Why Measles Hasn’t Been Eradicated

“The reason it keeps me up at night, the thing that distinguishes measles from most other diseases out there, is that it is the most contagious disease that we know. It spreads with unbelievable efficiency, in a nonimmune or a partially immune population.”

Adam Radner, MD, MPH Professor of Pediatrics and Microbiology, New York University Grossman School of Medicine  

“Misinformation or distrust of vaccines can be like a contagion that can spread as fast as measles.”                                                                                                                                         Theresa Tam, Chief Public Health Officer of Canada

Eradication means “the complete and permanent worldwide reduction of a disease to zero, requiring no other interventions.” Of the 1,400 or so human pathogens, variola major (the smallpox virus) is the only one that has been eradicated. (Poliovirus—another horrific virus—is close to being eradicated. See, “Eradication of Polio: A Stone’s Throw Away” Germ Gems, March 18, 2025.)

Elimination, on the other hand, refers to “the reduction of a disease to zero in a specific geographic area, requiring ongoing preventative measures to prevent reintroduction.” Measles was declared eliminated in the U.S. in 2000, a status shared with 81 other countries. But right now, the U.S. is experiencing an outbreak of measles, and measles is close to losing its elimination status.

We are a far cry from eradicating measles globally. Nonetheless, in this this week’s Germ Gems post, I compare our current measles predicament with that of smallpox, the only eradicated virus, for insights as to how this might be accomplished once again.

Smallpox and measles vaccines. Smallpox was a global scourge since at least as early as 1350 BCE. The earliest descriptions of smallpox-like diseases dates back to the Persian physician Rhazes in 910 CE, who provided a clear distinction between smallpox and measles.

William Jenner, a British physician, is credited with developing the first smallpox vaccine in 1796. In the early 1950s, 150 years after the introduction of smallpox vaccination, there were still some 50 million cases per year globally. Due to a combination of successful vaccination programs and surveillance efforts, smallpox was eradicated thirty years later.

By the time the World Health Organization (WHO) declared smallpox eradicated in 1980, the disease had killed more people than all wars combined. In the 20th century alone, smallpox claimed the lives of more people than influenza (including the 1918-19 pandemic that killed more than 50 million), HIV/AIDS, and tuberculosis all together.  

Measles is not nearly as lethal as smallpox—death from neurological or respiratory complications occurs in one in 1,000 infected people. But what the measles virus (aka Rubeola) lacks in virulence is made up for by its extraordinary contagiousness; it is one of the most contagious viruses in humans.

In 1954, John Enders and Thomas Peebles isolated the measles virus. This was a crucial first step in developing a vaccine. In 1973, measles virus was combined with mumps and rubella virus to create the measles, mumps, and rubella (MMR) vaccine. (In 2005, varicella, the chickenpox virus, was added to make the combined MMRV vaccine.) The MMR vaccine is safe and highly effective—two doses are about 97% effective at preventing measles.

The current measles outbreaks. In January 2025, the current U.S. measles outbreak began in a tightknit Mennonite community in Gaines County, Texas. It then spread like wildfire. The Centers for Disease Control and Prevention (CDC) reported that as of June 12, 2025, the U.S. had confirmed 1,197 measles cases across 35 jurisdictions. About 96% of the cases were in unvaccinated people or those unaware of their vaccination status.

At the same time that the U.S. is battling the measles virus (aka Morbillivirus hominis), measles cases are surging worldwide. Both the WHO and the CDC have reported a significant increase in measles cases globally, with 10.3 million people infected in 2023. This rise is attributed to reduced vaccination coverage and increases in vaccine hesitancy, particularly in countries where measles elimination efforts have been affected by the COVID-19 pandemic. (The U.S. isn’t alone in the fight against vaccine misinformation. See, “Measles: the urgent need for global immunization and preparedness,” The Lancet, May 3, 2025.

Because of the global rise in measles cases, the CDC currently recommends that all international travelers should be fully vaccinated at least two weeks prior to their departure flight unless they’re protected because they’ve had measles in the past. (Adult travelers born before 1957 are presumed to be immune to measles but those born in 1957 or later should verify their vaccination status.)

What set the smallpox eradication campaign apart from measles eradication? The smallpox vaccine, a version of which Jenner first developed in 1796, remained the mainstay of smallpox vaccination campaigns until routine worldwide administration was discontinued in 1980. So effective vaccines against both smallpox and measles were in use for quite some time. What then explains why only smallpox was eradicated? (For an overview of the saga leading to the eradication of smallpox, See, “Smallpox Is Gone, but the Lessons Learned Live On,” Germ Gems, September 30, 2020).

In his book, Smallpox: the Death of a Disease, Donald A. Henderson, the physician and epidemiologist who led the WHO’s Smallpox Eradication Unit, describes what it took to give smallpox the boot.  In the final chapter of his book, Henderson concludes, “I believe that the important longer-term contribution of smallpox eradication to world health was its demonstration of how much could be achieved through community-wide vaccination programs.”

In addition to an effective vaccine, two other factors played a critical role in the eradication of smallpox: an effective surveillance-containment strategy, which was provided by Dr. William Foege and his colleagues at the WHO and the CDC, and of utmost importance, trusted leadership. The team that conquered smallpox faced a number of powerful naysayers, but they demonstrated incredible perseverance, imagination, courage, and a refusal to let bureaucracy stop them.

Current state of leadership in efforts to control the spread of measles. Like smallpox virus, the measles virus doesn’t infect animal species other than humans, eliminating the possibility of animal reservoirs, and, as pointed out, a highly effective vaccine is available. Thus, in the early years of the 21st century, it appeared there was a good chance that measles could be eradicated.

In 2016, the Region of the Americas was declared free of endemic measles transmission. This was the first time a whole WHO region reached this milestone. But, since 2016, there’s been a resurgence of measles globally, including outbreaks in countries that had previously eliminated it. This resurgence is largely attributed to declining vaccination rates and increasing vaccine hesitancy. 

The Internet is teeming with misinformation and disinformation about vaccines, both about their efficacy and safety. And the current administration is sowing the seeds of distrust in the world’s public health organizations and in science in general.

Earlier this year, the current administration pulled the U.S. out of the WHO. Then, Robert F. Kennedy, Jr. became the Seretary of Health and Human Services. Knowing of the pivotal role played by leaders of the CDC and WHO in the eradication of smallpox, having RFK Jr. at the helm of the CDC seems disastrous for the prospects of measles eradication.

Just this past week, Mr. Kennedy “retired” all 17 members of the CDC’s Advisory Committee on Immunization Practices (ACIP)—a group of highly accomplished experts. To defend his vaccine policy changes and his newly appointed ACIP members which include anti-vaxxers, RFK Jr.  sent a document to Congress that mischaracterizes studies and contains “junk science,” that is, unpublished or disputed scientific studies.

Upon learning of Kennedy’s dismal of the CDC’s ACIP, Dr. Robert Schooley, a distinguished infectious diseases specialist at the University of California San Diego School of Medicine, wrote the article “Have You No Sense of Decency, Secretary Kennedy?” published on June 13, 2025 in MedPage Today outlining why the Secretary’s decision should be reversed. I share Dr. Schooley’s view.