UK was inundated by new coronavirus cases from abroad, genetic analysis reveals

Tom Clarke

Former Science Editor

Forget the UK's "patient zero", new cases of coronavirus were pouring into the country by the hundreds each week during March, helping to explain why our outbreak grew so large so quickly, new analysis of virus genomes reveals.

The study also finds the vast majority of cases arrived in the UK from mainland Europe at a time when much of our travel advice was focused on Asia.

The finding reinforces the conclusion of leading scientists like Professor Neil Ferguson on Wednesday that many lives could have been saved if the lockdown had happened sooner.

Scientists from the UK - Coronavirus Genomics Consortium - have been collecting the genetic sequences of more than 25,000 virus samples from UK cases of the virus.

Because the virus makes copying errors in its genetic code as it reproduces, it slowly mutates as time passes.

They used the changes in the virus genome to build a family tree of the SARS-CoV-2 virus from its origin in China to more than 7,000,000 confirmed human cases worldwide.

"The coronavirus evolves at around two-and-a-half mistakes a month and over the period of an outbreak that results in many different genomes of many different types being detectable," says Nick Loman, Professor of microbial genomics at the University of Birmingham and a member of the UK-COG.

They used their data on the virus' family trees, combined with data on travel patterns to the UK at the time to map where new cases of Covid-19 were coming from.

The majority (34%) came from Spain, a similar number (28%) from France, 14% from Italy, although these cases made up the bulk of early infections in February and 23% from other countries.

The bulk of cases - more than 80% - arrived in the UK between February 28 and March 29.

The UK implemented lockdown on March 23.

On Wednesday, Prof Ferguson told a committee of MPs "had we introduced lockdown measures a week earlier, we would have reduced the final death toll by at least a half".

The data also shows that while major public gatherings like football matches and the Cheltenham festival could have been important for spreading the virus, so many people were brining the virus into the country at the time, these events alone weren't significant drivers of our outbreak.

The study also found, that as our outbreak grew, the UK itself became a source of spread elsewhere.

It reveals the majority of cases contributing to Iceland's outbreak originated in the UK.

But the real benefit of sequencing Covid-19 genomes could be yet to come.

Now the numbers of infections has declined, its possible to use the virus' family tree to better trace and track new infections based on how related they are to one another.

"It's a very powerful tool," says Prof Loman.

"This allows us to look backwards and say, were all these cases at a particular gathering, is there an association with travel."

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