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Early data on Omicron show surging cases but milder symptoms

T WO WEEKS after the Omicron variant was identified, hospitals are bracing for a covid-19 tsunami. In South Africa, where it has displaced Delta, cases are rising faster than in earlier waves. Each person with Omicron may infect 3-3.5 others. Delta’s most recent rate in the country was 0.8.

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South Africa is not in lockdown, which may partly explain Omicron’s rapid spread. However, prior variants benefited from encountering lots of people with no immunity. By now, most South Africans have either recovered from covid or been vaccinated.

In such an environment, there are two ways Omicron could spread so fast. One is greater infectiousness, which depends on such factors as how easily it enters cells. The other is better evasion of immunity.

The Delta variant became dominant mainly because of its transmissibility. In contrast, Omicron seems to have advantages in both areas. Anecdotal evidence for its greater contagiousness is mounting: super-spreader events after which 35-78% of people tested positive have occurred in Norway, Denmark, Spain and Britain.

Moreover, Omicron has unprecedented capacity for reinfection. A recent study led by Juliet Pulliam of Stellenbosch University showed that the number of South Africans who test positive at least 90 days after their last positive test is more than you would expect based on earlier waves. And antibodies generated by Pfizer’s vaccine are less effective against Omicron than against earlier variants. However, they still achieved solid neutralisation in people with booster jabs or prior infections. Current vaccines may offer good protection against severe disease caused by Omicron.

Data on virulence are more heartening. In hospitals Omicron has not yet shown a pattern of worse disease in older people. Among covid-positive hospital patients in the South African city of Tshwane, 70% of those aged 50-69 and 90% of over-80s had severe cases during the Delta wave. This share is now around 30% for all ages.

The average severity of Omicron cases could rise. If it does not, one possible reason is that Omicron’s mutations yield milder illness. This would partly offset the impact of a surge in cases, though death rates could yet rise if wards are overwhelmed. Another explanation is that many older South Africans got jabs in recent months. If this is the cause, Omicron would pose a serious threat to the unvaccinated.

Sources: NICD, South Africa; Trevor Bedford; “Increased risk of SARS-CoV-2 reinfection associated with emergence of the Omicron variant in South Africa”, by J.R.C. Pulliam et al. (working paper)

This article appeared in the Graphic detail section of the print edition under the headline “Mixed signals”


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