I put the ‘R’ in the MMR vaccine, and I’m scared that, like measles, rubella could make a comeback under RFK Jr.

Secretary of Health and Human Services Robert F. Kennedy Jr.’s efforts to make the use of the MMR vaccine voluntary is not only letting loose measles, with all its acute and long-lasting effects, but also will allow mumps and German measles (rubella) to return to America.

MMR is called by those three letters because it contains vaccines against mumps and rubella, as well as measles. Mumps, for those who remember it, conjures up swollen and painful faces around the salivary glands, but it also frequently causes meningitis, inflammation of the testicles, and deafness.

The “R” in MMR — rubella — may not immediately conjure up such vivid images, but in the early 1960s, there was a huge outbreak of it, starting in Europe and spreading to the United States, where it caused an estimated 13 million cases.

Rubella causes a generalized rash, but it can also cause arthritis and other problems. Most importantly, if rubella infects a pregnant woman just before conception or in early pregnancy, it frequently passes to the unborn child, resulting in permanent neurological, cardiac, and other abnormalities.

Just before the rubella epidemic began, I had opened my virology lab at the Wistar Institute in Philadelphia. At the time, it was the only laboratory in the city capable of doing virologic and serologic studies of people infected with rubella. Soon, I was overwhelmed with pregnant women who had developed rashes and came to me wanting to know if it was rubella. Later, other women brought their ill babies to me to determine if their abnormalities were the result of congenital rubella infection.

The eventual total toll of infants in the United States with congenital rubella due to that single outbreak was estimated to be 20,000, as well as several thousand fetal deaths and an unknown number of spontaneous and induced abortions. Witnessing the anguish of the Philadelphia mothers was a sobering experience, to say the least.

Thereafter, I and others began to work on developing vaccines against the rubella virus, which were eventually licensed in the United States and Europe in the late 1960s and ‘70s.

To reduce injections, the rubella vaccine was eventually combined with the measles and mumps vaccines to make MMR. The triple vaccine was widely recommended by public health authorities in the United States and elsewhere, becoming part of routine childhood vaccination. The incidence of rubella soon dropped, and by 2011, rubella and congenital rubella were eradicated from the United States and the entire Western Hemisphere.

Europe and Australia are also virtually rubella-free, although it persists in other parts of the world, and could return to the United States due to Kennedy’s actions.

In that regard, it is important to understand that the public health success of vaccines depends on everyone with normal immune systems agreeing to be vaccinated.

Those who do not accept vaccination remain susceptible to the infections against which the vaccines were developed, and they serve as vectors for the spread of the infections to unvaccinated individuals, to infants too young to be vaccinated, and to immunosuppressed persons, such as those being treated for cancer, who cannot respond to vaccination.

I am horrified by the return of measles to the United States, having seen the disease and its complications when I was training in pediatrics so many years ago. The threat that rubella will also return is equally horrifying to me and my colleagues, who hoped measles, mumps, and rubella would never again threaten children and pregnant women in America or elsewhere.

Stanley A. Plotkin is a physician and an emeritus professor of pediatrics at the University of Pennsylvania.