Measles vaccine

The measles vaccine materialized when three separate rivers of knowledge converged in the early 1950s. John Franklin Enders' 1949 poliovirus breakthrough proved viruses could be grown outside the nervous system in cultures of human embryonic tissue. His discovery of the "cytopathic effect"—that viral infection produced visible cellular damage—allowed researchers to detect viruses by watching cells die. And his laboratory at Boston Children's Hospital had spent eight years refining culture techniques.

In 1954, as Enders received his Nobel Prize for polio work, pediatric fellow Thomas Peebles was dispatched to a suburban school during a measles outbreak. From the blood of 13-year-old David Edmonston, they isolated measles virus for the first time. The "Edmonston strain" would become the genetic ancestor of every measles vaccine used worldwide.

The wild strain was too virulent—it caused fever and rash in recipients. The solution was serial passage: propagating the virus through human kidney cells, amnion cells, embryonated hen's eggs, and finally chick embryo cultures. Each passage selected for viruses better adapted to non-human tissue and less capable of causing disease. Working with Samuel Katz, Enders turned the Edmonston strain into a vaccine licensed in March 1963.

But there was a problem: the sample sent to manufacturers was contaminated with avian leukosis virus from chicken embryos. Enter Maurice Hilleman, who would become the most prolific vaccine developer in history. When he discovered the contamination, he passed Enders' strain through clean chick embryos 40 more times, naming the result "Moraten" (more attenuated Enders). Licensed as Attenuvax in 1968, it remains the only measles vaccine used in the United States. In 1971, Hilleman combined it with mumps and rubella vaccines to create MMR.

Two doses provide 97% effectiveness. Measles deaths dropped 88% since 2000, saving nearly 59 million lives. Yet 11 million infections occurred in 2024, and the Americas lost elimination status in November 2025. The vaccine works—the virus is entirely preventable—but coverage has fallen below the 95% threshold needed for herd immunity.