The Continued Risk of Measles Outbreaks in the United States Resulting From Suboptimal Vaccination Coverage

In March 2024, the Centers for Disease Control and Prevention issued a health alert calling for vigilance to quickly identify potential measles cases and ensure that individuals are fully vaccinated against measles. ¹ As of October 24, 2024, a total of 271 measles cases had been reported by 32 jurisdictions, including 15 outbreaks (defined as ≥3 related cases) in 2024 with 71% identified as outbreak associated. ² From 2000 through 2024, a total of 4451 cases of measles were reported in the United States. ² While this total is orders of magnitude lower than the average annual measles incidence (approximately 530 000 cases) before the availability and widespread use of vaccines (eg, pre-1960s), ³ it is concerning because measles outbreaks can be difficult and costly to contain. ⁴ Moreover, the high transmissibility of measles could indicate that declining vaccine uptake in general could increase the potential for other vaccine-preventable disease outbreaks. ⁵ Most concerning in recent years was the continual measles outbreak that occurred in the United States during 2018-2019, which constituted approximately one-quarter of the total number of measles cases documented from 2000 through October 2024 and nearly resulted in the United States losing its declaration of measles elimination. ⁶

The potential for the rising number and increasing size of measles outbreaks is concerning because it represents a future public health burden in measles cases and fatalities, secondary sequelae, and greater health care utilization and costs, which the current public health infrastructure and funding may not be equipped to handle. Although many current measles outbreaks are associated with international travel,^1,7 future measles outbreaks could occur in local populations without a history of international travel, which may have lower protection against measles than assumed. Here, we outline some of the contributors to potential reductions in population-level protection against measles and the implications for current and future measles prevention strategies.

While annual estimates of measles vaccination coverage among US children are high, often exceeding 91%, ⁸ these estimates are concerning for 2 reasons. First, the conventionally accepted threshold of measles immunity to achieve community protection, or herd immunity, is 92% to 94%, ⁹ which is higher than the annual measles vaccine coverage estimates in the United States. Second, children who are not vaccinated or who are vaccinated late contribute to an accumulating pool of measles-susceptible children, as new birth cohorts of unvaccinated or undervaccinated children add to those not protected from measles infection. ¹⁰

We conducted a modeling study of measles susceptibility in 2016 to assess this accumulation of measles-susceptible children and adolescents by synthesizing age- and dose-specific vaccination data from 2008 through 2013. The modeling study estimated that 12.5% of US children and adolescents aged <18 years were susceptible to measles. ¹¹ This comprehensive modeling accounted for vaccine uptake, vaccine effectiveness, transplacental antibody transfer, and the role of immunosuppression due to childhood cancer treatment. It also addressed the potential limitations of relying on annual estimates of childhood vaccine coverage by accounting for the accumulation of susceptible individuals in multiple birth cohorts with incomplete vaccination. When we published an updated model in 2022 that included data from 2008 to 2017, the baseline model, not accounting for effects of the COVID-19 pandemic, showed an increase in the proportion of children and adolescents who were susceptible to measles, from 12.5% to 13.1%. ¹²

This increase alone is concerning given the high transmissibility of measles. However, it has also been well documented that childhood vaccine dose orders and uptake dropped substantially during the early months of the COVID-19 pandemic in 2020. ¹³ While vaccine ordering has rebounded since 2020, a deficit remains relative to pre–COVID-19 pandemic levels. ¹⁴ Despite a subsequent rebound in childhood vaccine uptake, our 2022 measles susceptibility model showed that the effects of incomplete catch-up vaccination could linger for years. In a sensitivity analysis, we estimated that the proportion of individuals who might be susceptible to measles could range from 13.5% if catch-up vaccination was incomplete to 21.7% if vaccine uptake were to stay at the levels of decline seen in 2020. ¹²

Concurrent with incomplete childhood vaccination coverage, documented decreases in parental confidence in measles vaccines, reduced perception of the need for measles vaccination and vaccine requirements for school entry, ¹⁵ and spillover vaccine hesitancy from the COVID-19 vaccination program have all occurred. ¹⁶ In our 2022 model of updated measles susceptibility estimates, we included decreases in measles vaccine coverage that could be related to increases in vaccine hesitancy. In that analysis, we estimated that a 10% relative decline in vaccine uptake because of greater vaccine hesitancy would increase the proportion of children and adolescents susceptible to measles from 13.5% to 21.4%. ¹²

Although the main model assessments presented from the 2016 and 2022 modeling studies^11,12 relate to measles vaccination coverage in children and adolescents, it is important to consider measles susceptibility in vaccinated and unvaccinated US adults. Although measles vaccine recommendations were updated in 1989 to include a second dose of measles, mumps, rubella (MMR) vaccine at ages 4 to 6 years, ¹⁷ many people born before this updated recommendation may have received only 1 dose of MMR vaccine, because that is all that would have been required for their school entry. Educational and occupational vaccine status checks are less common for adults than for children entering school, thereby reducing the opportunity to identify undervaccinated people in need of an additional MMR vaccine dose. (Notably, 1 of the authors [R.A.B.], born in the mid-1970s, was made aware of measles undervaccination with only 1 dose of MMR vaccine when starting graduate school in the 2000-2010 period.)

Taken together, these results raise concerns about population-level protection for adults as well as children and adolescents. Even a small change in measles susceptibility can lead to large changes in the risk of measles outbreaks. While 1 dose of MMR vaccine is 93% effective, a second dose increases the effectiveness to 97%, ¹⁸ offering a major boost for high levels of population-based protection.

More work is needed to generate granular estimates of measles susceptibility. As highlighted through the COVID-19 pandemic, a strong focus on descriptive epidemiology is needed to understand the distribution of risk factors in the population. ¹⁹ Our prior measles susceptibility model estimates were based on national-level, health care provider–verified vaccine data in the National Immunization Survey–Teen. As seen in studies of factors associated with COVID-19 and influenza vaccine uptake, at the county level, heterogeneity can occur in both vaccine coverage and the determinants of that coverage at substate levels. ⁸ Current data collection methods largely preclude these types of granular assessments. For example, National Immunization Survey–Child vaccination data from 2016 to 2019 are presented on the Centers for Disease Control and Prevention’s ChildVaxView website by urbanicity. However, even during those 4 years of data, estimates for rural areas were based on <11 000 children nationwide.^8,20 A need exists for consistent analysis of state- and substate-level estimates to understand areas of higher susceptibility and areas where more directed outreach and engagement are needed to address vaccine hesitancy and increase vaccine coverage. ²¹ Reductions in public health funding for systematic surveillance ²² —concurrent with increased vaccine hesitancy and decreased childhood vaccination coverage, as after-effects of the COVID-19 pandemic—are creating a higher risk for measles outbreaks in the United States.

The risk of measles outbreaks in the United States continues because of factors including vaccine hesitancy, lingering catch-up vaccination issues from the COVID-19 pandemic, and lack of consistently evaluated substate-level vaccine uptake data to support effective outreach activities. While the United States avoided losing its measles elimination status in 2019, we are seeing continued outbreaks globally ²³ and nationally, ² and we cannot let our guard down. A need exists to (1) prioritize consistent assessment of vaccine coverage data at substate levels to identify pockets of unvaccinated and undervaccinated individuals for which tailored vaccine information can be provided to support vaccine decision-making; (2) encourage individuals to check their vaccination status, especially if they were born before the 2-dose measles vaccination recommendation ¹⁷ in 1989; and (3) prepare for potential future disruptions to vaccine delivery systems arising from natural disasters or public health emergencies to ensure minimal effects on vaccine delivery and disease prevention activities.