Global Vaccine Safety

Six common misconceptions about immunization


"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 often receives considerable publicity. 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 of adverse events 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:

  • Most surveillance systems report events that are temporally associated with receipt of vaccine; these reports should not be interpreted to imply causality. In other words, an adverse report following vaccination 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 by the surveillance system, it does not mean it was caused by a vaccine.
  • 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 for a longer period 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 adverse event reports for a particular vaccine lot suggested that it was associated with more serious adverse events or deaths than are expected by chance, most countries have a system which results in the lot being recalled. All vaccines purchased through the UNICEF vaccine procurement system meet World Health Organization standards for safety and quality of production.

WHO gratefully acknowledges the permission of CDC Atlanta, to present an edited version of "Six common misconceptions about immunization".