Moderna Inc. has begun studying its Covid-19 vaccine in children aged 6 months to 11 years in the US and Canada, the latest effort to broaden the mass-vaccination campaign beyond adults. The Cambridge, Mass., company said on Tuesday that the first children have received doses in the study, which Moderna is conducting in collaboration with the National Institute of Allergy and Infectious Diseases and a division of the Department of Health and Human Services. The bulk of the US Covid-19 vaccination campaign so far has placed its effort into protecting adults, who are at higher risk of severe disease caused by the coronavirus rather than children. Moderna’s and Johnson & Johnson vaccines are authorized for use in adults 18 and older, while the vaccine from Pfizer Inc. and BioNTech SE is cleared for use in people 16 and older.Efforts have been initiated to test the Covid-19 vaccines in children, who can still become infected, both to protect them from the virus and further build the population-level immunity to move past pandemic restrictions.