Scientists believe coronavirus may have entered the country on 1,356 separate occasions at the beginning of the UK epidemic, which peaked in mid-March.
The COVID-19 Genomics UK (Cog-UK) consortium studied the genetic code of the virus and analysed data on inbound travel and the number of infections worldwide to determine how COVID-19 was introduced in the UK.
Its preliminary analysis shows there was “a very large number of importations due to inbound international travel”, with around a third of cases estimated to have come from Spain.
The study, which has not been peer-reviewed, states the coronavirus outbreak did not begin from one source – more commonly known as “patient zero” – but that there were at least 1,356 separate points of origin, which scientists warned is likely to be an underestimate.
The report states that until the beginning of March, the UK received around 1.75 million inbound travellers per week, which was 10 per cent higher at the end of February due to the half-term school holidays.
It estimates that 80 per cent of the earliest cases of COVID-19 that arrived in the UK happened between 28 February and 29 March.
The government advised against all non-essential overseas travel on 17 March and advised British travellers overseas to return to the UK on 23 March.
From Monday this week, travellers arriving in the country are required to self-isolate for 14 days or face a fine of up to £1,000 in England.
The study states: “The rate peaked in mid-March and most introductions occurred during March 2020.”
It continued: “There was a period in mid-March when inbound travel to the UK was still substantial and coincided with high numbers of active cases elsewhere.”
The UK was placed in lockdown on 23 March, which Neil Ferguson, professor of mathematical biology at Imperial College London, believes could have been implemented earlier to save thousands of lives.
The greatest number of inbound travellers arrived from Spain and France, which could account for 34 per cent and 29 per cent of imported cases respectively, the study said.
There were also high numbers estimated to be coming from Italy (14 per cent), Belgium (6 per cent) and the Netherlands (5 per cent), it added.
As 20,000 people were flying in from Spain per day in mid-March, the study suggested that events such as football matches were likely to have made little contribution to bringing the virus into the country.
It stated that initial cases of the virus coming into the UK from China, where the first case of coronavirus was reported, constituted “a tiny fraction” of all cases, estimated to be around 0.08 per cent.
The study found that the number of imported cases quickly declined to a low level in April.
What is the R number, and why is it relevant to coronavirus?
The reproduction number – often called the R value or R number – is a measure of a disease’s ability to spread. It tells us how many people a single infected person will pass on the disease to.
The R number for COVID-19 that’s being quoted in the media and government briefings is what’s known as the ‘effective’ reproduction number. This value can go up and down.
We can reduce R by making it harder for the disease to spread, by implementing measures such as social distancing, closing restaurants and non-essential shops, and encouraging people to stay at home.
Every disease also has what’s called a ‘basic’ reproduction number, R0, which is the fixed value of R if no measures are put in place. For example, measles is highly contagious, with a R0 as high as 18, while COVID-19 has a R0 of around three.
So if COVID-19 was allowed to spread through the population, an infected person would, on average, give the disease to three other people.
But if all these people are practising physical distancing, then the virus can’t spread so easily and the effective R value goes down.
The crucial thing is to keep R below 1. If we can do this, then the number of new cases dwindles and the outbreak will eventually come to a halt.
Conversely, if R rises above 1, then we run the risk of rapidly escalating case numbers that would require stronger measures to keep the virus under control.
Because of this, R is used by governments to assess how we are doing in our efforts to stop the spread of COVID-19, and to adjust our actions, if needed.
Alexander is the Online Editor at BBC Science Focus and is the one that keeps sciencefocus.com looking shipshape and Bristol fashion. He has been toying around with news, technology and science on internet for well over a decade, and sports a very fetching beard.