U.S. drug developer Novavax has started a mid-stage study of its experimental COVID-19 vaccine in South Africa. The company says the study will be conducted on about 2,665 healthy adults and will evaluate safety and immunogenicity in about 240 medically stable HIV-positive adults. South Africa is the world's fifth-worst affected country with 583,653 recorded coronavirus cases and 11,677 deaths. The Bill and Melinda Gates Foundation is giving a grant of 15 million U.S. dollars toward the clinical trial.