Deadly measles outbreaks have pushed US case counts to their highest level since the early 1990s, leaving the country close to losing the elimination status it achieved in 2000. The resurgence has reopened a public health fight once viewed as a landmark vaccination success.

In March 2000, the US Centers for Disease Control and Prevention assembled experts to assess measles after cases had remained exceptionally low for years. They concluded that the virus was no longer endemic in the United States.

Measles had not been eradicated, but it was considered eliminated within the country. The distinction reflected the profound effect of vaccination while acknowledging that imported cases and renewed transmission remained possible.

Dr. Yvonne Maldonado, a pediatric infectious diseases expert with Stanford University, represented the National Vaccine Advisory Committee at that meeting. She recalled leaving with a feeling of “cautious optimism.”

“We felt we were doing well, and we were very happy that we were doing well, but clearly we shouldn’t rest on our laurels,” Maldonado said. “With infectious diseases, you really can never know or feel comfortable that you completed the task.”

Authority over elimination status now rests with an international advisory group appointed by the Pan American Health Organization, an arm of the World Health Organization. At a private meeting this month, the committee will determine whether local measles transmission has continued in the US for at least a year.

Experts widely expect the country to lose its elimination status. Years of declining vaccination rates have created a well established threat that the US has increasingly struggled to contain.

“It’s really tragic and unnecessary, and it’s all completely preventable,” Maldonado said. She said two vaccine doses are highly safe and effective but warned that denial about measles and disinformation about vaccine safety have gained ground.

The 2000 milestone came after repeated demonstrations of vaccination’s power, but experts also warned that success was fragile. In a meeting summary published in the Journal of Infectious Diseases in May 2004, committee members credited millions of parents and children, hundreds of thousands of health workers, and thousands of policy makers and legislators.

They described elimination as an inherently unstable situation requiring very high immunization levels and strong surveillance. They also called for greater support for global measles control and work toward worldwide eradication.

The United States can restore elimination status after showing that outbreak transmission has been interrupted for at least 12 months. Public health experts who have worked to contain measles fear that present conditions could make recovering lost progress more difficult.

“Regaining elimination is going to be harder for us than it was the first time,” said Janet Hamilton, executive director of the Council of State and Territorial Epidemiologists. She pointed to weakened trust in public health and science following the Covid-19 pandemic.

The epidemiology of measles has also changed, with immune protection falling short among children and in some groups of adults. Hamilton said earlier elimination efforts benefited from an immune adult population, allowing officials to concentrate on vaccinating children.

The public health system is also less well funded, resourced, and supported than it has been previously. Dr. Walter Orenstein, who has led the CDC’s National Immunization Program, said large outbreaks once prompted major policy changes and presidential initiatives.

Orenstein cited the Vaccines for Children Program and the National Childhood Immunization Initiative, which encouraged school mandates and other state laws supporting broad immunization. CDC Director Dr. Erica Schwartz said in November that the agency would not fundamentally change its outbreak response if elimination status is lost.

Public health experts argue that prominent federal leadership is precisely what the moment requires. Dr. Jonathan Temte, who attended the 2000 meeting for the American Academy of Family Physicians, called for leaders at the highest levels to emphasize the MMR vaccine’s effectiveness and safety.

Temte said known adverse effects tend to be mild and that autism is not seen. Now professor emeritus and associate dean emeritus for public health and community engagement at the University of Wisconsin, he warned that controlling measles will remain difficult without that message.

Hamilton nevertheless said public health in the US remains resilient and accustomed to adapting. The vaccine already exists, its effectiveness is known, and the number of people requiring vaccination is manageable.

She also pointed to advances in behavioral sciences and understanding what motivates change. “Success now depends on how earnestly we focus on this moment and how much energy we really put into thinking carefully about the strategies that we need,” Hamilton said.