GEORGE OBULUTSA THE African Union (AU) has secured another 400 million doses of the AstraZeneca COVID-19 vaccine, a regional health leader said, in a push to immunize 60% of the continent's population over three years. As richer nations race ahead with mass immunisation campaigns, Africa is scrambling to obtain supplies for its 1.3 billion people. Only a handful of African nations have begun giving doses. John Nkengasong, director of the AU's disease control and prevention body, told an online briefing that in addition to 270 million doses previously secured, the bloc would get 400 million shots from the Serum Institute…