The study shows that Covid arrived in the UK in three waves in southern Europe

Covid-19 was introduced to the UK mainly from southern Europe, rather than China, during the first wave of the pandemic, according to the most comprehensive genomic analysis of transmission ever performed for any disease.

The study, conducted by Oxford and Edinburgh universities, looked at more than 1,000 infections brought to the UK by travelers from other countries before the end of April. Only 0.4% came directly from China. The largest sources were Spain (33%), France (29%) and Italy (12%).

The researchers analyzed the viral genome – the complete genetic code of the virus – in 50,000 samples taken from people infected with Sars-Cov-2 in the UK and other countries. They tracked the transmission chains by following slightly different lines that appear as the virus moves over time. The results were published Friday in the journal Science.

Although the study presents historical data from the first wave of Covid-19 last winter and spring, it shows how “genetic epidemiology” could be used to track the second rise now in full flood, said Professor Oliver Pybus of Oxford University, a study leader.

“This study shows that it is possible to track individual transmission lines of the virus accurately in time and space,” he said. “Performing weekly tests means that genomic tracking can become a key component of public health surveillance.”

The researchers are part of the Covid-19 Genomics UK consortium (Cog-UK), whose data have led to the identification of the most infectious variant B.1.1.7. They say that a detailed comparison of the behavior of the new variant with that of previous descendants will help us understand why it is spreading so fast now.

“Reconstructing where and when Covid-19 was introduced in the UK, we can see that previous travel and quarantine interventions could have helped reduce the acceleration and intensity of the first wave of cases in the UK,” said Prof. Pybus.

Although Covid-19 has its origins in China, only the first cases to arrive in the UK in late January came from there. The study shows three subsequent and much larger waves of introduction from continental Europe.

The first came in February and early March from Italy, the European country hit hardest by the pandemic. Then there was a greater increase in infection in Spain, followed by several cases in France in late March and early April.

By the end of April, with very few international trips and the UK blocked, the introduction of Covid-19 almost stopped and cases fell in the country. A new wave of imported infection – which was not included in this study – arrived in late August and early September in Spain and elsewhere.

Meanwhile, some viral lines continued to spread at a lower intensity in the UK during the summer. Unlike the current spread of the most transmissible variant B.1.1.7, the researchers found no evidence that any viral genealogy present had a selective advantage over others.

Researchers at the University of Glasgow published a separate but smaller study on Friday showing how travelers introduced Covid-19 to Scotland. He came to the same conclusions as the UK-wide analysis.

“The speed with which the virus has spread to Scotland and the UK as a whole, after multiple introductions, mainly from other European countries, has been extremely fast,” said Professor Emma Thomson, lead author of Nature Microbiology.

“It is possible that an earlier blockade in countries with a high burden of cases, such as Italy and other measures, such as quarantine of passengers in high-risk areas, could have prevented the escalation of the outbreak and more community transmission groups in course, ”she said.

“Our work offers unparalleled views on what happens in an individual epidemic,” said Louis du Plessis of Oxford University. “The UK distributes large volumes of genetic data on the virus on a weekly basis and if you do not have this level of surveillance, you will not know the real situation of the evolution and transmission of the virus.”