Sage modellers start to accept that Omicron is milder

23 December 2021

6:00 PM

23 December 2021

6:00 PM

Public health officials in Britain and South Africa were on different planets for about a fortnight. While those in South Africa kept presenting data suggesting that Omicron caused less severe disease than earlier variants, scientists in Britain continued to claim it was too early to say. Scenarios published by the London School of Hygiene and Tropical Medicine (LSHTM) last week pictured a frightening picture of January, suggesting that hospitalisations could peak above previous waves. An assumption was made that Omicron was just as likely to land you in hospital or kill you compared with Delta. As LSHTM admitted, quite a big assumption:-

Due to a lack of data, we assume Omicron has the same severity as Delta. If Omicronexhibits lower severity than Delta, this would decrease the projected number of severeoutcomes in our model.

A separate Imperial study, also published last Friday said that it found ‘no evidence of Omicron having lower severity than Delta, judged by either the proportion of people testing positive who report symptoms, or by the proportion of cases seeking hospital care after infection’. The research led to many calls for increased Covid restrictions in the run-up to Christmas, even some for a ‘circuit-breaker’ lockdown.

On Wednesday, however, UK science finally caught up South Africa’s. A fresh paper by Neil Ferguson’s team at Imperial has concluded, after all, that Omicron does cause less severe illness. The chances of infected people being hospitalised by the variant, it concludes, are reduced by 20 to 25 per cent compared with the Delta variant. The chances of having a hospital admission lasting more than a day are reduced by between 40 and 45 per cent. The study makes the point, however, that the greater transmissibility of Omicron could counteract the reduced severity of disease. The data collected for the study confirms the speed at which Omicron has taken over from Delta: on 1 December, there were 25,098 confirmed Delta cases and 164 cases of Omicron. Two weeks later, there were 13,819 cases of Delta and 15,809 cases of Omicron.

A preprint from Edinburgh and Strathclyde published yesterday also finds a sharp reduction in the risk of being hospitalised with Omicron relative to Delta — in this case it puts the reduction at two-thirds, although this is based on a tiny number of Omicron cases: just 15.

Meanwhile, a further South African study, this one by the National Institute for Communicable Diseases, finds an even greater gulf between the virulence of the Delta and Omicron variants — the risk of being hospitalised with the latter, it finds, is 80 per cent less than with the former. The authors make the point that some of the reduction in risk may be down to the population having greater immunity virus now compared with when Delta first emerged in April.

Given that we could by now have been in lockdown on the strength of scientists’ statements at the weekend, many will now be asking why it took British government scientists so long to accept South Africa’s research. It is still fairly early days — and none of these studies have yet been peer-reviewed — but the story of Omicron in Britain could be about to take a sharp turn.

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