Imperial College London study also finds Omicron five times more likely to cause reinfections, but some experts say there are still not sufficient data to draw conclusions.
British researchers say they have found “no evidence” that the Omicron coronavirus variant is milder than Delta, casting doubt on the cautious optimism of some experts that the new strain may not be as virulent and not overwhelm healthcare systems.
The study by Imperial College London (ICL) on Friday – released just as British officials reported record COVID-19 cases for the third consecutive day, with a rise to 93,045 new infections – also found that the risk of reinfection with Omicron is more than five times higher than Delta.
The non peer-reviewed research was based on UK Health Security Agency and National Health Service data on people who tested positive for COVID-19 in a PCR test in England between November 29 and December 11.
“We find no evidence (for both risk of hospitalisation attendance and symptom status) of Omicron having different severity from Delta,” the study said, although it added that data on hospitalisations remains very limited.
“Controlling for vaccine status, age, sex, ethnicity, asymptomatic status, region and specimen date, Omicron was associated with a 5.4-fold higher risk of reinfection compared with Delta,” the study, which was dated December 16, added.
In South Africa, whose scientists were the first to identify Omicron last month, early anecdotal accounts have suggested that the new variant is causing less severe illness than previous ones – but scientists say it is too early to draw firm conclusions.
The country reported a record number of daily infections earlier this week.
“We believe that it might not necessarily just be that Omicron is less virulent, but … coverage of vaccination [and] … natural immunity of people who have already had contact with the virus is also adding to the protection,” Health Minister Joe Phaahla told a news conference on Friday. “That’s why we are seeing mild illness.”
However, ICL said in a statement that protection afforded by past infection against reinfection with Omicron may be as low as 19 percent, according to its research.
The study, which involved AstraZeneca and Pfizer vaccines, also found a significantly increased risk of developing a symptomatic Omicron case compared with Delta for those who were two or more weeks past their second jab dose, and two or more weeks past their booster dose.
Depending on the estimates used for vaccine effectiveness against symptomatic infection from the Delta variant, this translates into vaccine effectiveness of between 0 percent and 20 percent after two doses, and between 55 percent and 80 percent after a booster dose.
“This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination,” study lead Professor Neil Ferguson said in ICL’s statement.
“This level of immune evasion means that Omicron poses a major, imminent threat to public health.”
But Dr Clive Dix, former Chair of the UK Vaccine Taskforce, said it was important not to overinterpret the data.
“The conclusions made are based on making assumptions about Omicron where we still don’t have sufficient data,” Dix said. “For example, we have no data on the cellular immune response which is now probably driving effectiveness of vaccines.”
He added: “This is a crucial missing assumption in the modelling.”
Some of the conclusions are different from the data emerging from South Africa, where vaccines are holding up well against severe disease and death at present, he said.
“There is a huge amount of uncertainty in these modelled estimates and we can only be confident about the impact of boosters against Omicron when we have another month of real-world data on hospitalisation ICU numbers and deaths,” he said.
An earlier study by Britain’s SIREN looking at reinfection risk in health workers, which was carried out before Omicron emerged, found that a first coronavirus infection offered 85 percent protection from a second for the following six months.
The data analysed by ICL was based on 333,000 cases, including 122,062 of Delta and 1,846 which were confirmed as the Omicron coronavirus variant through genome sequencing.
The new findings could accelerate the imposition of tighter restrictions across several European countries in a bid to stem the new variant’s spread.
“Given the rapid spread of the Omicron variant to date, it is now highly likely that this will replace the circulating Delta variant globally in thecoming weeks,” ICL Professor Azra Ghani said.
“One remaining uncertainty is how severe the disease caused by the Omicron variant is compared to disease caused by previous variants. Whilst it may take several weeks to fully understand this, governments will need to put in place plans now to mitigate any potential impact. Our results demonstrate the importance of delivering booster doses as part of the wider public health response.”
Meanwhile, the UK Health Security Agency said Omicron cases were doubling in less than two days in all regions of England, apart from the southwest. The variant is already estimated to account for more than 80 percent of new cases in London.
The variant has not caused large-scale hospitalisations or deaths as yet, though there is always a lag between infection and severe disease.