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Toppage > Measles, Epidemiological week 1-20, 2026 (as at 20 May 2026)

Measles, Epidemiological week 1-20, 2026 (as at 20 May 2026)

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◆Measles, Epidemiological week 1-20, 2026 (as at 20 May 2026)

Measles is an acute viral infection caused by the measles virus and is characterized by high fever, generalized rash, and catarrhal symptoms. It is highly contagious and is transmitted primarily through airborne, droplet, and contact routes. Measles pneumonia, which commonly complicates infections in infants and young children, and measles encephalitis, occurring in approximately 1 in 1,000–2,000 cases, are major causes of measles-related mortality. In addition, individuals, particularly those who had measles during infancy, may develop subacute sclerosing panencephalitis (SSPE) years to decades later. Although there is no specific treatment, vaccination can prevent measles and reduce the risk of severe disease. Japan was verified by the Regional Verification Commission for Measles Elimination in the Western Pacific, World Health Organization (WHO), as having achieved measles elimination in 2015, and this status has been maintained through 2024. The Guidelines on Prevention of Specified Infectious Diseases Concerning Measles (Ministry of Health, Labour and Welfare (MHLW) Notification No. 442, 2007), as amended, are in place to maintain the elimination status. This report summarizes the latest epidemiological situation of measles in Japan based on the National Epidemiological Surveillance of Infectious Diseases.

The number of measles cases reported during weeks 1 to 20 of 2026 (as at 20 May 2026) was 498, exceeding the number reported during the same period in any year from 2020 to 2025. By week of diagnosis, case counts ranged from 0 to 5 cases per week during weeks 1 to 4, but increased thereafter to between 28 and 39 cases per week during weeks 11 to 14 and reached the highest level at 62 to 71 cases per week during weeks 15 to 17. Although the number of cases has subsequently decreased to between 17 and 23 cases per week from week 18 onward, continued close monitoring is warranted, as case reporting may have been affected by the extended holiday period. Of the 498 reported cases, 495 were laboratory-confirmed and met the pathogen-based diagnostic criteria required for notification, while the remaining three were clinically diagnosed cases. Among the 495 laboratory-confirmed cases, 319 were classified as typical ‘measles’, presenting all three clinical features (rash, fever, and catarrhal symptoms), while 176 were classified as ‘modified measles’, presenting one or two of these clinical features. There were 322 males and 176 females, with a median age of 26 years (range: 0–65 years).

Cases were reported from 27 prefectures. The prefectures with the highest numbers of reported cases were Tokyo (244 cases), Kanagawa (46 cases), Saitama (38 cases), Kagoshima (34 cases), and Chiba (31 cases). Marked increases in reported cases were observed in Tokyo from week 15 onward, while 1 to 10 cases per week continued to be reported from Kanagawa, Saitama, and Chiba. A total of 360 cases, including 58 with unknown prefecture, were presumed to have been infected within Japan; 39 cases were presumed to have been infected overseas (Indonesia: 14; New Zealand: 7; India: 4; Vietnam: 3; Taiwan: 2; United Kingdom: 1; South Korea: 1; Singapore: 1; Philippines: 1; United States: 1; Malaysia: 1; Indonesia/Singapore: 1; Vietnam/Thailand: 1; Finland/Italy/France: 1); 9 cases were presumed to have been infected either within Japan or overseas [Chiba/New Zealand: 4; Chiba/South Korea: 1; Tokyo/Vietnam: 1; Tokyo/Indonesia/Singapore/Hong Kong: 1; Aichi/Turkey/Canada/Germany: 1; Japan (prefecture unknown)/Vietnam: 1]; and the place of infection was unknown for 90 cases.

Vaccination history, as confirmed by medical institutions and public health centers, was analyzed by age group: 1–5 years (age group that would have completed the first dose of routine MR vaccination) and ≥6 years (age group that would have completed the second dose of routine MR vaccination). Among the 13 cases aged 1–5 years, 9 cases (69%) were unvaccinated, 3 cases (23%) had received one dose, and 1 case (8%) had received two doses. Among the 478 cases aged ≥6 years, 63 cases (13%) were unvaccinated, 62 cases (13%) had received one dose, 158 cases (33%) had received two doses, and 195 cases (41%) had unknown vaccination status. Of the 159 cases with two documented doses, 69 were typical measles, 89 were modified measles, and 1 was classified as clinically diagnosed measles. Of the 72 unvaccinated cases, 68 were typical measles, and 4 were modified measles. Among cases aged ≥1 year who met the pathogen-based diagnostic criteria, 68 of 313 typical measles cases were unvaccinated, whereas among 175 modified measles cases, only 4 were unvaccinated and 89 had received two documented doses.

Although some measles cases with a two-dose vaccination history were also reported, the benefit of vaccination cannot be assessed by simply comparing the number of cases by vaccination status alone. Given that, particularly among younger age groups, the number of individuals with a two-dose vaccination history substantially exceeds the number of unvaccinated individuals, the risk of developing measles is considered to be relatively low among those who have received two doses. In Japan, routine measles vaccination was introduced in 1978, and since fiscal year 2006, the current routine immunization schedule, consisting of two doses in the first and second stages, has been implemented.

As at 20 May 2026, measles virus genotype information was available for 341 of the 498 cases reported to the National Epidemiological Surveillance of Infectious Diseases. The detected genotypes were B3 in 132 cases (39%) and D8 in 209 cases (61%).

Changes in measles epidemiology have been observed worldwide, including increased cases in the European Region in 2024 and the loss of measles elimination status in Canada in 2025. Individuals planning international travel should confirm the epidemiological situation at their destination, review their vaccination history, and receive any necessary vaccinations. In addition, within the country, several local governments have issued public advisories in response to cases of infection in schools and the occurrence of patients at facilities where large numbers of people gather. To prevent domestic transmission, completing the two-dose measles-rubella (MR) vaccination schedule under the Immunization Law remains the most important measure for both individual protection and maintaining herd immunity. Early case detection and rapid public health response are also essential. Key measures to prevent secondary transmission include accurate diagnosis, rapid contact tracing and response, and timely information sharing with healthcare providers and the public. Rapid communication between local governments is particularly important when patients travel widely or have contacts across prefectural borders.

In areas where measles cases have been reported, and at medical institutions that may examine international travelers, infection prevention and control measures should be further strengthened. All healthcare personnel, including administrative staff, who may have contact with measles patients or those exposed to them should have their vaccination and infection histories reviewed, and vaccination should be administered when necessary. Vaccination should also be considered within 72 hours of exposure to a measles case for susceptible contacts, as it may help prevent disease onset. Individuals who develop symptoms such as fever after contact with a measles case should call the medical facility before visiting, and avoid using public transportation whenever possible to prevent secondary transmission.

Measles is an airborne disease and cannot be effectively prevented by hand hygiene and face mask use alone. It is highly contagious and infectiousness begins even before the onset of fever, and outbreaks have been reported in which a single measles case served as the index case, leading to an increase in the number of patients within the same facility or community. Continued occurrence of cases is therefore anticipated. In addition to rapid response, maintaining high two-dose MR vaccination coverage among children at the target ages for routine vaccination remains essential. Individuals without a history of measles or measles vaccination are encouraged to consult their healthcare provider regarding immunization.

International mass gathering events, including the 20th Asian Games (Aichi–Nagoya 2026), are scheduled this year and are expected to increase the potential risk of infectious diseases. Given the likelihood of contact with numerous unspecified individuals, especially in densely populated spaces, the verification of vaccination history is therefore recommended.

For detailed information and the latest updates regarding measles surveillance (as at 20 May 2026), please refer to the resources listed below:

 

Department of Infectious Disease Surveillance
National Institute of Infectious Diseases, Japan Institute for Health Security

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