Assessment of the Status of Measles Elimination in the United States, 2001-2014

Abstract

We assessed the status of measles elimination in the United States using outbreak notification data. Measles transmissibility was assessed by estimation of the reproduction number, R, the average number of secondary cases per infection, using 4 methods; elimination requires maintaining R at <1. Method 1 estimates R as 1 minus the proportion of cases that are imported. Methods 2 and 3 estimate R by fitting a model of the spread of infection to data on the sizes and generations of chains of transmission, respectively. Method 4 assesses transmissibility before public health interventions, by estimating R for the case with the earliest symptom onset in each cluster (Rindex). During 2001-2014, R and Rindex estimates obtained using methods 1-4 were 0.72 (95% confidence interval (CI): 0.68, 0.76), 0.66 (95% CI: 0.62, 0.70), 0.45 (95% CI: 0.40, 0.49), and 0.63 (95% CI: 0.57, 0.69), respectively. Year-to-year variability in the values of R and Rindex and an increase in transmissibility in recent years were noted with all methods. Elimination of endemic measles transmission is maintained in the United States. A suggested increase in measles transmissibility since elimination warrants continued monitoring and emphasizes the importance of high measles vaccination coverage throughout the population.

Keywords: United States; measles; reproduction number; transmissibility.

Figure 1.

Reported numbers of measles cases (A) and outbreaks (B) and numbers and percentages of cases that were directly imported and not directly imported (C), by year, United States, 2001–2014. An outbreak of measles was defined as a chain of transmission with ≥3 confirmed cases. Directly imported cases (black portions of columns) are those arising in persons who acquired measles outside of the United States and brought their infection into the United States. Cases not directly imported (gray portions of columns) include persons who acquired measles in the United Sates—that is, cases that are epidemiologically linked to an imported case, cases for which viral genetic evidence indicates an imported genotype, and cases with unknown sources (no epidemiologic or virological link to importation). The dark line with squares indicates the proportion of cases that were imported in each year.

Figure 2.

Estimates of the effective reproduction number (R) for measles according to the proportion of cases imported (A), the distribution of chain sizes (B), and the distribution of chain durations (C) and estimates of the reproduction number of the index case (R_index) according to the likelihood-based estimation method (D), by year, United States, 2001–2014. Bars, 95% confidence intervals.