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Hong Kong seems to have given the world a lesson in how to effectively curb COVID-19. With a population of 7.5 million, it has reported just 4 deaths. Researchers studying Hong Kong’s approach have already found that swift surveillance, quarantine and social-distancing measures, such as the use of face masks and school closures, helped to cut coronavirus transmission — measured by the average number of people each infected person infects, or R — to close to the critical level of 1 by early February. But the paper, published1 this month, couldn’t tease apart the effects of the various measures and behavioural changes happening at the same time.