Dec. 10, 2020 7:27 pm ET SYDNEY—A Covid-19 vaccine candidate in Australia has been scrapped because recipients returned false positives on certain HIV tests, underscoring the difficulties scientists face in rapidly developing an inoculation for the coronavirus. The vaccine candidate, called v451, was being jointly developed by the University of Queensland and CSL Ltd. , an Australia-based biopharma company that also runs blood-collection centers in the U.S. The Australian government had agreed to buy 51 million doses. The episode illustrates why governments have sought access to multiple vaccines and more doses than needed for their populations. Some vaccines might not prove effective, adverse reactions might arise and manufacturing constraints could limit how many doses can be produced.