Letters To the editor (Dr. Nasreldin Tantawi, Tabuk)

I would like to comment on "Preventing vaccine failure" by Dr. Mohammed Khalil from Suleimania Children's Hospital, Riyadh (Saudi Epidemiology Bulletin, Vol. 1, No. 3). It is still possible to have an occurrence of outbreaks of a vaccine-preventable disease, like measles, despite high vaccine coverage.

First: It is well known that the widespread use of measles vaccine clearly has had a major impact on the number of reported cases of the disease. The success in interrupting measles transmission in some places, coupled with the dramatic success of the smallpox eradication program that was based on vaccination of less than 100% of susceptibles (theory of herd immunity), offered hope of interrupting measles transmission. However, the continued transmission of measles in

many areas despite high levels of vaccination raised doubts about the influence of herd immunity. In 1977, Walter A. Orenstein evaluated the influence of prompt response to epidemics in a series of settings. He concluded that aggressive response to outbreaks was a crucial adjunct to high immunization levels and disease surveillance in stopping transmission.

Second: How should we look at vaccine failure? In fact, the issue of vaccine effectiveness is often raised because a considerable percentage of reported measles cases have a history of adequate vaccination. The answer could easily be obtained from the following mathematical model which Dr. Orenstein and I worked on at the U.S. Centers for Disease Control and Prevention's measles eradication program (indigenous measles):

This model shows that as you increase vaccine coverage, the total number of susceptibles decreases, while the number and proportion of vaccine failure increase. That is why we expect an increasing proportion of measles among vaccines (vaccine failure) as we increase vaccine coverage. This vaccine failure (primary failure) should not raise too much worry. The prime concern should be to raise and maintain high vaccine coverage, set up a strong surveillance system, and provide rapid response to epidemic occurrence.