The reason the majority of kids get MMR+V is because it’s preferred over the single shot based on past data. MMRV earned approval from the Food and Drug Administration in 2005, but after a few years, it became clear that there was a slightly increased risk of febrile seizures when it is given as a first of two doses in toddlers 12 to 15 months (there was no increased risk for the second recommended dose, given at 4 to 6 years).

That increase is slight—there were 7 to 8.5 seizure cases for every 10,000 first-dose MMRV vaccinations, compared to 3.2 to 4.2 in 10,000 first-dose MMR + V vaccinations, analyses found. That difference works out to an extra one febrile seizure per 2,300 to 2,600 children. And febrile seizures are generally harmless—however alarming they may be for a parent to observe. A febrile seizure is simply a seizure associated with fever, and they can be spurred by almost anything that can cause a fever, such as the flu or an ear infection. In almost every case, children fully recover, with no long-term effects. By age 5, about 5 percent of all children have had such a febrile seizure for one reason or another.

Still, given the comparative increase over MMR+V, in 2009, the ACIP of the time combed through the data and decided that MMR+V should be preferred over MMRV. But, MMRV was still considered safe and effective and was left as an option for parents in consultation with their doctors. No new data has changed that view among experts since then.

Vulnerable toddlers

Between 2015 and 2025, use of MMRV among children in King County held steady at 15 percent for the decade, despite the ranked recommendation. And that 15 percent had clear demographic characteristics: Children who got an MMRV as a first dose were more likely than other vaccinated children to be in minority racial and ethnic groups. By comparison, significantly more of them were getting a “catch-up dose” after the initial window of 12 to 15 months, getting them instead between 16 and 47 months. Children getting an MMRV were also more than three times more likely than other vaccinated children to be eligible for a federal program that offers free vaccines to children in low-income families. They were nearly four times more likely to get vaccinated at a safety-net clinic.