Measles outbreak in a unique sociocultural context: epidemiological investigation and control measures in Northern Brazil, 2025.

After the recertification of measles elimination in Brazil, outbreaks associated with imported cases may occur in pockets of susceptible individuals, including communities with low vaccination coverage. This study describes an outbreak investigation conducted in 2025 in Campos Lindos, Tocantins State, Brazil. In total, 25 cases were confirmed, mostly among unvaccinated Old Believers (a traditional community of Russian origin) following viral introduction linked to international travel, with transmission occurring predominantly within the community. Outbreak response measures, including ring vaccination and active case finding, interrupted transmission after 12 weeks with no new cases. The rapid response was effective and reinforces the importance of identifying susceptibility pockets in the population to prevent measles reintroduction.