COVID infections are more likely to spread in households than any other indoor setting, according to new research.

Academics at Imperial College London found that the risk of passing the virus on is significantly higher when people spend more than five days together in the same household, compared to five days or less.

The study also provides the some of the clearest evidence yet that individuals with asymptomatic infection are much less likely to infect others than people with symptoms.

The researchers found that the chance of an asymptomatic infected person infecting a close contact was 3.5 per cent compared to 12.8% in the case of a symptomatic person.

Households show the highest transmission rates among indoor settings compared to social gatherings, travel, healthcare, workplace and casual close contacts.

Hayley Thompson, a co-author and public health researcher at Imperial College London, said: "With many countries continuing to recommend 'stay-at-home' measures, the upcoming festive season and with cases isolating inside households, our results show it is likely that this location will continue to be important in sustaining transmission."

Professor Neil Ferguson, co-author and former UK SAGE advisor, added: “This analysis provides some of the first evidence that asymptomatic infections are substantially less infectious than symptomatic cases.

"It also reinforces growing evidence of the importance of household transmission, especially in the context where symptomatic cases are not isolated outside the home.”

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The findings are based on a global analysis of transmission, and come amid ongoing debate over the safety of Christmas gatherings.

In Scotland, three households of up to 11 people be allowed to stay together for up to five days, from December 23 to 27, to celebrate the holiday.

Understanding where transmission of SARS-CoV-2 takes place is critical to effectively targeting interventions, but evidence of transmission rates in different settings has been limited.

The researchers analysed data from 45 studies covering a number of settings including households, social gatherings with friends and family, travel, healthcare, workplace and casual close contacts, such as strangers during shopping at a supermarket.

They used this to assess the 'secondary attack rate' of each setting - that is, the probability of onward infection from one infected individual among a defined group of close contacts, such as a family group.

The researchers found that households show the highest transmission rates with a SAR risk of 21.1%.

The team also find that the chance of one household member infecting another is significantly higher when the duration of household exposure is more than five days compared to five days or less.

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The research team found moderate evidence of less transmission occurring both from and to individuals under 20 years of age in the household context, but insufficient data to determine whether this held in other settings.

Researchers found limited data reporting transmission in workplaces, schools, and care-= homes, highlighting the need for further research in these settings.

The researchers also caution that the majority of studies included in the pooled analysis came from China, where strict control policies were implemented.

Professor Linda Bauld, the Usher chair of public health at Edinburgh University, said: "This new report hasn’t yet been published in a journal and gone through peer review, so we need to be cautious about its findings. But it does flag some serious points for us to consider about plans for Christmas.

"The researchers analysed the results from many studies in different countries to find out where the virus that causes Covid-19 is passed on to others. They found that infections occur most commonly within households, particularly where people live together or have an extended stay rather than just visiting.

"There are two key messages from this study for Scots to consider over the Christmas period. The first is that household mixing over Christmas in people’s homes is not without risk. In fact it could be worse than meeting up with relatives in a café of restaurant, for example."

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Prof Bauld said people forming a Christmas households bubble should keep contact to a minimum and "avoid overnight stays if possible". 

She added: "The review found that staying in the same house over several days with members of another household will increase infection risk.

"So perhaps meet up for a coffee or a meal in other people's houses, even if you have to travel a distance to get there - but keep it short."

The work is presented in the latest report from the WHO Collaborating Centre for Infectious Disease Modelling.