Washington (AFP) – Joe Biden has tested positive for Covid-19 for a second time and is returning to isolation, his White House doctor said Saturday, attributing the result to “rebound” positivity from treatment the US president received. Biden “tested positive late Saturday morning, by antigen testing,” following four consecutive days of negative tests, and “will reinitiate strict isolation procedures,” presidential physician Kevin O’Connor wrote in a memorandum. “This in fact represents ‘rebound’ positivity,” O’Connor said, referring to a situation in which patients treated with the drug Paxl…