Waning of vaccine-induced immunity: is it a problem in Africa?

Strebel et al. misinterpreted the authors' paper on the role of schools in measles transmission. As Strebel et al. noted, the main reason for the outbreak was low vaccine coverage among children aged 5-14 years, together with a marked reduction in the incidence of measles over the past 10 years. Because of the high measles vaccine coverage in younger age groups, many children in Niakhar have gone through their first 5 years of life without being infected with the measles virus. The waning of vaccine-induced immunity has played a role. Strebel et al. believe that there is no indication of waning immunity in the authors' paper and that there is a downward bias in vaccine efficacy due to faulty methodology. Their argument, however, misses the point. The children's ages at vaccination with standard vaccine were completely different in those age groups, with the median age being 295 days for those under age 5 years and 1017 days for those aged 10-14 years. Whether waning immunity will translate into declining vaccine efficacy with age depends upon whether misclassification of vaccination status and measles history is the same in all age groups. Other observations support the existence of waning immunity. The phenomenon of waning vaccine-induced immunity needs to be examined for measles and other vaccine-preventable diseases.