Almost every day a new study is published that shines light on the way in which the new coronavirus is spread. It will be years before the precise dynamics of transmission are nailed down, but the broad outline of how the disease moves is becoming clear.
The latest findings come not from epidemiological estimates but from analysis of dozens of actual Covid-19 clusters unpicked by contact and trace teams from around the world. These studies are like a forensics report from a crime scene. While a good psychological profiler can point detectives in the right direction, the CSI team gives them the smoking gun.
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An analysis of such studies was recently posted on Twitter by Dr Muge Cevik, an infectious diseases clinician and researcher from St Andrews University. It was promoted by Sir Jeremy Farrar, the head of the Wellcome Trust and a member of the Scientific Advisory Group for Emergencies (Sage). "If you read one thread, make it this one," Sir Jeremy said.
Dr Cevik starts with a big Chinese study that traced 2147 close contacts of 157 confirmed Covid-19 cases. The overall infection rate was 6 per cent, but it was much higher among friends (22 per cent) and family members (18 per cent). In terms of location, the main risk factors were homes (13 per cent) transport (12 per cent) and dinner and entertainment (7 per cent).
Broadly similar findings emerge in several other papers. Risk of infection is much higher within households or other enclosed environments in which contact is close and sustained. In the outdoors, it falls to something in the 0-5 per cent range.
Transmissibility also appears to be affected by age and the type of relationship people have with one another. A Chinese study of 392 contacts of 105 confirmed cases found the attack rate in children was just 4 per cent within the home, compared with 17.1 per cent in adults, for example. The infection of spouses was super high, at 28 per cent.
Emphasising the link with age, another study found that household members over 60 were much more likely to become infected (18 per cent) than those under 20 (5 per cent).
Children, it seems, are not only better able to resist the infection within the home but also less likely to bring it back with them. A study, funded by the Australian Research Council, of 31 household clusters including children found that only 10 per cent had been sparked by children.
"Whilst Sars-CoV-2 can cause mild disease in children, the data available to date suggests that children have not played a substantive role in the intra-household transmission of Sars-CoV-2," said the authors.
Using these and other studies, Dr Cevik concludes that they suggest (not prove) the following:
• Close and prolonged contact is required for transmission of the virus.
• Risk is highest in enclosed environments such as houses, care facilities, public transport, bars and other indoor spaces where people congregate.
• Casual, short interactions are not the main driver of the epidemic.
• Susceptibility to infection increases with age.
Cevik does not look at how the virus passes from person to person, but plenty of other research suggests it moves in droplets expelled from the infected. Droplets may land directly on the mucus membranes of others (eyes, nose, mouth) or indirectly via shared surfaces (plates, desks, doorknobs). If the virus were truly airborne, household infection rates would almost certainly be far above 20 per cent.
Is this good or bad news for the British Prime Minister, who is working hard this weekend on the UK exit strategy? And what sort of policies might it point to?
On the upside, it suggests lockdown can be relaxed with much less risk in outside spaces. In truth, Britain has been more liberal on this from the start than places like Spain, France and Italy, and the evidence suggests it can almost certainly go further. The trick will be to find mechanisms to stop people from inadvertently forming crowds by turning up at the same places at the same time.
The evidence also points to the power of good hygiene. Household attack rates are high, at 20 per cent – but that leaves 80 per cent who escape despite living under the same roof. There is some evidence that UK infections started to fall before lockdown because of hand-washing and other hygiene messages. These should almost certainly be ramped up again, and extended. Think hand-cleansing facilities at every doorway, public or private.
On the downside, our winters are cold and wet, forcing us to congregate inside. Cevik says this points to the need to "redesign our living and working spaces and rethink how to provide better, ventilated living and working environments for those who live in deprived and cramped areas". That will be a hard task, but an essential one.
It is winter the Government will be most worried about. It can give us some outdoor freedom now, but it knows that won't mean much beyond September. The NHS also comes under much greater strain in winter, and Covid-19 symptoms will become harder to detect when mixed with flu and winter sniffles.
Ministers will therefore want to do everything possible now to drive down the virus to levels at which contact tracing teams can keep it in abeyance before winter arrives. Anything else and they risk a second major outbreak two to three months after the nights draw in.