Variant watch: UK govt study backs lower Omicron severity risk

In its latest technical briefing on Sars-CoV-2 variants, released late on Thursday, the UK Health Security Agency (UKHSA) said there was a 50-70% reduced risk of hospital admission for Omicron infections compared to the Delta variant.

The government of UK has now corroborated what three studies found last week: the Omicron variant is significantly less likely to send people to the hospital.

The UKHSA said the reduction in risk “is likely to be partly a reduction in intrinsic severity of the virus and partly to protection provided by prior infection”. In picture - Shoppers wear face mask on Oxford Street in London. (AP)

In its latest technical briefing on Sars-CoV-2 variants, released late on Thursday, the UK Health Security Agency (UKHSA) said there was a 50-70% reduced risk of hospital admission for Omicron infections compared to the Delta variant.

In an accompanying risk assessment that UKHSA released, it said the reduction in risk “is likely to be partly a reduction in intrinsic severity of the virus and partly to protection provided by prior infection”.

“We cannot confidently quantify the relative contributions of these two factors at present. Even at the reduced hospitalisation risk observed, the combined growth advantage and immune evasion properties of Omicron have the potential to lead to very high numbers of admissions to hospital,” it added, reiterating a warning that experts all over the world have said.

But the findings are now consistent in four studies, and they indeed offer hope for some cautious optimism. The first study was from South Africa, where government experts found an up to 70% lower risk of hospitalisation for people with Omicron variant compared to those who had a Delta infection.

The third study was by the Imperial College of London, which found a 45-50% reduction of risk on average (including people with vaccines and past infection). Crucially, this study estimated the intrinsic severity (relevant for unvaccinated, previously uninfected people) could be 0-30% lower.

UKHSA, as the others, said the estimates were still preliminary. “These analyses are preliminary because of the small numbers of Omicron cases currently in hospital and the limited spread of Omicron into older age groups as yet. There is insufficient data to comment on severity of illness once in hospital or mortality,” the risk assessment document said.

More insight into vaccine hit...

A crucial new detail that the UKHSA report adds to the understanding of the Omicron variant is that vaccine boosters (the UK uses mRNA doses for boosters) begin to wane from one month onwards and show as low as 30-50% efficacy from 10 weeks after the third doses. “This waning is faster for Omicron than for Delta infections,” the technical briefing document said.

But it added that protection against severe disease and death is likely to continue. “There are insufficient severe cases of Omicron as yet to analyse vaccine effectiveness against hospitalisation, but this is expected to be better sustained, for both primary and booster doses,” it said.

Experts told HT on Thursday that vaccine efficacy against severe disease is likely to be helped by how vaccines train what is known as the cellular immunity. T cells, part of this immunity, retain a more durable ability to find and kill cells infected with Sars-CoV-2, even if it is of the Omicron variety.

The other component seen as key is what is known as humoral immunity, which has antibodies. These antibodies bind to the virus before they infect cells.

Lab studies earlier this month discovered that Omicron has heavily mutated in regions that antibodies typically bind to, but not in regions that T cells recognise to destroy them. Thus raising the theoretical hope that even if an infection is not prevented, which is the job antibodies, an Omicron disease will mostly remain mild or asymptomatic – as has been reported anecdotally.

...and into reinfections rates

As reported in the other studies, and expected since the new variant is more resistant to antibodies that will also include those from a past infection, Omicron is more likely to cause a reinfection than other variants.

“The population reinfection rate has increased sharply and disproportionately to the number of first infections. 9.5% of Omicron infections have been identified to have previous confirmed infections, which is likely to be a substantial underestimate of the proportion of reinfections. The first infections of the individuals with Omicron reinfections occurred in both the Alpha and Delta waves and are likely to have been undetected if in the first wave,” the technical briefing document said.

Here, too, the risk of hospital admission in people who had a prior infection is likely to be lower than their chances of being infected in the first place. The UKHSA report pointed to the relevant finding by the Imperial College researchers, who concluded “the reduced risk of hospitalisation in those previously infected estimated as 55 to 70%”