The U.S. immunization program is one of the best public health success stories. Physicians who provide care for children are familiar with the routine childhood immunization schedule and administer a measles-containing vaccine at age-appropriate times. Thanks to its rigorous implementation and acceptance, endemic measles (absence of continuous virus transmission for > 1 year) was eliminated in the U.S. in 2000. Loss of this status was in jeopardy in 2019 when 22 measles outbreaks occurred in 17 states (7 were multistate outbreaks). That year, 1,163 cases were reported.1 Most cases occurred in unvaccinated persons (89%) and 81 cases were imported of which 54 were in U.S. citizens returning from international travel. All outbreaks were linked to travel. Fortunately, the outbreaks were controlled prior to the elimination deadline, or the United States would have lost its measles elimination status. Restrictions on travel because of COVID-19 have relaxed significantly since the introduction of COVID-19 vaccines, resulting in increased regional and international travel. Multiple countries, including the United States noted a decline in routine immunizations rates during the last 2 years. Recent U.S. data for the 2020-2021 school year indicates that MMR immunizations rates (two doses) for kindergarteners declined to 93.9% (range 78.9% to > 98.9%), while the overall percentage of those students with an exemption remained low at 2.2%. Vaccine coverage greater than 95% was reported in only 16 states. Coverage of less than 90% was reported in seven states and the District of Columbia (Georgia, Idaho, Kentucky, Maryland, Minnesota, Ohio, and Wisconsin).2 Vaccine coverage should be 95% or higher to maintain herd immunity and control outbreaks.
Why is measles prevention so important? Many physicians practicing in the United States today have never seen a case or know its potential complications. I saw my first case as a resident in an immigrant child. It took our training director to point out the subtle signs and symptoms. It was the first time I saw Kolpik spots. Measles is transmitted person to person via large respiratory droplets and less often by airborne spread. It is highly contagious for susceptible individuals with an attack rate of 90%. In this case, a medical student on the team developed symptoms about 10 days later. Six years would pass before I diagnosed my next case of measles. An HIV patient acquired it after close contact with someone who was in the prodromal stage. He presented with the 3 C’s: Cough, coryza, and conjunctivitis, in addition to fever and an erythematous rash. He did not recover from complications of the disease.
Prior to the routine administration of a measles vaccine, 3-4 million cases with almost 500 deaths occurred annually in the United States. Worldwide, 35 million cases and more than 6 million deaths occurred each year. Here, most patients recover completely; however, complications including otitis media, pneumonia, croup, and encephalitis can develop. Complications commonly occur in immunocompromised individuals and young children. Groups with the highest fatality rates include children aged less than 5 years, immunocompromised persons, and pregnant women. Worldwide, fatality rates are dependent on the patients underlying nutritional and health status in addition to the quality of health care available.3
Measles vaccine was licensed in 1963 and cases began to decline (Figure 1). There was a resurgence in 1989 but it was not limited to the United States. The cause of the U.S. resurgence was multifactorial: Widespread viral transmission among unvaccinated preschool-age children residing in inner cities, outbreaks in vaccinated school-age children, outbreaks in students and personnel on college campuses, and primary vaccine failure (2%-5% of recipients failed to have an adequate response). In 1989, to help prevent future outbreaks, the United States recommended a two-dose schedule for measles and in 1993, the Vaccines for Children Program, a federally funded program, was established to improve access to vaccines for all children.