Ask the Expert: COVID-19 Vaccine Development


Tim Farrell, MD

Timothy W. Farrell, MD
Professor of Medicine and Geriatrics Division Associate Chief for Age-Friendly Care, University of Utah School of Medicine

You may have questions about how the COVID-19 vaccines were developed. We asked Timothy Farrell, MD, to share his expertise. 

Q. What are vaccine trials? Why should we trust them?

A. Safety has been key to the development and approval process for COVID-19 vaccines.

Clinical trials are research studies that examine new treatments, to see if they are safe and effective. Clinical studies were carefully designed and controlled for the COVID-19 vaccines. Tens of thousands of people participated in the vaccine clinical trials. After the trials were finished, the U.S. Food and Drug Administration (FDA) and independent, expert advisory boards reviewed the clinical trial data to make sure that the data are correct.

Q. Who was included in the vaccine trials?

A. Currently, three vaccines have completed a phase 3 clinical trial in the U.S.  A phase 3 trial is a late-stage trial to determine effectiveness and monitor adverse reactions in participants.

As of April 2022, both the Pfizer-BioNTech and Moderna vaccines have received full FDA authorization. Johnson & Johnson/Janssen (J&J) received FDA Emergency Use Authorization. The FDA issues an emergency use authorization only if the benefits of a vaccine are greater than its risks. Under an emergency use authorization, vaccines are still considered investigational.

The Pfizer-BioNTech COVID-19 vaccine (now also known as Comirnaty) began a phase 3 clinical trial in late July 2020. They enrolled about 44,000 participants worldwide. Approximately 42 percent of the global participants and 30 percent of US participants have racially and ethnically diverse backgrounds. In addition, 41 percent of the global and 45 percent of U.S. participants were 56 to 85 years of age. The results of the study were consistent across the participants’ age, gender, race, and ethnicity. (See the FDA’s Pfizer-BioNTech fact sheet for recipients and caregivers here.)

The Moderna Covid-19 vaccine (now also known as Spikevax) began a phase 3 clinical trial (known as the COVE study) in October 2020. They enrolled about 30,000 US participants ages 18 and older, including people at high risk for severe complications of COVID-19 disease. The COVE study includes more than 7,000 Americans over the age of 65. It also includes more than 5,000 Americans who are under the age of 65 but have high-risk chronic conditions, such as diabetes, severe obesity, and cardiac disease. Those chronic conditions put them at increased risk of severe COVID-19. These medically high-risk groups represent 42 percent of the total participants in the Phase 3 COVE study. The study includes more than 11,000 participants from communities of color. That represents 37 percent of the study population, which is similar to the diversity of the entire U.S. This includes more than 6,000 participants who identify as Hispanic or Latinx, and more than 3,000 participants who identify as Black or African American.

According to the results published in The New England Journal of Medicine, people who received two doses of the Moderna vaccine, taken 28 days apart, reported no major side effects. In people over 65 years of age, the Moderna COVID-19 vaccine was over 86 percent effective against COVID-19.  (See the FDA’s Moderna fact sheet for recipients and caregivers here.)

The Johnson & Johnson/Janssen (J&J) vaccine phase 3 trial, known as the ENSEMBLE trial, enrolled 43,783 adult volunteers in the U.S., Latin America, and South Africa. The global participants were 59% White/Caucasian; 45% Hispanic and/or Latinx; 19% Black/African American; 9% Native American; and 3% Asian. In the United States, the participants were 74% White/Caucasian; 15% Hispanic and/or Latinx; 13% Black/African American; 6% Asian; and 1% Native American.

Forty-one percent of participants in the study had conditions that raised the risk for serious COVID-19 disease, including obesity, type 2 diabetes, high blood pressure, and HIV.  The protection provided by the vaccine was similar across race and age groups, including adults over 60 years of age. (See the FDA’s Johnson & Johnson fact sheet for recipients and caregivers here.)

While all three vaccines are highly effective against getting severe COVID-19., the Pfizer and Moderna vaccines are preferred because they are more effective than the J&J vaccine.

Q. Did the vaccine trials investigate variants?

A. All three vaccine trials investigated how well the vaccines worked to protect people from the variants of COVID-19 that were active at the time. The Delta and Omicron variants had not yet developed when the initial trials were conducted, so they were not studied then. However, scientists have since studied the effectiveness of the vaccines against the Delta and Omicron variants. All three vaccines provide effective protection against serious illness and death from most variants of COVID-19. The Johnson & Johnson vaccine prevents fewer COVID-19 hospitalizations, ICU admissions, and death than mRNA vaccines (Pfizer-BioNTech or Moderna). 


Last Updated May 2022