OTTAWA—Prime Minister Justin Trudeau says Canada has been warned of manufacturing problems plaguing the Johnson & Johnson vaccine. The viral vector vaccine developed by J&J’s subsidiary, Janssen Pharmaceuticals, was authorized by Health Canada as safe and effective last week. Canada pre-ordered 10 million doses of the vaccine, which is the first and only one in Canada’s vaccine plan that needs only one dose. But Trudeau says Canada still doesn’t have a date for when it should receive the first deliveries. “We have heard in many conversations with Johnson & Johnson that there are challenges around production of the Janssen vaccine, but we will continue to engage with them and we look forward to receiving doses as soon as possible,” he said Tuesday at a news conference in Ottawa. “And as soon as we get confirmation of doses being sent to Canada, we will let everyone know.” Canada’s vaccine rollout has …