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In an interview with Fox News’ Sean Hannity, Health and Human Services Secretary Robert F. Kennedy Jr. made several unsupported or misleading claims about the measles vaccine, which the Centers for Disease Control and Prevention has said is safe and “the most important tool to prevent” the disease. Meanwhile, a measles outbreak in Texas continues to expand.
- Kennedy said that the “vaccine wanes about 4.5% per year.” Although antibody levels can fall, there’s no evidence that overall vaccine protection declines that quickly. It would mean thousands of vaccinated people should have contracted measles in the latest outbreak.
- The health secretary made the unsupported claim that the measles vaccine leads to “deaths every year” and misleadingly said it causes “all the illnesses” of the disease. The vaccine can cause some similar symptoms but is much safer than getting measles.
- Kennedy misleadingly said the measles vaccine “does not appear to provide maternal immunity.” Evidence suggests that vaccinated mothers pass fewer protective antibodies on to their babies than previously infected moms, but in both cases this protection wanes before the infant’s first year. The best way to ensure babies don’t contract measles is to vaccinate everyone around them.
The measles vaccine, which is part of the measles, mumps and rubella, or MMR, vaccine, is highly effective, according to the CDC. Two doses of the vaccine are 97% effective at preventing measles, likely for life — and even one dose is 93% effective. As with any medical product, the MMR vaccine is not 100% safe. But the vast majority of people experience no or only mild and temporary side effects, and getting the vaccine is much safer than getting measles. As we’ve recently reported, measles is an extremely contagious viral disease, which can be deadly or cause long-term health issues.
During the interview, which was held at a fast-food restaurant chain and aired on March 11, Kennedy continued to downplay the seriousness of a measles outbreak that is expanding, although he did say the vaccine stops “the spread of the disease” and said the government should “encourage” but not “force” vaccination. There are now more than 300 cases.
MMR Vaccine Waning
Minimizing the current outbreak, Kennedy emphasized that measles outbreaks happen every year — incorrectly adding that there are sometimes “hundreds” of them — and he placed partial blame on the MMR vaccine.
“Part of that is that there are people who don’t vaccinate, but also the vaccine itself wanes,” he said, of why there are outbreaks every year. “The vaccine wanes about 4.5% per year. So that means older people are essentially unvaccinated. They aren’t — their immune system is not protected.”
That’s wrong. The measles vaccine offers strong and long-lasting protection against the virus. Although some studies show that the concentration of measles antibodies in vaccinated people decreases over time, overall vaccine immunity is not waning that quickly. If it were, far more vaccinated people would get measles.
Furthermore, older vaccinated people aren’t the ones contracting measles. Most people 65 years and older had measles as children and also lived during a time when they would have had measles exposures afterward. This means their measles immunity is likely quite robust.
Outbreaks primarily occur because not enough people are vaccinated. Rarely, vaccinated people can develop infections, but they typically do not get as sick and are not driving spread of the disease. A 2024 systematic review found that only about 10% of vaccinated people who get infected because of waning immunity passed the virus on to anyone at all — a “remarkably low” rate that cannot sustain an outbreak.
As of March 20, 95% of the measles infections so far this year are among people who are either unvaccinated or with an unknown vaccination status, according to the CDC. In Texas, where an outbreak began in January and is thought to have spread to neighboring states, out of the 309 cases identified, only two are vaccinated people, as of March 21. In New Mexico, only four of the 42 cases are in vaccinated people. There are four probable cases in Oklahoma, all of them in unvaccinated people. Most of the infections, according to CDC data, have been in people 19 years old or younger.
“In Texas and a growing number of states across the country, declining vaccination rates are fueling a staggering increase in measles illnesses, measles hospitalizations, and the first death from the disease in years— all primarily among unvaccinated populations,” Dr. Bruce A. Scott, president of the American Medical Association, said in a statement on March 5. A day later, New Mexico reported that an unvaccinated adult who died also tested positive for measles.
It is not entirely clear what Kennedy meant when he said that “the vaccine wanes” about 4.5% per year, or what his source is for such a statement. We reached out to HHS to ask for support for his claim, but we didn’t hear back.
Dr. Michael Mina, an infectious disease expert who previously was a professor at Harvard School of Public Health, told us that if Kennedy was referring to clinical protection provided by the vaccine, that would be “just wrong.”
“If we had 4.5% waning, then, you know, everyone who is an adult would be susceptible” to measles, he said, since at that rate it would take only about 22 years to reach zero protection — and there would be thousands of measles cases in vaccinated people in the latest outbreak alone. “And that’s just not what we see.” (The first of two doses is given at 12 to 15 months.)
A number of studies show that the concentration of measles antibodies does begin to drop as more time elapses after vaccination, including a study in Slovakia that reported an average annual decline in measles antibodies of 4.8% in people 10 to 33 years old after a second dose.
But it’s not necessarily clear what antibody level is protective, and circulating antibodies aren’t the only way the immune system protects against the disease.
There are long-lived antibody-producing cells in the bone marrow, Mina said, which serve as a kind of “cushion of protection.” Although total antibody levels may decline over time, the smaller amount of antibodies produced by these cells can last decades, and may on its own provide sufficient protection. Calculations based on total antibody level declines, then, may overestimate when a person would become susceptible to measles infection due to antibody waning.
In addition, unlike the coronavirus, Mina said, the measles virus infects much more slowly, traveling through lymph nodes, which are full of immune cells. If the measles virus “bumps up against even a few anti-measles” cells, he said, “those cells will rapidly expand and defeat the virus.” As a result, measles immunity may still be very strong even in a person whose measles antibodies have fallen substantially.
Other studies, using modeling or case data that reflect what happens during outbreaks, have found that vaccine protection against measles does wane, but is much slower and less significant than Kennedy claimed.
A 2024 study in England, for example, used a mathematical model to reproduce the way that measles spread in England between 2010 and 2019, using scenarios that did or didn’t include waning. Waning was needed to explain the number of cases in vaccinated people, but it was very slow — about 0.04% per year.
“So the protection remains around 99% 20 years after vaccination,” Alexis Robert, a mathematical modeler in infectious disease dynamics at the London School of Hygiene & Tropical Medicine and the corresponding author of the study, told us in an email.
A 2023 study in France similarly concluded protection from the vaccine waned 0.22% per year, while a study in Germany found that “only a small percentage (maximum 1%) of cases could be ascribed to waning immunity.”
Robert noted that the Slovak study’s 4.8% per year of antibody waning could be misleading, as the authors also estimated that at around 30 years of age, only about 10% of people vaccinated with two doses of the vaccine would have antibody levels below the threshold of protection — “a far better” level of protection than what the 4.8% number might suggest.
Indeed, Robert said that although there are differences in the various studies, “all converge towards a very high and long-lasting immunity brought by measles vaccines,” especially after two doses. “The vast majority of adults who received two doses of vaccine in their childhood will therefore have protection against measles infection,” he said.
Waning of vaccine protection may become a larger issue in the future. This is because as widespread vaccination has eliminated measles, people have not had as many subsequent exposures to the disease that can boost immunity.
In fact, although it is often stated that measles infection provides more durable immunity than vaccination — a claim Kennedy repeated to Hannity — Mina said it isn’t clear that’s necessarily true. It could be that the regular measles exposures that have occurred while measles continued to circulate is what makes natural immunity appear longer-lasting, not the infection itself.
Many people born in the 1980s and ‘90s have only been vaccinated and have not been exposed to measles, since they grew up in an era with few measles infections. For this reason, it’s possible that when today’s 30- and 40-year-olds are 50 or so, their measles immunity will be low enough that it would be a good idea to recommend a measles booster, Mina said. But right now, there simply aren’t “big measles outbreaks happening amongst 40-year-olds who are vaccinated.”
No Evidence the MMR Vaccine Causes Deaths ‘Every Year’
In addition to being highly effective, the MMR vaccine is also very safe. According to the CDC, although mild side effects are expected with any vaccine, most people who receive an “MMR vaccine do not have any serious problems with it.”
“Getting MMR vaccine is much safer than getting measles, mumps, or rubella,” the agency adds.
Speaking of informed choice and emphasizing that no one should be forced to take a vaccine, Kennedy told Hannity, “There are adverse events from the vaccine. It does cause deaths every year. It causes — it causes all the illnesses that measles itself causes, encephalitis and blindness, etcetera. And so people ought to be able to make that choice for themselves. And what we need to do is give them the best information, encourage them to vaccinate.”
There isn’t evidence that the measles vaccine causes “deaths every year.”
“There have been no deaths shown to be related to the MMR vaccine in healthy people,” the Infectious Diseases Society of America website says. “There have been rare cases of deaths from vaccine side effects among children who are immune compromised, which is why it is recommended that they don’t get the vaccine.”
According to a 2015 article by the CDC that reviewed historical information and epidemiological data on deaths following vaccination starting in the early 1990s, there were at least six case reports of deaths linked to the MMR vaccine in severely immunocompromised people. The CDC does not recommend the vaccine for people who are immunocompromised, have a history of life-threatening allergies, are pregnant, or have had other health complications such as bruising or bleeding easily.
The study was prompted by online claims about deaths from the vaccine that misused the Vaccine Adverse Event Reporting System. As we’ve explained numerous times, VAERS encourages reports of any health event that occurs after vaccination. But VAERS reports aren’t vetted for accuracy and don’t mean that a vaccine caused a particular problem. The system is commonly exploited by people who want to mislead others about vaccine safety.
“A review of the VAERS data reveals that many of the death reports for MMR vaccine involved children with serious preexisting medical conditions or were likely unrelated to vaccination (e.g., accidents),” the study said. “These complete VAERS reports and any accompanying medical records, autopsy reports and death certificates have been reviewed in depth by FDA and CDC physicians and no concerning patterns have emerged that would suggest a causal relationship with the MMR vaccine and death.”
Misleading Claim on Side Effects
As for Kennedy’s claim that the vaccine “causes all the illnesses that measles itself causes,” that’s misleading. Because the MMR vaccine uses a weakened but live measles virus, sometimes the vaccine can cause a mild rash or fever. But the virus is attenuated and does not pose the same risk of complications caused by the wild virus and a natural infection.
Research also shows that measles infections have short- and longer-term effects on the immune system that can make people susceptible to other infections for several years after recovery — an effect that does not occur with vaccination.
As we’ve explained, even a mild case of measles makes patients feel miserable. According to the CDC, children with measles commonly develop ear infections or have diarrhea; as many as 1 in 20 develop pneumonia and 1 in 5 unvaccinated patients are hospitalized. For every 1,000 children who catch measles, 1 will develop encephalitis, or swelling of the brain, which can cause permanent disability, and around 1 to 3 will die.
In contrast, as the CDC explains, the most common side effects of the measles vaccine are pain at the injection site, fever, a mild rash, and swollen glands in the cheeks or neck. Other less common side effects include a temporary disorder that affects the body’s ability to stop bleeding, known as immune thrombocytopenic purpura. It’s not life-threatening and occurs less often than with a measles infection.
Rarely, the vaccine can cause febrile seizures, with approximately 1 case in 3,000 to 4,000 doses, but they are not associated with long-term effects. Serious allergic reactions to the vaccine occur “extremely rarely,” the CDC says.
There have been three reported cases of measles inclusion body encephalitis, a serious brain swelling caused by measles in people with a weakened immune system, in vaccinated people, according to the CDC. In one of the three cases, “the measles vaccine strain was identified as the cause.”
A Cochrane review published in 2021 concluded there’s no evidence of an association between MMR vaccines and encephalitis. The risk of febrile seizure and ITP are “very small,” it said.
While there are a few case reports of an eye condition called optic neuritis following a measles vaccine, it is very rare and doesn’t cause blindness. As a 2016 case report notes, in the previous 30 years, there had only been six case reports of the condition following measles vaccination.
“It should be stressed that this is a temporary clinical situation, which resolves after administration of a high dose of corticosteroid in the initial phase of the therapy,” the report reads.
In contrast, measles infection is a leading cause of blindness in children, and the disease can harm people’s eyes and vision in several ways, according to the American Academy of Ophthalmology.
Maternal Immunity
As part of his lengthy criticism of the MMR vaccine, Kennedy also discussed maternal immunity, or the transfer of protective antibodies to a baby through the placenta or breast milk.
“One of the problems is it does not appear to provide maternal immunity,” he told Hannity, referring to the MMR vaccine. “You don’t want a very — 1-year-old kid getting measles. That’s very dangerous. They were protected by breast milk, and by maternal immunity. And women who get vaccinated do not provide that level of maternal immunity that the natural measles infection did.”
Studies have found that previously infected mothers tend to transfer more antibodies and protect their infants longer than vaccinated mothers. That’s because their antibody levels are usually higher, which could be due to living in an area where measles is endemic, which periodically boosts immunity. But that doesn’t mean that vaccinated mothers offer no protection, nor does it mean that it would be better for infants to get immunity from infected mothers, as that would require measles virus to be circulating.
While the protection from a vaccinated mother’s antibodies “may not be quite as long lasting as from mothers who have been infected,” Robert said, “infants in vaccinated populations are protected by the high levels of vaccination around them.”
“The best way to avoid infants being exposed to measles is to maintain low to no circulation of measles, which is achieved with high vaccine coverage,” he added.
Dr. Natasha S. Crowcroft, a senior technical adviser for measles and rubella for the World Health Organization and a professor at the University of Toronto who has studied measles maternal immunity, echoed that advice, noting that “any infant” can get measles when exposed and suffer “devastating complications,” including a fatal progressive brain disorder. “The best way to protect infants too young to be vaccinated is by everyone else being vaccinated against measles,” she told us in an email.
In the U.S., the recommendation is for babies to receive their first MMR shot between 12 and 15 months of age and a second shot between ages 4 to 6. Doses are not typically given before 1 year because maternal antibodies can prevent younger babies from responding fully to the vaccine. But during an outbreak or for international travel, babies should get an extra, early dose starting at 6 months of age (a second dose can also be given earlier to children older than 12 months).
Crowcroft said that breastfeeding can transfer some antibodies, but the amount is “much smaller” than the transfer through the placenta that occurs before or during birth.
“The antibody levels for the infant fall over time regardless of breastfeeding, and they fall below what we consider to be protective levels a little earlier in infants born to vaccinated mothers compared with infants born to mothers who previously survived measles,” she said.
A measles infection during pregnancy is associated with increased risk for the mother and the fetus, including miscarriage and stillbirth, according to the American College of Obstetricians and Gynecologists.
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