‘No evidence’ that Omicron is less severe than Delta, say Imperial researchers

There is at present “no evidence” that the Omicron coronavirus variant is any less severe than the Delta strain, according to early findings from researchers at Imperial College London, which also highlighted the elevated risk of reinfection posed by Omicron and the need for booster shots to combat it.

The research, based on UK infection data, casts doubt on the hopes of some experts that a change in the virulence of the new variant would ease the pressure on health systems despite Omicron’s high levels of infectiousness.

“The study finds 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,” said the research team, led by Professor Neil Ferguson, an infectious disease modeller and government science adviser.

However, they cautioned that hospitalisation data “remains very limited at this time”. The study said data suggested “at most, limited changes in severity compared with Delta”.

Early reports from medics in South Africa’s Gauteng province, the centre of the Omicron outbreak there, had raised hopes that the variant’s mutations might have led to a change in the virus’ biology causing it to be less severe.

The UK recorded its highest ever daily caseload on Thursday for the second consecutive day, as more than 88,376 people tested positive within a 24-hour period. In London, where the variant is already dominant, hospital admissions are on the rise following a tripling in Covid-19 cases over the past week.

Professor Azra Ghani, an epidemiologist at Imperial College London and one of the study’s authors, stressed the “uncertainty” surrounding whether or not Omicron was less severe than previous strains.

“Whilst it may take several weeks to fully understand this, governments will need to put in place plans now to mitigate any potential impact,” she said, adding that the results demonstrated “the importance of delivering booster doses as part of the wider public health response”.

Addressing the question of severity, an analysis of 120,000 Delta cases and 15,000 suspected Omicron cases found that people infected with Omicron were no more likely to be asymptomatic than those infected with Delta.

This was supported by the finding that Omicron infections were no less likely than Delta to result in hospitalisation, though this assessment was based on only 24 Omicron hospitalisations compared with more than 1,000 for Delta.

The research team also estimated that Covid booster shots could provide about 85 per cent protection against severe illness from Omicron, and upwards of 90 per cent protection against death from the variant, each at 60 days after the third dose.

Without a booster dose, protection against severe disease with Omicron could wane to about 20 per cent for those who received the Oxford/AstraZeneca vaccine, and 40 per cent for BioNTech/Pfizer by six months after the second dose. However, the authors noted that this was a worst-case scenario and the decline could be moderated by the greater longevity of cellular immunity.

Omicron’s impact on vaccine efficacy was assessed in two different ways, but both found that two-dose protection against infection had been substantially reduced relative to Delta, and there was now likely to be “very limited remaining protection against symptomatic infection afforded by two doses of [the AstraZeneca vaccine]”.

By six months after the second dose, two doses of AstraZeneca most likely offer between zero and 5 per cent protection against symptomatic infection, and two doses of BioNTech/Pfizer about 20 per cent, the research showed, down from about 30 and 60 per cent respectively against Delta.

On Thursday, 812,044 jabs were administered across the UK, of which about 90 per cent were third doses, marking the most successful day for the booster rollout on record.

The research also found that Omicron posed five times greater risk of reinfection compared with the Delta strain, meaning the protection against reinfection could be as low as 19 per cent.

Before Omicron hit, previous infection gave 85 per cent protection against reinfection over six months, according to the Siren cohort study of UK healthcare workers.

Ferguson said the study highlighted how “Omicron poses a major, imminent threat to public health”, providing further evidence for the “substantial extent” to which the new variant could evade immune protection provided by prior infection and vaccination.