Biogen Inc. agreed to pay $22 million to resolve U.S. allegations that the company illegally used two charities that help cover Medicare patients’ out-of-pocket drug costs as a means to pay them kickbacks to use the Cambridge, Mass-based drugmaker’s multiple sclerosis medicines.

Initially traced to about 100 cases of Covid-19 that spread across the United States, the number of infections linked to a Biogen corporate conference held in February 2020 at the Boston Marriott Long Wharf Hotel has grown exponentially and researchers estimate the total could be as many as 300,000.