A measles outbreak at an October 2024 wedding in New Brunswick contributed to the virus reestablishing itself in Canada, according to a recent analysis in The BMJ, a London-based medical journal.
The outbreak began after several wedding guests from Ontario were exposed to measles by another guest from Thailand. The Ontario residents brought the virus home, where it continued to spread, ultimately resulting in a multijurisdictional outbreak that lasted almost a full year, with 2,064 confirmed infections and 313 probable cases.
As the outbreak progressed, the authors said identifying specific sources of exposure became increasingly difficult, because transmission spread widely within the most affected communities.
“In under-immunised communities in Ontario, longstanding low immunisation uptake combined with the absence of measles circulation since the pre-elimination period of the mid-1990s, resulted in a large and highly susceptible cohort,” wrote the authors, led by Sarah E. Wilson, MD, the deputy chief of communicable disease control at Public Health Ontario.
Ultimately, Canada lost its measles elimination status in November 2025. The country had achieved the status in 1998 after adopting a two-dose schedule of the measles, mumps, and rubella (MMR) vaccine. One shot is 93% effective against measles, and two doses bump protection up to 97%.
As the study notes, Ontario reported more than 5,000 cases in 1991 alone, but just 101 cases total from 2013 through 2023.
Under-vaccination in Ontario led to an international outbreak
Nearly three out of every four cases of the outbreak were associated with a household that had more than one case, and 93% of cases were among those who were unvaccinated or had an unknown immunization status, according to the study.
Also, some 90% of these cases occurred among historically under-vaccinated groups, which include some Mennonite, Amish, and Netherlands Reformed communities.
“Frequent travel and social connections between affected communities meant that some cases had multiple plausible exposures across jurisdictions, complicating efforts to determine where transmission had occurred,” said the authors.
Over time, cases emerged in other regions in Canada, as well as in the United States, Mexico, and the Netherlands. Genome sequencing showed these cases were caused by the same strain of measles first identified in Ontario in November 2024.
Canada’s experience underscores the fragility of hard-won public health achievements and the need for countries to recommit to elimination.
To serve the ethno-religious and cultural groups affected by the outbreak, public health authorities translated information into Low German and then shared it through community-specific channels, such as radio, newspapers, and WhatsApp groups.
Messaging emphasized personal autonomy, the importance of protecting vulnerable people by preventing transmission, increasing awareness of measles symptoms, and the value of both the MMR vaccine and timely medical treatment.
Language barriers, concerns around stigma, and mistrust of government complicated the public health response, the authors said. Therefore, Low German–speaking community healthcare workers were valuable in facilitating communication, access to care, and informed decision-making on vaccination.
An ongoing measles outbreak in Pennsylvania has greatly affected that state’s Amish and Old Order Mennonite residents. Citing patient privacy, state health officials remain tight-lipped on aspects of outreach but say vaccine uptake has been strong in these communities in response to the outbreak.
The state health department has also noted that measles has reached unvaccinated Pennsylvanians who are not part of these communities. Indeed, public data on school immunization rates show that under-vaccination is a threat that cuts across many segments of Pennsylvania, not just Amish and Mennonite residents.
US might soon lose measles elimination status
Canada’s loss of measles elimination status is a troubling sign of a global trend in increasing susceptibility to infectious diseases, epidemiologist Jennifer Nuzzo, DrPH, director of Brown University’s Pandemic Center, wrote in an editorial The BMJ published with the study.
“Canada’s experience underscores the fragility of hard-won public health achievements and the need for countries to recommit to elimination,” she said.
She added that an outbreak occurring in a high-income country like Canada, with its substantial resources and access to effective vaccines, is particularly concerning.
However, Brazil and Venezuela, both of which lost their measles elimination status, were re-verified as measles-free in 2024 and 2023, respectively, by the Pan American Health Organization (PAHO).
Next week PAHO will meet in Washington, DC, and is scheduled to discuss whether the United States will lose its elimination status. The country gained elimination status two years after Canada, in 2000, but has reported nearly 5,000 cases this year—though the true number of infections is believed to be much higher.
Source: www.cidrap.umn.edu
