'We are starting a clinical trial where 50 patients will be treated with the antiviral drug Paxlovid and 50 will be given a placebo. This is to probe whether persistent virus infection may be causing long Covid in a subset of people. We’re not giving them the conventional five-day course, but a 15-day course of treatment, to see if that can eliminate the source of the problem, which is the remnants of replicating virus'