PULSE MASS MEASLES VACCINATION ACROSS AGE COHORTS

Although vaccines against measles have been routinely applied over a q uarter of a century, measles is still persistent in Israel, with major epidemics roughly every 5 years. Recent serological analyses have sho wn that only 85% of Israelis aged 18 years have anti-measles IgG antib odies. Considering the high transmissibility of the virus and the high level of herd immunity required for disease eradication, the Israeli vaccination policy against measles is now being reevaluated. Motivated by theoretical studies of populations in perturbed environments, we e xamined the possibility of replacing the conventional cohort vaccinati on strategy by a pulse strategy-i.e., periodic vaccination of several age cohorts at the same time. Numerical studies of a deterministic age -structured model suggest that vaccination, which renders immunity to no more than 85% of the susceptible children aged 1-7 years, once ever y 5 years will suffice to prevent epidemics in Israel, where infection rate is highest amongst schoolchildren. The model suggests that by us ing such a strategy the density of susceptible individuals is always k ept below the threshold above which recurrent epidemics will be mainta ined. Analysis of simpler, non-age-structured, models serves to clarif y the basic properties of the proposed strategy. Our theoretical resul ts indicate that the advantages and disadvantages of a pulse strategy should be seriously examined in Israel and in countries with similar p atterns of measles virus transmission.