Measles was officially declared eliminated in the United States in 2000, meaning that the transmission of this infection was no longer endemic. Since then, measles has mostly occurred as outbreaks — either because of imported cases or among those who come in contact with these cases. And indeed, there is a growing measles outbreak in the Pacific Northwest, which has led the governor of Washington to declare a state of emergency.
Since the disease was eliminated, an average of 124 cases have been reported in the United States per year. That’s relatively low, compared with the 5,307 cases per year in the 12 years before elimination. But several of the recent outbreaks have attracted public attention. While there have been outbreaks involving multiple states, such as what was known as the Disneyland outbreak in 2015 with 147 cases in seven states, there has not been a recent sustained national-level measles epidemic in the United States.
Are these recent measles cases and outbreaks truly sporadic, or are we on the verge of the return of widespread measles? While recent measles outbreaks have been contained, the frequency and size of these outbreaks is alarming. A return of widespread measles is not inevitable, but to be sure we prevent it, we need to address vaccine refusal directly.
A national outbreak, or an outright national-level measles resurgence, would not be out of the ordinary for a Western country. In recent years, there have been several large sustained outbreaks in Europe. In Italy, approximately 5,000 measles cases were reported from February 2017 to January 2018. Similarly, large national-level outbreaks have occurred in Britain, Germany and France. In 2008, the World Health Organization reported approximately 60,000 measles cases from countries included in its European region. While most European countries, including Britain, have been certified as having eliminated measles, measles is still considered endemic in Italy, Germany and France.
It’s not just luck that the United States hasn’t seen a similar resurgence. There are many things the United States does right in vaccine policy, compared with Europe. For example, the United States has a patchwork of school-entry vaccine requirements that work. These requirements, based in state laws, have contributed to maintaining high immunization rates and keeping rates of vaccine noncompliance low. The U.S. Centers for Disease Control and Prevention aggressively monitors and responds to emerging outbreaks — an epidemiological firefighting function it performs with state and local health departments. In Europe, on the other hand, the effectiveness of public health agencies is uneven, and the European Centre for Disease Prevention Control, a much smaller and newer agency, compared with the American CDC, lacks the resources and mandate to perform a similar function. U.S. professional medical societies such as the American Academy of Pediatrics have been at the forefront of vaccine advocacy — leveraging the fact that physicians are the most trusted source of vaccine information, even among vaccine refusers.
But while a national measles resurgence in the United States has been kept at bay, we cannot be complacent. With increases in vaccine refusal, the risk of larger outbreaks remains. Imported measles cases will often find those who are susceptible to the disease, causing these outbreaks even after measles has been eliminated. If vaccine refusal is left unchecked, more people will be susceptible to this disease, leading to larger outbreaks and possibly resumption of sustained transmission.
Notably, each year there are children not vaccinated against measles because of parental hesitancy or refusal. These nonimmunized children join the ranks of all other susceptible children from years past, increasing the population of susceptible people. With the slow and steady accumulation of people who haven’t been immunized, we may only be delaying a large measles outbreak. In fact, in an epidemiological study we published in 2016, we estimated that 1 in 8 children younger than 18 are susceptible to measles.
We epidemiologists are always mindful of what’s known as herd immunity (often called community protection) threshold — calculated as the proportion of individuals who need to be immune to prevent outbreaks. In mid-adolescence, when children have had multiple years to catch up on vaccines they didn’t get earlier, immunization levels are still dangerously close to dropping under the herd immunity threshold for measles. Similar findings have been subsequently reported by other researchers, highlighting the need for interventions to improve measles vaccination rates.
While the risk of a national measles resurgence or a large multi-state outbreak is real, it is not guaranteed. But if we want to prevent it, we need a coherent response to vaccine hesitancy. Fortunately, an evidence-based blueprint exists in the form of recommendations published in 2015 by the National Vaccine Advisory Committee, an independent committee charged with the advising the Department of Health and Human Services. These recommendations focus on evidence-based strategies for increasing confidence in vaccines. Unfortunately, these recommendations have not been fully implemented.
In the aftermath of the last measles resurgence in the United States in 1989-1991, there was a remarkably bipartisan effort to address the main cause of that resurgence: vaccine access. President Bill Clinton and congressional Republicans and Democrats came together to establish the Vaccines for Children program to remove affordability as a barrier to vaccination. This program was effective in addressing inequities in immunization coverage. Preventing the next potential resurgence of measles will require a similar broad-based response.