International measles incidence and immunization coverage.

Measles is exquisitely sensitive to immunization programs. We investigated the decline in measles incidence after immunization with 1 or 2 doses of measles-containing vaccine (MCV), with or without supplementary immunization activities (SIAs). Using data from the World Health Organization, we modeled the impact of measles immunization using a negative binomial regression model. All countries offer measles immunization, and 192 of 193 countries offer a second dose of MCV (MCV2), using either a routine second dose, SIAs, or both. The incidence of measles fell from a median of 70.9 cases/100,000/year when coverage with a first dose of MCV (MCV1) was in the range of 0%-39% to a median of .9 cases/100,000/year when MCV1 coverage was 90%-100%, in both cases with no MCV2. Further reductions followed the introduction of MCV2 and SIAs. Modeling showed that each 1% increase in MCV1 coverage was followed by a 2.0% decrease in incidence in the same and following years (95% confidence interval [CI], 2.0%-1.9%, and 2.1%-1.9%, respectively). For a second dose, a rise of 1% in MCV2 coverage was followed by a decrease in measles incidence by .4% (95% CI, .3%-.5%) in the same year and .3% (95% CI, .2%-.5%) in the following year. SIAs were followed by decreases of measles incidence by 40.3% (95% CI, 46.3%-33.8%) in the same year and 45.2% (95% CI, 51.1%-48.7%) in the following year. A herd immunity effect was demonstrated with MCV1 coverage of >80%, and SIAs are an extraordinarily effective strategy for measles control.