Misconceptions about Immunization

Misconception #3:
There are "hot lots" of vaccine that have been associated
with more adverse events and deaths than others. Parents
should find the numbers of these lots and not allow their
children to receive vaccines from them.

This misconception got considerable publicity recently when vaccine safety was the subject of a television news program. First of all, the concept of a "hot lot" of vaccine as it is used in this context is wrong. It is based on the presumption that the more reports to VAERS a vaccine lot is associated with, the more dangerous the vaccine in that lot; and that by consulting a list of the number of reports per lot, a parent can identify vaccine lots to avoid.

This is misleading for two reasons:

1. VAERS (the Vaccine Adverse Events Reporting System) is a system for reporting events that occur after the administration of any vaccine. VAERS reports should not be interpreted to imply causality. In other words, a VAERS report does not mean that the vaccine caused the event. Statistically, a certain number of serious illnesses, even deaths, can be expected to occur by chance alone among children recently vaccinated. Although vaccines are known to cause minor, temporary side effects such as soreness or fever, there is little, if any, evidence linking vaccination with permanent health problems or death. The point is that just because an adverse event has been reported to VAERS does not mean it was caused by a vaccine.

2. Vaccine lots are not the same. The sizes of vaccine lots might vary from several hundred thousand doses to several million, and some are in distribution much longer than others. Naturally a larger lot or one that is in distribution longer will be associated with more adverse events, simply by chance. Also, more coincidental deaths are associated with vaccines given in infancy than later in childhood, since the background death rates for children are highest during the first year of life. So knowing that lot A has been associated with x number of adverse events while lot B has been associated with y number would not necessarily say anything about the relative safety of the two lots, even if the vaccine did cause the events.

Reviewing published lists of "hot lots" will not help parents identify the best or worst vaccines for their children. If the number and type of VAERS reports for a particular vaccine lot suggested that it was associated with more serious adverse events or deaths than are expected by chance, the Food and Drug Administration (FDA) has the legal authority to immediately recall that lot. No vaccine lot in the modern era has been found to be unsafe on the basis of VAERS reports.

All vaccine manufacturing facilities and vaccine products are licensed by the FDA. In addition, every vaccine lot is safety-tested by the manufacturer. The results of these tests are reviewed by FDA, who may repeat some of these tests as an additional protective measure. FDA also inspects vaccine-manufacturing facilities regularly to ensure adherence to manufacturing procedures and product-testing regulations, and reviews the weekly VAERS reports for each lot searching for unusual patterns. FDA would recall a lot of vaccine at the first sign of problems. There is no benefit to either the FDA or the manufacturer in allowing unsafe vaccine to remain on the market. The American public would not tolerate vaccines if they did not have to conform to the most rigorous safety standards. The mere fact is that a vaccine lot is still in distribution indicates that the FDA considers it safe.


This information is adapted from material that the U.S. Centers for Disease Control and Prevention published in 1996 to help physicians reassure their patients.

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This page was posted on November 11, 1997.

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