Preventing vaccine failure

Vaccination failure is a failure in giving a particular vaccine to all or part of the target group and it is measured by vaccination coverage, while on the other hand vaccine failure is a failure in the induction of expected immunogenic response after giving the vaccine and it is measured by seroconversion rate. This means that even

90% coverage does not mean that 90% of the target population are protected. For example, if the measles vaccine coverage is 90% and the seroconversion rate is also 90%, this means that only 81% are protected [vaccine coverage (90%) x vaccine seroconversion (90%) = 81%). In another way, almost 20% of the target population are not protected and this means that a big pool of susceptible will be accumulated each year and an outbreak can occur. To remedy this situation, the Ministry of Health has encouraged research in the field of vaccine failure, besides sustaining the effort to assure a high coverage not only for the primary vaccine series but also for booster doses. In 1990, a national cluster survey was conducted to determine vaccination coverage in Saudi children before their first birthday [1]. The results are shown in the table below.

In the same period, a study was conducted to evaluate the seroconversion rate after measles vaccine. At that time, Schwarz measles vaccine was given at 9 months. The seroconversion rate was only 85% [2]. A vaccination trial was performed using Edmonston-Zagreb (E-Z) measles vaccine and seroconversion rate was 95% after E-Z at 6 months with persistent high measles antibody at 15 months [3]. Accordingly, the measles immunization strategy in Saudi Arabia was changed to use E-Z at 6 months with a compulsory second dose given as MMIR at 12 months. For poliomyelitis immunization, a national post-vaccination serosurvey was conducted during the same period and seropositivity after the third dose of oral polio vaccines was 79% (type 1), 88% (type 2) and 65% (type 3) [4]. At that time, OPV was given at 3, 4 and 5 months. Alternative strategies are under investigation.

The interpretation of the seroconversion results should be taken within the context of the epidemiological pattern of the disease, knowing that the protective antibody levels for some dis-eases are not known yet.