Worried about splitting up the MMR vaccine into three separate shots? You should be, this doctor says

By Katia Hetter, CNN

(CNN) — The combination measles, mumps and rubella vaccine — better known as the MMR shot —should be split into three separate vaccines, US Centers for Disease Control and Prevention Acting Director Jim O’Neill said this week. His post on X included similar language posted on X by President Donald Trump in late September.

This proposal has raised alarms for many public health experts, who worry that splitting up the shots has no clear scientific justification but would increase the number of health care visits required to get vaccinated for measles, mumps and rubella; reduce access to the vaccines; and lead to lower vaccination rates.

Since the MMR combination vaccine shot has been approved in the US since 1971, I wanted to understand more about the diseases the MMR vaccine was designed to prevent and what are advantages and disadvantages of separated shots? Importantly, what could happen if vaccination rates fall, and who would be at highest risk?

To guide us through these questions, I spoke with CNN wellness expert Dr. Leana Wen. Wen is an emergency physician and clinical associate professor at the George Washington University. She previously served as Baltimore’s health commissioner.

CNN: Many Americans may not have had personal experience with measles, mumps or rubella. What are these diseases and what complications are they associated with?

Dr. Leana Wen: All three are contagious viruses that, until the development of vaccines, caused serious illness and sometimes death.

Before the introduction of the measles vaccine, every year, measles led to an estimated 48,000 hospitalizations and 400 to 500 deaths, primarily among children, according to the CDC. Mumps can cause complications include brain swelling, testicular inflammation, infertility and deafness.

Rubella can harm the developing baby if a woman acquires the infection during pregnancy. It increases the risk of miscarriage and stillbirth. Babies born with congenital rubella syndrome can have serious birth defects including heart, liver and spleen damage, loss of hearing and eyesight and lifelong intellectual disability. According to the CDC, during the last major rubella epidemic in the US, in 1964-65, 11,000 pregnant women lost their babies. More than 2,000 newborns died. About 20,000 were born with congenital rubella syndrome.

CNN: Why was a combination vaccine developed against all three viruses?

Wen: The MMR vaccine was developed to protect against measles, mumps and rubella with a single injection rather than three separate shots. Combining these vaccines streamlines the childhood immunization schedule, reduces the number of doctor visits and needle sticks, and helps ensure that children receive full protection on time. Currently, the CDC recommends that children receive two doses of the MMR vaccine, one at 12 to 15 months and another at 4 to 6 years old. If they had to have to separate shots, they would require 6 shots instead of 2.

The combined vaccine has been extensively studied and consistently shown to provoke an immune response comparable to what’s seen when the individual vaccines are given separately, and without raising the risk of serious side effects.

CNN: What are advantages and disadvantages of separated shots?

Wen: In theory, separating the measles, mumps and rubella vaccines could allow for more flexibility if a family had a specific reason to delay or skip one of the components. Some parents also believe that spreading out shots might reduce the likelihood of side effects such as fever or mild rash, although research has not shown differences in side effects.

In practice, separating the vaccines has several drawbacks. It requires multiple clinic visits and more injections, which increases the chance that one or more doses will be delayed or missed. That delay leaves children unprotected for longer periods of time against highly contagious diseases.

Furthermore, there is no evidence that the separate vaccines are safer or more effective than the combined MMR shot, and changing the current vaccine schedule adds unnecessary complexity for families and providers. For these reasons, health authorities—including the CDC itself and the American Academy of Pediatrics—recommend the combined MMR vaccine as the standard approach for immunization.

CNN: Are separate shots available now if someone chooses to get them?

Wen: No, separate shots for measles, mumps and rubella are not currently available in the US. Only the combined MMR vaccine is approved and in use.

The acting CDC director, Jim O’Neill, has asked manufacturers to begin developing individual vaccines for each virus, but that process will take years. Producing new versions of these vaccines requires not only manufacturing but also rigorous clinical testing to confirm their safety and effectiveness. This will be very challenging when these diseases are now rare thanks to widespread immunization. After that, the vaccines must still go through the full review by the US Food and Drug Administration before they can be offered to the public.

Many public health experts have questioned the motivation behind this push, noting that the existing MMR vaccine is safe, effective and well-established, and there is no scientific justification for calling for separate shots. Some worry that this proposal could further fuel vaccine hesitancy by reinforcing unfounded fears about combination vaccines and ultimately leading to fewer children receiving timely protection against serious diseases.

CNN: What could happen if vaccination rates fall, and who would be at highest risk?

Wen: If vaccination rates for measles, mumps and rubella decline, these diseases can quickly return. When community immunity drops, outbreaks become more frequent and can spread rapidly through schools, households, and entire regions.

This is what has already happened in the US, where vaccination rates have already been falling. In 2025, there have already been 1,544 confirmed measles cases. This is the highest number since measles was declared eliminated 25 years ago.

Those most at risk include newborns, who are too young to be vaccinated and among the most medically fragile. People with weakened immune systems are also highly vulnerable. This group includes patients undergoing cancer treatment, transplant recipients and individuals taking immune-suppressing medications. They face a higher likelihood of severe illness if exposed to infections, and vaccines may not protect them as effectively, or may not be safe for them to receive, because their immune systems cannot generate a full response.

Older adults are another group at increased risk. Age-related changes in immunity and chronic health conditions make them more prone to serious complications. In some cases, immunity from prior vaccination may have diminished over time, leaving them more susceptible if vaccine coverage falls and infections spread more widely.

The concept of community immunity depends on collective participation. When most people stay up to date on their vaccinations, it protects not only themselves but also those around them who are most vulnerable to severe disease and death.

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