In Aberdeen it was pubs, restaurants and golf courses. In Prague it was a nightclub. In California it was a church. Why does Covid-19 appear to spread more easily in some places than others? Prof Devi Sridhar, the chair of global public health at the University of Edinburgh, says there are some key factors that make indoor settings higher risk, in particular poor airflow, difficulties in distancing from others, and prolonged exposure. According to a study published in June by researchers including Dr Sebastian Funk of the London School of Hygiene and Tropical Medicine, most clusters of infection are linked to indoor settings ranging from factories to shopping areas. The research also suggested that some settings may have particular risks: for example, in meat-packing factories the chilly climate, prolonged close proximity of workers and the need to shout over the noise of the machines may have increased the risk of transmission. Similarly, a research paper from Japan noted: “Many Covid-19 clusters were associated with heavy breathing in close proximity, such as singing at karaoke parties, cheering at clubs, having conversations in bars, and exercising in gymnasiums.” Meanwhile, the UK government is encouraging diners to return to restaurants through the “eat out to help out scheme”. How concerned should we be? According to data from the Office for National Statistics, nearly four in 10 adults (37%) said they would feel comfortable or very comfortable eating indoors at a restaurant this week, up slightly from 34% last week. Of the 92% of survey respondents who went out in the seven days to 2 August, one in five went to eat or drink at a restaurant, cafe, bar or pub. “Pubs and bars are quite high risk: it goes back to the conditions inside – poor ventilation and a crowded setting,” says Sridhar, adding that there is growing concern about the extent to which Covid-19 may be spread by fine particles known as aerosols as well as larger respiratory droplets. Clusters have been linked to restaurants, including a case in China where it has been suggested that droplets were blown between tables by the air conditioning. “It’s probably going to depend a bit on your ventilation and airflow, but I think a crowded restaurant is not a good idea,” Sridhar says. “Anything that is outdoors, even outdoor hospitality, I am quite relaxed about, especially if there is a breeze.” . Identifying the source of an outbreak is hindered by weaknesses in the current tracing systems in the UK . “We don’t currently do backward contact tracing on each case, so we don’t routinely know where each case picked up their infection,” says Prof John Edmunds, an epidemiologist at the London School of Hygiene and Tropical Medicine. “Public Health England investigate the complex cases – these are the ones that are linked with an institution such as a workplace, school or care home. However, for the rest of the cases – the majority of them – we have rather poor information as to where they might have been infected. This obviously hinders evidence-based decision-making about how best to organise local restrictions when there is a community flare-up.” Funk says understanding where and how infections occur is important as it could make it easier to trace cases, advise the public where they may need to take extra precautions such as wearing face coverings, and decide what measures to take should an outbreak occur. For example, there have been few recorded outbreaks in schools. Funk says this may be because children are less likely to show Covid-19 symptoms than adults, meaning clusters may not be picked up. “Kids don’t get very ill if and when they get infected. What we don’t know is whether they also have a lower chance of getting infected and whether they are also not as good at transmitting the virus as others.” Dr William Hanage, a professor of the evolution and epidemiology of infectious disease at Harvard University, says more information on effective ways to prevent transmission is helpful, but he stresses that outbreaks tend to occur because of similar factors. “It doesn’t take much digging to find examples of outbreaks in all kinds of places, and there’s a tendency to cherry-pick from them according to political priorities. But it is a mistake to fixate on a single outbreak, and better to recognise that they become more likely whenever people gather together indoors in poorly ventilated spaces.” The key issue, he adds, is the degree to which authorities try to prevent or encourage such gatherings. “I don’t understand how the British government maintains the cognitive dissonance required to suggest that relaxing restrictions in Europe leads to ‘second waves’ while encouraging British people to go to work or school or pubs will not have a similar impact.”