Holidaymakers to Spain are responsible for the majority of the UK's coronavirus cases, a new study suggests.
The study tracked a coronavirus variant, called 20A.EU1, that originated among Spanish farm workers in a super-spreading event.
The variant is known to have spread from farm workers to local populations in Spain in June and July. From there it spread to holidaymakers who took it back home across Europe.
In September the strain was found in 50 per cent of English Covid cases, as well as 80 per cent of Scottish and Welsh cases.
Experts behind the study, which has not yet been peer-reviewed in a medical journal, said there is currently no evidence that the strain spreads faster than other strains of coronavirus.
There is also no suggestion that the strain causes more severe disease, or would affect how a vaccine works.
Dr Emma Hodcroft, an evolutionary geneticist at the University of Basel and lead author of the study, told BBC Radio 4’s Today programme: “We know there was a super-spreading event among agricultural workers in Spain, that then the virus was able to jump into the local population and start moving more generally around Spain.
“This happened in kind of June and July, right when travel was picking up again in Europe, and of course Spain’s a wonderful holiday destination and many people headed there.
“What we think happened is that rising of cases in Spain combined with that increase in holiday travel allowed the virus to move to many different countries across Europe and, when it got there, it was able to spread quite successfully.”
Dr Hodcroft said it was the movement of people that had allowed the variant to spread, rather than any suggestion that the strain was particularly powerful or dangerous.
“That’s definitely what we think,” she said, adding that there were failures in the travel system which enabled it to spread.
“We are in the process of working with labs to more closely inspect the mutations, but we actually think that it’s really behaviour here that was the key point, and a few failures in the travel system over the summer that we really hope that we can learn from in the future so that next time, when we start opening up travel again, we won’t have to risk having cases rise again.
“I think there was actually three real failures here. So, first, the cases were rising in Spain earlier than in most of Europe, but we still allowed people to go travel there.
“And on top of that, we didn’t really do much screening of passengers at airports, and it’s very likely that possibly people didn’t follow the quarantine as much as they were supposed to.
“And then, finally, if the variant did get back to another European country, those countries weren’t able to cut that off quickly with just a few people, and instead it had a good environment where it could spread more widely so I think these are all things we could address.”
She said the strain was not the most prominent variant in all countries, particularly in France and in Belgium.
“Certainly, it is associated with the second wave, but we don’t think it’s responsible for it,” Dr Hodcroft said.
“It’s not very different from the the variants that circulated in spring. It only has six more mutations, most of which we absolutely are sure don’t do anything.
“Most importantly, we really don’t think that mutations have any impact on any immunity someone might have from being infected, or on a vaccine, and we have no indications that it changes the clinical course of the disease.”