Canada's measles case count has passed 4,500, with the western province of Alberta — where about 5 million people live — recording more cases this year than the United States, figures updated Thursday showed.
World Health Organization numbers released this month show Canada accounts for about half the confirmed measles cases across the Americas this year.
Canada eradicated measles in 1998, but the virus has stormed back, particularly among unvaccinated members of certain Mennonite Christian communities.
The most populous province of Ontario, Canada's most populous province with about 16 million residents, has recorded 2,366 cases, according to federal government data updated this week, which put the national case count at 4,638.
Alberta's government, which releases its weekly figures on Thursdays, said it had registered 1,790 cases, making it the hardest-hit area per capita.
The United States, confronting its worst measles epidemic in 30 years, has confirmed 1,375 cases, the Centers for Disease Control and Prevention said this week.
The Pan American Health Organization, WHO's regional office, said this month that 71% of confirmed cases occurred in unvaccinated people, with an additional 18% among people whose vaccination status was not known.
Canadian experts have pointed to several factors driving the outbreak, including the proliferation of vaccine misinformation.
Canadian physicians have criticized U.S. Health and Human Services Secretary Robert F. Kennedy Jr., who has spent decades spreading false information about vaccines.
But the bulk of the Canadian epidemic has occurred among Anabaptist Christian communities — of which Mennonites are one — where vaccine hesitancy is historic.
The beginning of the outbreak has been linked to a Mennonite wedding in the eastern province of New Brunswick.
Outside of Ontario and Alberta, which have larger Mennonite communities, cases have been isolated, with British Columbia the third-hardest-hit province with 190 cases.
The only suspected measles-related death in Canada thus far in the outbreak was that of a newborn baby whose mother was unvaccinated, but officials noted that the baby was born prematurely and had other medical conditions.