'Super-seeding' drove UK spread of Alpha variant: Study

The rapid spread of the Alpha variant of SARS-CoV-2 through Britain last autumn was driven by large numbers of infected people "exporting" the variant to multiple parts of the country, according to a large study.

The researchers, including those from Oxford University in the UK, noted that the explosive spread of the Alpha variant resulted not only from the biological changes in the virus but was also enhanced by the "super-seeding event".

The study, published on Thursday in the journal Science, maps the spread of the variant, also known as lineage B.1.1.7, from its origins in Kent and Greater London in November 2020 to all but five counties in the UK by January 19 this year.

"At the beginning of December 2020, the epicentre of Covid-19 transmission in England shifted rapidly from the North West and North East to London and the South East, as the Alpha variant took hold," said study lead author Moritz Kraemer, from Oxford.

"As people travelled from London and the South East to other areas of the UK they 'seeded' new transmission chains of the variant. This continued as a national 'super-seeding' event which did not start to slow until early January," Kraemer said.

The researchers noted that although travel was curbed, after restrictions were introduced on December 20, this was compensated for by the continued exponential growth in Alpha variant cases.

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The rapid spread of the Alpha variant across the UK led to initial reports that it could be up to 80 per cent more transmissible than the original strain.

The study shows mobility significantly affected its spread and early growth rates.

"Estimates of Alpha's transmission advantage over previous strains were initially 80 per cent, but declined through time," said Professor Oliver Pybus, from Oxford.

"We found Alpha's emergence was a combination of virus genetic changes and transient epidemiological factors. An initial wave of Alpha variant export to places in England with low rates of infection, from the massive outbreak in Kent and Greater London, explains why at first it spread so fast," Pybus said.

The researchers said Alpha variant does contain genetic changes which makes it more transmissible, and it is likely the variant was 30 to 40 per cent more transmissible than the initial strain.

"The Alpha variant began by spreading mostly within London and the South East, even during the November lockdown in England. Once this was lifted, it spread rapidly across the country, as human movement increased significantly,” Verity Hill, co-author and researcher from the University of Edinburgh, said.

"Our ability to be able to trace the origins of the Alpha back to a point source in the South East of England has important implications for how new variants arise and how they will spread across the UK," Hill added.