Why is the new Covid variant spreading? | David Spiegelhalter and Anthony Masters

The UK’s fine performance in sequencing Sars-CoV-2 genomes allows Public Health England to publish detailed analyses on the progress of variants and the latest report represents the changing of the guard. The B.1.1.7 lineage, first identified in Kent, had been dominant in the UK, but the B.1.617.2 lineage, first identified in India, comprised 58% of the most recent sequences, up from 44% the week before. There are strong regional differences, with under 10% of cases in Yorkshire and the Humber being the Indian-identified variant, while in north-west England that share is over 60%.

The main concern is about increased risk of transmission and reports also include estimates of what is known as the “secondary attack rate” (SAR), which simply means the proportion of an infected person’s contacts who also get infected. Using NHS test-and-trace data for recent non-travel cases, the estimated SAR for the B.1.1.7 variant was 8.1% (+/- 0.2%), while for the variant identified in India it was substantially higher at 13.5% (+/- 1.0%) – although these are likely to understate the true values due to the limitations of contact tracing.

Between January and March 2021, it was estimated that, of contacts of a non-traveller infected with the B.1.1.7 variant, around one in 10 got infected. This is an average of a very widely dispersed distribution: most cases do not infect anyone else, while at the opposite end of the spectrum there can be “super-spreader” events, often in prolonged indoor gatherings with poor ventilation. It is increasingly accepted that viral spread is mainly airborne rather than by droplets and “fresh air” has finally been added to the “hands, face, space” mantra.

Nearly 7 million contacts in England have been legally obliged to self-isolate for at least 10 days. Even if the proportion of contacts infected were as high as 20%, it would still mean that over 5 million of these were not infected and yet were unable to shorten their isolation by testing.

Where are we now? Confirmed cases are rising, with the virus in a race against the vaccines and the prospects are extremely uncertain.

David Spiegelhalter is chair of the Winton Centre for Risk and Evidence Communication at Cambridge. Anthony Masters is statistical ambassador for the Royal Statistical Society


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