The rules suggest homes are Covid hotspots but the data isn’t so clear

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On September 14 the new “rule of six” came into effect in parts of the UK. Households will have to get used to their worlds shrinking again – social gatherings, inside and out, of more than six people are now illegal in England, Scotland and Wales.
The government says the new rules are to help police identify and disperse illegal gatherings, including raves and parties – but the rules will have the biggest effect on households, while pubs and restaurants remain open, and many people are being encouraged to stop working from home. At first, the government’s focus on households make sense: the latest Covid-19 surveillance report shows that, for people who tested positive and reported their contacts, the vast majority of them reported contacts who also live in their. But this data doesn’t tell the whole story about where Covid-19 is being transmitted.

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The household transmission numbers come from the UK’s contact tracing system: NHS Test and Trace. Once a person tests positive for coronavirus, the service will get in contact via a text, email or phone call, and ask them to share details of the places they have visited and the people they have had recent contact with.
These individual cases are categorised as non-complex. Complex cases are those in places like a health or care setting such as a hospital, care home or prison, aren’t managed at an individual level; instead, local public health specialists give advice directly to the affected institution. Therefore, information on individual contacts associated with these situations is not available.
Over the past five weeks, the reports show that, of all the contacts that Test and Trace followed up, just under 60 per cent had been exposed in their home. Since many of these cases occur within the home it’s likely we would have traced these contacts anyway. “We don’t need track and trace to tell people within the same household to isolate,” says Daniel Lawson, senior lecturer in data science at Bristol University. “So this is adding only 40 per cent value in some sense – though this is a critical 40 per cent as those cross-household infections are those worth fighting against.”
Additionally, between ten and 20 per cent had been exposed visiting an infected household. Places like Bolton, Oldham and Salford are marked as areas “in which overall infection rates are high, with household transmission a key infection pathway”.

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These figures dwarf contacts in other settings – “leisure and community” settings, which include eating and drinking out, attending arts events, and going to church, were much lower than household visitors for the week ending September 6. “Public services”, like hairdressers, barbers, tattooists and nail bars, were practically nil. “I would expect that to be a biased sample of the population,” says Lawson. “Really we just don’t know about what is missing from the data set – that’s the big problem.”
One of the key problems is that household tracing has been way more accurate than other venues – basically, we know who we live with. Compare households to pubs, for instance. From September 18, venues will be forced to collect and share data on visitors, or risk a fine. Since July 4, when pubs reopened, tracking has only been recommended. Customers may also have slipped in and out of premises, not wanting to name who they were with, or simply forgot.
This all means that some pubs won’t have a clue about who is actually meeting in their venues. If people eating out to help out caused Covid-19 cases to rise, we might not see the correlation in Test and Trace. Complex cases, like in a school or prison where there is accurate data about who is there, are easier to trace in this situation than non-complex ones.
Correspondingly, the household transmission rate is probably inflated. “If the data is saying above 50 per cent transmissions in the household that sounds pretty high,” says Thomas House, a reader in Mathematical Statistics at the University of Manchester. This is not the rate we would expect to see for a country that has been out of lockdown for several months, he says, where people have spent the summer visiting crowded venues.

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One of the ways out of this confusion would be the development of a reliable contact-tracing app. This would drastically improve Test and Trace, removing the reliance on memory or dodgy paper forms. But England’s app has a troubled birth – its first iteration was trialled on the Isle of Wight in May, then abandoned. A new app, which will let people scan barcode-like QR codes to register visits to hospitality venues, will be launched across England and Wales on September 24.
“A good app would almost eliminate the need for manual track and trace,” says Lawson. “Obviously, somebody needs to do the follow up, but the process of filling in forms based on memory is mostly redundant – it’s more confirming the list that the app provides you, if the app is implemented in such a way that makes that possible. The problem, of course, is that discussion of the app has gone a bit quiet.“
Will Bedingfield is a staff writer for WIRED. He tweets from @WillBedingfield
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