Vaccine Safety: Six Common Misconceptions : Part 2

by CDC

(Page 2 of 4)

2. The majority of people who get disease have been vaccinated.

This is another argument frequently found in anti-vaccine literature - the implication being that this proves vaccines are not effective. In fact it is true that in an outbreak those who have been vaccinated often outnumber those who have not - even with vaccines such as measles, which we know to be about 98% effective when used as recommended.

This apparent paradox is explained by two factors. First, no vaccine is 100% effective. To make vaccines safer than the disease, the bacteria or virus is killed or weakened (attenuated). For reasons related to the individual, not all vaccinated persons develop immunity. Most routine childhood vaccines are effective for 85% to 95% of recipients. Second, in a country such as the United States the people who have been vaccinated vastly outnumber those who have not. How these two factors work together to result in outbreaks in which the majority of cases have been vaccinated can be more easily understood by looking at a hypothetical example:

In a high school of 1,000 students, none has ever had measles. All but 5 of the students have had two doses of measles vaccine, and so are fully immunized. The entire student body is exposed to measles, and every susceptible student becomes infected. The 5 unvaccinated students will be infected, of course. But of the 995 who have been vaccinated, we would expect several not to respond to the vaccine. The efficacy rate for two doses of measles vaccine can be as high as >99%. In this class, 7 students do not respond, and they, too, become infected. Therefore 7 of 12, or about 58%, of the cases occur in students who have been fully vaccinated.

As you can see, this doesn't prove the vaccine didn't work - only that most of the children in the class had been vaccinated, so those who were vaccinated and did not respond outnumbered those who had not been vaccinated. Looking at it another way, 100% of the children who had not been vaccinated got measles, compared with less than 1% of those who had been vaccinated. Measles vaccine protected most of the class; if nobody in the class had been vaccinated, there would probably have been 1,000 cases of measles.

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:

A report made to VAERS does not mean that the vaccine, or other vaccines from the same group or lot caused the event. VAERS is a national system for reporting health problems that happen around the same time of the vaccination. Only some of the reported health conditions are side effects related to vaccines. A certain number of VAERS reports of serious illnesses or death do occur by chance alone among persons who have been recently vaccinated.

VAERS reports have many limitations since they often lack important information, such as laboratory results, used to establish a true association with the vaccine. For all serious and other clinically significant events (life-threatening events, hospitalization, permanent disability, death), follow-up with the health care provider and/or the parent or vaccinated individual is conducted in an attempt to collect supplemental information on the reports. Because of the limitations of this type of reporting system, causality is difficult to determine. Regardless of the cause, VAERS is interested in hearing about any health concerns that happen around the time of vaccination. In summary, scientists are not able to identify a problem with a vaccine lot based on VAERS reports alone without scientific analysis of other factors and data.

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. To date, no vaccine lot in the modern era has been found to be unsafe on the basis of VAERS reports.

About the Author

www.cdc.gov
The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.