By Trevor Hunnicutt and Carl O'Donnell WASHINGTON (Reuters) -The Biden administration plans to donate 500 million Pfizer coronavirus vaccine doses to nearly 100 countries over the next two years, three sources familiar with the matter told Reuters on Wednesday. The United States is likely to distribute 200 million shots this year and another 300 million in the first half of next year to 92 lower-income countries and the African Union, they said. The donations will go through the COVAX vaccine program that distributes COVID-19 shots to low- and middle-income countries. The program is led by the...