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, APA reports quoting Reuters.
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 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 Dec. 16, added.
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reinfection with Omicron may be as low as 19%, Imperial College (ICL) said in a statement, noting that the study had not yet been peer reviewed.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.
"This level of immune evasion means that Omicron poses a major, imminent threat to public health."