kolata, G. (2021, Mar. 29). The Moderna and Pfizer-BioNTech Vaccines are Very Effective in Real-World Conditions at Preventing Infections, the C.D.C. Reported. The New York Times. https://www.nytimes.com/2021/03/29/world/pfizer-moderna-covid-vaccines-infection.html 

Much about the novel coronavirus, i.e., COVID-19, is still not fully understood. As research progresses and our knowledge of the virus increases, information can change rapidly. We strive to update all of our articles as quickly as possible, but there may occasionally be some lag between scientific developments and our revisions.