1/ Some people are looking at this map and saying: "it must have come from Europe". Here are a few reasons why that is unlikely.

2/ Though B.1.1.7 was especially prominent in Kent, the 2nd sequence ever sequenced was found in London, a day after the 1st. Due to superspreading it isn't possible to pinpoint for sure whether it originated in Kent or London. (If London, there's no argument for the continent).
3/ And suppose we did believe the new variant was introduced from abroad. Would we expect it to be introduced by sea or air? Far more arrivals to the UK in August/September occurred by air compared to sea.
4/ Genetic evidence. Suppose this highly transmissible variant had arisen in some other country, and made its way through France to the UK. It would also have left lots of descendants in Europe. We can look at a genetic tree of sequences and see that that is not the case.
5/ The grey/orange sequences here are UK sequences and the others are from an array of other countries. Specifically I've highlighted a lineage in Denmark which represents spread from a single introduction to Denmark, ultimately from the UK.
6/ Given there is evidence of spread from a single point in the UK, if the UK was not the source of the outbreak we'd expect all these UK grey sequences to group together, like these Danish sequences do.
7/ We'd expect to see at least some European "outgroups", looking like more distant relations on the family tree. But we don't.
8/ You can explore the tree more at https://t.co/XpB0fPkgo1
9/ One doesn't even necessarily need to invoke these phylogenetic arguments. We've seen how quickly this strain has taken over in the UK. We know it has not yet taken over in France. If it had existed in France before the UK, it would be the dominant strain there already.

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Brief thread to debunk the repeated claims we hear about transmission not happening 'within school walls', infection in school children being 'a reflection of infection from the community', and 'primary school children less likely to get infected and contribute to transmission'.

I've heard a lot of scientists claim these three - including most recently the chief advisor to the CDC, where the claim that most transmission doesn't happen within the walls of schools. There is strong evidence to rebut this claim. Let's look at


Let's look at the trends of infection in different age groups in England first- as reported by the ONS. Being a random survey of infection in the community, this doesn't suffer from the biases of symptom-based testing, particularly important in children who are often asymptomatic

A few things to note:
1. The infection rates among primary & secondary school children closely follow school openings, closures & levels of attendance. E.g. We see a dip in infections following Oct half-term, followed by a rise after school reopening.


We see steep drops in both primary & secondary school groups after end of term (18th December), but these drops plateau out in primary school children, where attendance has been >20% after re-opening in January (by contrast with 2ndary schools where this is ~5%).

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