Omicron more likely to reinfect than delta, no milder: study

Researchers found a significantly increased risk of developing a symptomatic omicron case compared to delta

The risk of reinfection with the omicron coronavirus variant is more than five times higher, and it has shown no sign of being milder than delta, a study showed, as cases soar across Europe and threaten year-end festivities.

The results of the study by Imperial College London were 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 Nov. 29 and Dec. 11.

"We find no evidence (for both risk of hospitalization attendance and symptom status) of omicron having different severity from delta," the study said. Although it added that data on hospitalizations 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 Dec. 16, added.

The Pfizer Covid-19 vaccine is administered at a pop-up clinic offering vaccines and booster shots in Rosemead, Calif., Nov. 29, 2021. (Frederic J. Brown/AFP via Getty Images / Getty Images)

The protection afforded by past infection against reinfection with omicron may be as low as 19%, Imperial College (ICL) said in a statement, noting that the study had not yet been peer reviewed.

The researchers found a significantly increased risk of developing a symptomatic omicron case compared to delta for those who were two or more weeks past their second vaccine dose, and two or more weeks past their booster dose.


The study involved AstraZeneca and Pfizer vaccines.

Depending on the estimates used for vaccine effectiveness against symptomatic infection from the delta variant, this translates into vaccine effectiveness of between 0% and 20% after two doses, and between 55% and 80% 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.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, speaks during a news conference in the James S. Brady Press Briefing Room at the White House in Washington, D.C., Dec. 1, 2021. The U.S. Centers for Disease Control (Jim Lo Scalzo/EPA/Bloomberg via Getty Images / Getty Images)

"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 over-interpret the data.

"The conclusions made are based on making assumptions about omicron where we still don't have sufficient data," Dr Dix said. "For example, we have no data on the cellular immune response which is now probably driving effectiveness of vaccines."

"This is a crucial missing assumption in the modeling."

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 modeled estimates, and we can only be confident about the impact of boosters against omicron when we have another month of real-world data on hospitalization 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% protection from a second for the following six months.

The data analyzed by Imperial College 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.


Imperial College's Professor Azra Ghani, who co-led the study, described it as "essential for modeling the likely future trajectory of the omicron wave and the potential impact of vaccination and other public health interventions."

The new findings could accelerate the imposition of tighter restrictions across a number of European countries in a bid to stem the new variant's spread.