Mark Jit

SPI-M modellers: a response to our critics

(Andrew Parsons/No. 10 Downing Street)

In late November, scientists in South Africa and Botswana identified a new variant of the virus that causes Covid. This variant — later named Omicron — was spreading rapidly throughout South Africa, particularly in Gauteng, its most populous province. As well as reporting these concerning epidemiological facts, South African scientists worked quickly to identify Omicron’s genetic sequence and to alert the world with further bad news: Omicron has a large number of mutations in the region of the virus that our immune system recognises after we are vaccinated. In other words, it would be harder for antibodies to latch onto the new variant and destroy it. The final key piece of information from South Africa is that given the extremely severe Covid epidemic they have experienced, it is reasonable to assume almost everyone in Gauteng should have already been immune as a result of either infection or vaccination.

Put together, these three pieces of information suggested the extremely troubling possibility that Omicron might have an inherent transmission advantage compared to the previously dominant Delta strain; it might also be better at evading immunity, infecting even people who are protected from other Covid strains because they had been vaccinated or previously infected. This could potentially create major problems in the UK, where we are to a large extent relying on immunity — again, either through vaccination or prior infection — to manage the pandemic after relaxation of lockdown measures over the summer.

Helped by the quick work of South African scientists, Covid modelling groups across the UK were been able to quickly begin analysing what Omicron would do once it reached the UK. One question was particularly difficult to answer: how severe is Omicron? The speed of its spread had been so dramatic that no one had access to detailed hospitalisation and death data required to answer this question adequately.

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