Although the incidence of measles cases in the United States is low, it is increasing – and it appears the majority of cases occur in unvaccinated people, especially in the very young.

So, the importance of maintaining high vaccine coverage remains essential, concluded the authors of an analysis of confirmed U.S. measles cases, because endemic measles was eliminated nationwide in 2000.

In an analysis to determine all confirmed cases of U.S. measles from January 2001 to December 2015, 1,789 measles cases were reported among U.S. residents aged 0-89 years; 69.5% were in unvaccinated persons, and 17.7% were in persons with an unknown vaccination status. Among those people 30 years or older, 48% had unknown vaccination status. More than one-fourth of the measles cases were imported.

Incidence of measles was highest in infants aged 6-11 months, followed by toddlers aged 12-15 months. Measles rates fell with age, starting at 16 months, Nakia S. Clemmons and her associates of the division of viral diseases at the Centers for Disease Control and Prevention, Atlanta, said in a research letter in JAMA ( 2017 Oct 3;318[13]:1279-81 ).

Higher incidence per million population occurred over time, from 0.28 in 2001 to 0.56 in 2015. Imported cases decreased from 47% in 2001 to 15% in 2015. Vaccinated patients decreased from 30% of U.S. measles cases in 2001 to 20% in 2015.

“The concurrent increase in incidence and declines in the proportion of imported and vaccinated cases (signifying relative increases in U.S.-acquired and unvaccinated cases) may suggest increased susceptibility and transmission after introductions in certain subpopulations,” the investigators said.

“The declining incidence with age, the high proportion of unvaccinated cases, and the decline in the proportion of vaccinated cases despite rate increases suggest that failure to vaccinate, rather than failure of vaccine performance, may be the main driver of measles transmission, emphasizing the importance of maintaining high vaccine coverage,” they added.