Taking hydroxychloroquine does not protect people who have been close to someone with coronavirus from becoming infected, a study suggests. Donald Trump told the world he was taking one pill a day to safeguard himself against the coronavirus, on the advice of his doctor. However, a study published in the New England Journal of Medicine suggests the drug is no more effective in protecting people exposed to the virus than a placebo. The trial, which was carried out in the USand Canada, recruited people who were at moderate to high risk of contracting Covid-19. Most of them were deemed at high risk because they had been closer than 2 metres from somebody with the virus for more than 10 minutes without wearing any personal protective equipment. The hope was that the drug could be used to protect people where somebody in their family was infected. This was a randomised, controlled trial – the so-called “gold standard” – and was set up to compare what happened in people given hydroxychloroquine within four days of exposure and those given dummy pills instead. The researchers enrolled 821 mostly younger and healthy adults with an average age of 40 who had no symptoms at the time. Within four days of exposure, each received a delivery from a courier of a package containing either placebo or hydroxychloroquine. The pills were to be taken over five days, starting with a stronger dose on day one. About one in eight (107 out of 821) of the participants developed Covid-19 over the 14-day follow-up period. Both confirmed cases and probable cases – those not tested but judged on symptoms – were included in the study owing to some lack of availability of diagnostic testing in the US. Among those who received hydroxychloroquine, 49 developed Covid-19 (or compatible symptoms such as fever or cough), compared with 58 in the group that received the placebo. The difference is not considered to be significant. Two patients had to treated in hospital, one in each group and there were no deaths. People given hydroxychloroquine were more likely to report side-effects such as nausea and stomach pain – 40% v 17%. But there were no serious reactions and no heart rhythm disturbances, which is a known issue with the drug. “While we had hope this drug would work in this context, our study demonstrates that hydroxychloroquine is no better than placebo when used as post-exposure prophylaxis within four days of exposure to someone infected with the new coronavirus,” said Dr Todd Lee, an associate professor of medicine, division of infectious diseases at McGill University in Canada and one of the lead authors of the study. “Our study’s results set politics aside and provide unbiased evidence to guide practice in the prevention of Covid-19 and reinforce the importance of randomised clinical trials as we work together nationally and internationally to combat the novel coronavirus,’’ said Dr Ryan Zarychanski, an associate professor of internal medicine at the University of Manitoba, Canada. Other trials taking place will show whether there is a place for the drug in preventing infections in other settings. A very much larger trial is underway in healthcare workers, aiming to recruit 40,000 around the world, led by the Mahidol Oxford Tropical Medicine Research Unit in Bangkok.