Why are measles cases increasing in US and how dangerous is the disease?
Executive summary
measles-outbreak">Measles cases in the United States have surged since 2024–2025 and continued into 2026 largely because pockets of under-vaccinated communities allowed imported cases to seed large outbreaks, with a single epicenter in South Carolina driving much of the national increase [1] [2]. The disease remains highly contagious and capable of severe outcomes—hospitalizations, encephalitis, long-term complications such as SSPE, and deaths were reported in 2025—making renewed vaccination and targeted public‑health outreach the central tools to stop transmission [3] [4].
1. What’s behind the sudden rise: importation meets under‑vaccination
Epidemiologists point to a familiar two‑step pattern: measles is repeatedly imported via international travel and then spreads where immunity gaps exist, producing chains of transmission that grow into outbreaks; CDC and PAHO data show many recent U.S. cases are outbreak‑associated and tied to travel‑linked introductions [5] [1]. Public‑health agencies and reviews cite declines in childhood vaccine coverage in some communities, high exemption rates in certain schools or counties, and social clustering of unvaccinated people as the fuels that allowed the 2025 surge and 2026 spillover to unfold [6] [7].
2. Why South Carolina became the epicenter and why that matters nationally
South Carolina’s outbreak—centered in Spartanburg County—has produced hundreds of cases, broad community circulation and large numbers of public exposure sites, which dramatically increased the national tally because many jurisdictions’ cases were outbreak‑associated with that cluster [2] [1]. Local outbreaks like this matter for national elimination status because sustained transmission over months can threaten the technical definition of ‘elimination’ used by public‑health bodies, a concern flagged by ASTHO and PAHO amid the 2025–2026 wave [6] [8].
3. How dangerous is measles now: real risks, who’s most vulnerable
Measles is among the most contagious human viruses and can cause severe disease—before widespread vaccination it caused thousands of hospitalizations and hundreds of deaths annually in the U.S.; recent outbreaks have again produced hospitalizations, neurologic complications and at least three deaths in 2025 [3] [4]. The greatest clinical risk is concentrated among young children (a quarter of 2025 cases were under age five), the unvaccinated, and medically immunocompromised or malnourished people, and measles can also cause rare but devastating delayed complications such as subacute sclerosing panencephalitis (SSPE) years after infection [3] [7] [4].
4. Why vaccines remain the decisive factor and what public health is doing
Two doses of MMR remain highly effective at preventing measles and, when coverage is high, produce community immunity that prevents outbreaks; public‑health bodies are therefore prioritizing catch‑up vaccination campaigns, mobile clinics, school verification and community engagement with trusted messengers to rebuild coverage in under‑vaccinated pockets [7] [6]. Jurisdictions have reported surges in vaccine uptake during outbreaks—evidence that visible transmission prompts protective behavior—but experts note that boosting trust and access is essential to converting short‑term vaccination spikes into sustained coverage [9] [6].
5. Uncertainties, alternative explanations and limitations in the reporting
Surveillance lags, differences in how jurisdictions classify outbreak‑associated cases, and evolving denominators for vaccine coverage complicate precise attribution of every case to a single cause, so while the convergence of importations and under‑vaccinated communities is well supported in CDC, PAHO and public‑health commentary, finer points—such as the role of specific social networks, detention centers, or precise post‑pandemic behavioral shifts—require local investigation beyond the national summaries provided here [1] [5] [9]. Reporting across sources is consistent that vaccination gaps are central, but granular causal inference about every outbreak driver is limited by the publicly available surveillance summaries [7] [10].