OTTAWA, Jan 22 (Reuters) - Canadian Prime Minister Justin Trudeau said on Friday he had talked to Pfizer Inc Chief Executive Albert Bourla about shortages of the firm’s COVID-19 vaccine and said the next few weeks would be challenging. Trudeau told reporters that despite the current shortfall in deliveries Bourla had assured him the firm would be able to stick to its commitment to ship 4 million doses to Canada by the end of March. (Reporting by David Ljunggren)