GORDON BAY, Canada — Three months ago, Canada, which has no domestic manufacturer of COVID-19 vaccines, lagged far behind the United States in immunizations. Only 3% of its population was fully vaccinated. Canadians watched glumly as friends and relatives south of the border lined up for shots, while residents of Toronto and Montreal suffered repeated lockdowns.

No longer. Last month, Canada blew past the United States in the share of its population that’s fully vaccinated — 58% as of Friday, versus 49% in the U.S. — to take first place among the seven big industrial democracies. (The United States ranks sixth, ahead of only Japan.)

How did Canada, the country that most closely resembles the United States, do so much better, even though it had to wait longer for Pfizer-BioNTech and Moderna to deliver their vaccines?

The simple answer is that in Canada, the pandemic didn’t become a politically polarized issue, as it did in the United States.

Canada’s major political parties, including the opposition Conservatives, joined early in full-throated support of mass vaccination. Leading politicians didn’t dismiss immunization as unnecessary, deride mask mandates or attack scientists.