Scientists overestimated the transmissibility of the Kent or alpha Covid variant because they were did not know how much its spread was aided by public behaviour and movement, a new study shows.
The emergence of the strain was responsible for a huge surge in cases in south-east England despite a nationwide lockdown.
But while it spread faster and more easily than the previously dominant form of the virus, scientists were unable to ascertain exactly how much more dangerous it was.
A Sage meeting on Dec 21 saw experts put forward suggestions ranging from 34 per cent to 74 per cent more transmissible. A study published in January by World Health Organisation academics claimed it was 70 to 80 per cent more transmissible.
But a new study led by the University of Oxford assessed the spread of the variant from Kent in November last year to almost every area of the UK by mid-January. By piecing together its spread, researchers managed to prise apart the roles of increased transmissibility and public mobility.
Dr Moritz Kraemer, the lead author on the study, said that once the variant had emerged it was carried around the UK in "super-seeding" events.
Co-author Prof Oliver Pybus said it was impossible for previous studies to determine what proportion of new cases were due to genetic mutations for increased transmissibility and how much of the spread was down to movement.
"Estimates of alpha’s transmission advantage over previous strains were initially 80 per cent, but declined through time," he said. "It is likely the alpha variant was 30 per cent to 40 per cent more transmissible than the initial strain.
"Early estimates were higher because we did not know how much its growth was exacerbated by human mobility and by how many contacts different groups of people have."
The results are published in Science.