Immunization Issues

Vaccine Adverse Event Reporting System

Updated: Febrero 12, 2009

The Vaccine Adverse Event Reporting System (VAERS) is a national system for reporting any possible adverse reactions after any U.S.-licensed vaccine is administered.1 VAERS is operated by the Centers for Disease Control (CDC) and the Food and Drug Administration (FDA).

VAERS accepts voluntary reports from everyone—physicians, nurses, parents, patients, or anyone else. They are encouraged to report any untoward event that occurs after a vaccination (from a mild fever to hospitalization to death) whether they believe that the vaccination was the cause or not. Vaccine manufacturers are required to report to the FDA all potential adverse events of which they become aware.2

The CDC established Clinical Immunization Safety Assessment centers in 2002 to have experts follow up on the medical records from specific cases of adverse events reported to VAERS. For example, recently researchers followed up on VAERS reports such as transverse myelitis and Guillain-Barré Syndrome (both illnesses of the nervous system) after human papillomavirus vaccination.3

The CDC and FDA use VAERS reports to look for early warning signals. They then generate hypotheses about possible unrecognized adverse events that might be caused by a vaccine that can be tested by epidemiologic studies.

The primary strengths of VAERS include its national scope and its ability to detect rare events in a cost-effective and timely manner. Because VAERS is a centralized reporting system, isolated rare events that would ordinarily escape attention may become apparent.

For example, the VAERS system rapidly detected unexpected cases of intussusception associated with the first rotavirus vaccine (Rotashield) in 1999. After the signal was discovered, intussusception occurring after the vaccine was then examined using the Vaccine Safety Datalink. It was determined to have occurred at a rate of about 1 in 10,000 children after the first dose of vaccine. The recommendation to use this vaccine was then withdrawn.1

Detection of increases or decreases in known adverse events is another important goal of vaccine safety surveillance. For example, VAERS documented the disappearance of vaccine-associated paralytic polio after the recommendation to switch from the oral polio vaccine to the inactivated poliovirus vaccine; there have been no cases reported since 1997.2

However, when analyzing data from VAERS, researchers, the public, and the media need to take into account the system’s limitations. The information in a VAERS report is not necessarily complete, nor is it checked for its accuracy in most cases. Reports of particular interest, including all deaths and hospitalizations, are followed up by epidemiologists at the CDC or FDA. Sometimes follow-up of these reports reveals errors in the information, such as reports of events that occurred before the immunization. Also, in 24% of VAERS reports the causes of death on follow-up were different from what was originally reported. In one case a vaccinee that had been reported dead on the VAERS form, had in fact not died.2

One wrong assumption when interpreting VAERS data is that all VAERS reports represent side effects caused by vaccines. Rather, it is a system designed to look for adverse events—things that occurred about the same time as vaccination—that then may require additional investigation to see if in fact there is an association with vaccine administration.

In some media reports and on some Web sites, VAERS reports are misrepresented as verified cases of vaccine-caused injuries.

Whereas some events reported to VAERS are likely caused by vaccines (for example, injection-site swelling), others may be related to an underlying disease or condition, or due to drugs being taken—or it may be something that occurred simply by chance shortly after the vaccine was administered.

Adverse events are reported to VAERS in a biased manner. Because VAERS reporting is voluntary, reports are more likely to be submitted for those untoward events that the observer considers more likely to have been caused by the vaccine.

The number of reported adverse events is also influenced by publicity. Thus reports to VAERS of a particular adverse event may be the result of parents or physicians having heard about it in the media, independent of any possible causal relationship between the vaccine and the event in question. For example, in Canada—which has a similar reporting system—media reports of a mild allergic-type reaction after influenza vaccine resulted in a 3- to 5-fold increase in reports of all types of adverse events following influenza immunization compared with the previous year.2

In spite of its many limitations, VAERS data have proven to be very useful in ongoing vaccine safety monitoring, especially looking for signals of unexpected or rare adverse events. People trying to use this data to perform causation studies fail to understand the limits of the system, however, and often arrive at unwarranted conclusions.