The risk of reinfection with the Omicron coronavirus variant is more than five times higher, and it hasn’t been shown to be milder than Delta, a study showed, as cases rise across Europe and threaten end-of-year celebrations.

The results of the study by Imperial College London were based on data from the UK Health Authority and the National Health Service 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 hospitalization risk and symptom status) that Omicron has a different severity than Delta,” the study said. However, it added that data on hospital admissions remains very limited.

“When checking for vaccine status, age, gender, race, asymptomatic status, region, and sample date, Omicron was associated with a 5.4-fold higher risk of reinfection compared to Delta,” the December 16 study added.

The protection that a previous infection offers from re-infection with Omicron can be as low as 19%, Imperial College (ICL) said in a statement, noting that the study has not yet been peer-reviewed.

The researchers found a significantly increased risk of developing a symptomatic Omicron case compared to Delta for those two or more weeks after their second vaccination dose and two or more weeks after their booster dose.


The study included vaccines from AstraZeneca and Pfizer.

Depending on the estimates of vaccine effectiveness against symptomatic infections by the Delta variant, vaccine effectiveness results between 0% and 20% after two doses and between 55% and 80% after a booster dose.

“This study provides further evidence of the extent to which Omicron can evade previous immunity through infection or vaccination,” said study leader Professor Neil Ferguson in the ICL statement.

“This level of immune evasion means that Omicron poses a major, imminent threat to public health.”

Dr. However, Clive Dix, former chair of the UK Vaccine Taskforce, said it was important not to over-interpret the data.

“The conclusions are based on assumptions about Omicron that we do not yet have adequate data on,” said Dr. Dix. “For example, we don’t have data on the cellular immune response, which is now likely to drive vaccine effectiveness.”

“This is a critical missing assumption in modeling.”

Some of the conclusions differ from the data from South Africa, where vaccines are currently doing well against serious illness and death, he said.


“There is a great deal of uncertainty with these modeled estimates, and we can only be certain of the effectiveness of boosters against Omicron if we have another month of real-world data on the number of hospitalizations and deaths in the intensive care unit,” he said.

A previous UK SIREN study of reinfection risk in health workers, conducted before omicron’s emergence, found that a first coronavirus infection offered 85% protection from a second in the following six months.

The data analyzed by Imperial College was based on 333,000 cases, including 122,062 delta and 1,846 cases confirmed as Omicron coronavirus variant by genome sequencing.


Professor Azra Ghani of Imperial College, who co-directed the study, described it as “essential to modeling the likely future trajectory of the omicron wave and the potential impact of vaccinations and other public health interventions.”

The new findings could accelerate the imposition of stricter restrictions in a number of European countries to curb the spread of the new variant.