Federal regulators are expected to authorize the mixing and matching of COVID-19 booster doses this week in an effort to provide flexibility as the campaign for extra shots expands. The upcoming announcement by the FDA is likely to come along with authorization for boosters of the Moderna and Johnson & Johnson shots and follows the OK for a third dose for the Pfizer vaccine for many Americans last month. … The FDA was expected to say that using the same brand for a booster was still preferable, especially for the mRNA vaccines from Pfizer and Moderna that have proved most effective against the coronavirus.