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  1. FDA Voices

VAERS: A Critical Part of the National Vaccine Safety System

Vaccine Adverse Event Reporting System

By: Peter Marks, M.D., Ph.D., director of the Center for Biologics Evaluation and Research (CBER), FDA and Daniel Jernigan, M.D., MPH, director of the National Center for Emerging and Zoonotic Infectious Diseases, CDC

Monitoring vaccine safety is an important responsibility shared by the Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA). 

And there's no part of America's vaccine safety system more talked about than the Vaccine Adverse Event Reporting System (VAERS). 

Peter Marks
Peter Marks, M.D., Ph.D.

VAERS is an early-warning monitoring system for vaccine safety. It allows patients, pharmaceutical companies, medical personnel and other users to report concerns about medical events that occurred after someone received a vaccination. CDC and FDA experts partner to review reports and, when appropriate, to make changes to clinical recommendations. In some cases, these changes can include pausing or stopping the administration of a vaccine.

What VAERS doesn’t do, though, is tell us whether a vaccine caused a medical issue. That requires investigation.

VAERS has a proven track record of successfully helping to identify safety issues. 

What follows below explains how VAERS works, how successful it's been and addresses some of the common myths about the system.  

Daniel Jernigan
Daniel Jernigan, M.D., MPH

VAERS: A Critical Part of the National Vaccine Safety System 
Vaccines are a cornerstone of modern medicine, preventing debilitating diseases and saving countless lives. As we evaluate new vaccines, safety is one of the most important areas we monitor, because vaccines, like any medical intervention, are not without risks. 

To protect the public, the United States has built a multilayered system that monitors vaccine safety, including the Vaccine Adverse Event Reporting System (VAERS).
VAERS, established in 1990, is a collaborative effort between the Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA). When a potential issue is detected, the system raises the flag for scientific experts to investigate further.

While arguably the most visible piece of America’s vaccine safety surveillance system, VAERS isn’t always well understood. Some critics expect it to do things it isn’t designed to do, while others use the publicly available, unverified information to inaccurately claim that VAERS reports show that vaccines definitively caused certain adverse - or harmful – health outcomes. 

How Does VAERS Work?

Both the CDC and the FDA take every adverse event seriously. We investigate all events that potentially indicate a safety concern. 

VAERS relies on individuals, including healthcare providers, vaccine manufacturers, and the public, to submit reports of adverse events following vaccination. Some of these reported events may be true adverse reactions to a vaccine, while others may not be related. These reports, which range from mild reactions, like soreness and fatigue, to more severe complications, are reviewed by VAERS staff within days to identify potential patterns of concern. 

The fundamental question in vaccine safety is if an event is directly caused by a vaccine. To understand whether an adverse event is related to the administration of a vaccine most often requires careful examination of the facts underlying multiple reports to VAERS. Further studies are often conducted involving comparisons of the rate of an event to rates of the same event in populations that have not been vaccinated, or who have been vaccinated at a different point in time. Since most events that occur after vaccination also occur in people who are not vaccinated, these comparisons are critical to sorting out whether vaccines may have caused a particular event.

Our experts also look carefully to see if there are differences in reported adverse events between what men and women, or between different ethnic groups to determine whether particular populations may be at risk.

After an adverse event is reported, the information is processed and sent to the CDC and the FDA. Physicians and scientists there collaborate on evaluating the reports. In the case of reports related to the COVID-19 vaccines, for example, FDA and CDC experts assessed relevant data from serious reports and shared important findings with each other. (Serious reports are defined by the following outcomes: death, a life-threatening adverse event, inpatient hospitalization or extension of existing hospitalization, a persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions, or a congenital anomaly/birth defect.) 

Scientists perform further analysis using other safety systems such as the CDC's Vaccine Safety Datalink and Clinical Immunization Safety Assessment Project, or in the FDA’s Biologics Effectiveness and Safety system and data obtained in collaboration with the Centers for Medicare & Medicaid Services. In addition to information obtained by VAERS investigators, these systems are better able to assess health risks and shed light on whether the vaccine caused the adverse event. 
During the height of the COVID-19 pandemic, the CDC and the FDA had hundreds of people working on vaccine safety, and serious reports were followed up within five days. This includes reports of serious events that were in the original list identified during clinical trials or new potential safety concerns that warrant further review. The five-day timeline for follow-up remains true to this day.

Not long after COVID-19 vaccines were authorized in late 2020, there was a backlog of reports, due in part to requirements under FDA’s emergency use authorizations that require manufacturers and vaccination providers to report any serious adverse event, regardless of whether it was believed the vaccine caused the event. During that initial time, a backlog of serious reports was cleared within a month and reports of non-serious events were cleared within three months.  

What are the limitations of VAERS? 

Reports sent to VAERS may include incomplete, inaccurate, coincidental, and unverified information. Only after experts have reviewed all the facts and looked at all available data will they make a determination about the cause of the event.

Investigators may or may not reach out to the individual who submits a report to VAERS. VAERS experts may contact health systems or facilities for information related to a report to ensure they have all available information. This has sometimes led to confusion for individuals who have reported serious events, as they assume the ball was dropped. Even though the person who submitted a report may never hear from VAERS, without fail, work is happening to obtain more information about reported life-threatening events.

How has VAERS performed?

VAERS has worked – again and again. 

It has helped identify notable COVID-19 vaccine safety concerns. After VAERS detected an increase in rare, life-threatening allergic reactions just weeks after the first vaccines were administered, for example, the CDC and the FDA provided information and guidance to help prevent and manage them. Just days after VAERS detected that six out of the more than 6 million patients who received the Janssen vaccine had developed blood clots, the CDC and the FDA paused the use of the vaccine to better understand this adverse event. And after detecting myocarditis following the mRNA COVID-19 vaccines, the CDC provided advice to healthcare providers about the potential risk and recommended that some people, primarily teen and young adult males, space out their vaccines. Additionally, the FDA announced revisions to the patient and provider fact sheets to reflect updated information about adverse events, including, for example, information about the suggested increased risks of myocarditis and pericarditis following vaccination.

VAERS is constantly evolving, with ongoing efforts to enhance its data quality and analysis capabilities. 
While not without limitations, VAERS has proven to be vital in detecting both potential and actual safety issues and informing vaccine policy decisions that protect the health of the American public. That’s a job the CDC and the FDA take seriously. We know that failure to detect and communicate a serious health issue related to a vaccine could impact confidence not only in COVID-19 vaccines but in all vaccines. 

Bottom line: VAERS works and has a track record that proves it.

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