Modeling the impact of subclinical measles transmission in vaccinated populations with waning immunity

An increasing body of evidence suggests that a substantial proportion of in dividuals who respond to measles vaccine display an antibody boost accompan ied by mild or no symptoms on exposure to wild virus. It is unknown whether this emerging class of individuals can support transmission. The epidemiol ogic consequences of vaccinated individuals able to transmit virus are inve stigated using a mathematical model. Parameters for this model are estimate d using regression analysis on a Canadian serologic data set. The authors c onfirm that neutralizing antibodies are decaying significantly in absence o f circulating virus. Based on a protective threshold plaque reduction neutr alization (PRN) titer of 120, the authors estimate the mean duration of vac cine-induced protection in absence of reexposure to be 25 years (95% confid ence interval (CI) 18, 48). After long-term absence of circulating virus, t he mathematical model predicts that 80% (95% CI 65, 91) of all seroconverte d vaccinees have titers below the protective threshold. In this case, elimi nation of measles virus cannot be achieved by a single-dose routine vaccina tion strategy if the basic reproduction number in vaccinated individuals ex ceeds 1.24 (95% CI 1.10, 1.53). For this reason, there is a need to establi sh the intensity and duration of infectiousness in vaccinated individuals.