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June 15, 2021

How do we know COVID-19 vaccines are safe?

The answers on this site were developed with and vetted by the CDC

Millions of people in the United States, including 96% of medical doctors, have received COVID-19 vaccines, and these vaccines have undergone the most intensive safety monitoring in U.S. history.


Vaccines are authorized by the U.S. Food and Drug Administration (FDA), which sets strict standards for clinical trials and rigorously evaluates scientific data submitted by vaccine developers. Once vaccines are made available to the public, the FDA continues to monitor vaccines very closely for safety.

Researchers began developing vaccines for COVID-19 more than a year ago in January 2020, based on decades of understanding immune response and how vaccines work. Thousands of volunteers participated in clinical trials that started that spring, making sure we can trust the vaccines to be safe and effective.

Based on the results, the FDA has authorized multiple vaccines for public use. In December 2020, the FDA authorized two versions of COVID-19 vaccines (Moderna and Pfizer-BioNTech) for the American public. A third (Johnson & Johnson) was added in February 2021 and is currently available for use in the United States after a pause in April 2021. Doctors and medical experts with many years of experience regulating vaccines evaluated information about the safety, effectiveness, and manufacturing quality of the vaccines prior to making their decision.

After a vaccine is authorized by the FDA and made available to the public, experts continue to closely monitor the vaccines for ongoing safety and to help us learn more about questions like how long vaccines will provide protection.

Ted Obi -- What’s A Clinical Trial | COVID-19 Vaccine Education Initiative | Ad Council
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How were the vaccines made so quickly?

The science behind the breakthrough had a head start. Researchers had already made progress developing vaccines for other types of coronaviruses: they applied lessons learned after the 2003 SARS epidemic and the 2012 MERS outbreak. They also learned a lot from creating a vaccine for Ebola — which isn’t a coronavirus but has taught us more about viruses.

The rapid spread of COVID-19 made developing these vaccines an international priority, unlocking billions of dollars in funding to ensure safety while moving with urgency to save lives.

Many researchers and medical experts have come together to develop the vaccine while still meeting the FDA’s rigorous requirements for safety and effectiveness. While regulators have streamlined some steps in the vaccine authorization process, the vaccines still needed to meet the agency’s rigorous, scientific standards for safety, effectiveness, and manufacturing quality.

After a vaccine is authorized by the FDA and made available to the public, experts continue to keep track of data to help ensure ongoing safety and help us learn more about questions like whether vaccinated people can still get infected without having symptoms.

How the FDA evaluated safety data (FDA)
How are vaccines tested for safety?

Every vaccine must go through rigorous testing and inspection to ensure it is safe.

Vaccines for COVID-19 followed a 3-phase process where there are several stages required before FDA authorization:

Phase 1: The vaccine is tested in a small number of generally healthy adults, usually between 20 and 80 people. It’s evaluated for safety, dosage, and any side effects. Experts also look at what type of immune response is created.

Phase 2: If there are no safety concerns from Phase I studies, the vaccine is given in various dosages to hundreds of adults who may have a variety of health issues and come from different backgrounds to make sure it is safe. These studies provide additional safety information on common short-term side effects and risks, examine the relationship between the dose given and the immune response, and may provide initial information regarding the effectiveness of the vaccine.

Phase 3: Experts broaden the study to include thousands of adults, from a variety of ages and backgrounds. They see how many people who got the vaccine were protected from the disease, compared to those who received a placebo.

After a vaccine is authorized by the FDA and made available to the public, FDA continues to monitor its safety very closely. FDA also continues to oversee the production of the vaccine, including periodic facility inspections, (such as periodic facility inspections) to ensure continuing safety.

Vaccine testing and approval process (CDC)
Did the clinical trials include people like me?

Researchers made sure that the trials included adults of diverse backgrounds, races, ethnicities, and geographic areas. They collaborated with faith leaders, community organizations, and health clinics to reach volunteers from many different walks of life across the United States.

Medical experts and doctors want to make sure the vaccines work safely and effectively for as many people as possible. People may respond differently to vaccines based on factors like age, gender, and health conditions — so it is important to have a diverse group of participants in clinical trials.

COVID-19 has hit hard in the Black and Hispanic communities. Historically, these populations haven’t always been included in clinical research, but with COVID-19 vaccines researchers made sure volunteers included people of color, as well as people over the age of 65 who are at higher risk of complications from the virus.

About COVID-19 vaccine trials (Duke Health)
Dr. Bright – Clinical Trials
How did the COVID-19 vaccines get authorized?

Vaccines are authorized by the FDA, which sets strict standards for clinical trials and rigorously evaluates scientific data submitted by vaccine developers.

In December 2020, the FDA authorized two versions of COVID-19 vaccines (Moderna and Pfizer-BioNTech) for the American public. A third (Johnson and Johnson/Janssen)was added in February 2021.

Doctors and medical experts with many years of experience regulating vaccines evaluated information about the safety, effectiveness, and manufacturing quality of the vaccines prior to making their decision. They determined that the known and potential benefits of the vaccines outweigh any known or potential risks.

After a vaccine is authorized by the FDA and made available to the public, experts continue to keep track of data to help ensure safety and help us learn more about questions like whether vaccinated people can still get infected without having symptoms.

How emergency use authorization works (FDA)
Do vaccines impact fertility?

There is currently no evidence that any vaccines, including COVID-19 vaccines, cause fertility problems. If you are trying to become pregnant now or want to get pregnant in the future, you may receive a COVID-19 vaccine when one is available to you. Like with all vaccines, medical experts are studying COVID-19 vaccines carefully for side effects and will report findings as they become available.

Vaccine recommendations for people who are pregnant or breastfeeding