When Will the Measles Outbreak End?
Paul Offit, MD, offers a glimpse of when the outbreak might end as well as where we are with herd immunity, the MMR vaccine’s efficacy, and insights on breakthrough cases.
The Johns Hopkins Bloomberg School of Public Health’s Center for Outbreak Response Innovation has a measles outbreak tracker. Much like they did during the COVID-19 pandemic, and were tracking cases of the virus, the institution is tracking measles cases across all states.1
As of today, May 13, there are 1,014 cases of measles across 31 jurisdictions in the US according to the Hopkins tracker.1 For comparison, in 2024, there were 285 cases across 33 jurisdictions.2
According to the Centers for Disease Control and Prevention (CDC) there have been 14 outbreaks (defined as 3 or more related cases) reported in 2025. In 2024, there were 16 outbreaks reported during 2024.2
One of the bigger questions around the outbreak is when will it end? Paul Offit, MD, the director of the Vaccine Education Center and an attending physician in the Division of Infectious Diseases at Children's Hospital of Philadelphia (CHOP), is a veteran of the 1990-1991 Philadelphia measles outbreak where his hospital and the other children’s hospital in Philadelphia were overwhelmed with cases. During that outbreak, there were more than 1400 cases, with a majority in unvaccinated children, and led to 9 children dying.3 And, Offit says if his past experience is any indicator, there is a seasonality to measles and does believe there will be an end in sight soon.
“The measles epidemic now will die down because it's essentially a winter disease, and by mid-spring, it's gone. That's what happened in Philadelphia,” Offit said. We basically didn't see any more measles beyond mid-May, so I think we're getting there. My sense is that the incidence of measles is starting to decline. But, if we continue to choose not to vaccinate our children, it'll be back again next winter.”
The measles vaccine is highly effective with 1 dose of the MMR vaccine being shown to be 93% effective, and with 2 doses, being 97% effective.
“The measles vaccine is so good that all you need is immunological memory, because measles is a long incubation period disease. From the time when you're exposed to the time that you develop symptoms—2 weeks, sometimes as long as 3 weeks—that's plenty of time for those memory cells to become antibody secreting cells, and protect you even against mild disease,” Offit said.
In terms of herd immunity, he believes it may be gone in the US.
“I think we're there,” Offit said. “I've talked to people on the ground who have told me that if you look at the death rate, you look at the doubling rate, and you look at the hospitalization rate, it's probably at least 3000 cases, and maybe as many as 5000 cases in the US. So I think we've already exceeded where we were in 2019 where we had 1200 cases. I think that we are no doubt going to soon lose our status as a country that eliminated measles.”
Although rare, breakthrough infections are being reported, and Offit says for those who are vaccinated, cases are typically very mild.
“People who are vaccinated that nonetheless develop symptoms are invariably mild, because the vaccine is so remarkably effective, even as a single dose, it's effective at preventing all manner of illness, mild, moderate, or severe disease.”
This is the second segment of a 2-part interview with Offit. In the first segment, he discussed if measles has become a binary choice for families between vaccination and treatment.
