The novel omicron variant of coronavirus is spreading faster than the delta variant and seems to be reducing vaccine efficacy according to preliminary data, the World Health Organization said Sunday.

Omicron had spread to 63 countries as of Dec. 9, the WHO said in an update on the variant, with faster transmission observed in South Africa, where delta is less prevalent, and in the UK, where delta is dominant.

“Based on currently limited evidence, omicron appears to have a growth advantage over delta,” the WHO said.

Omicron is likely to outpace delta where there is community transmission of the COVID-19 variant, according to the world health body.

It said that whether omicron’s observed rapid growth rate in countries with high levels of population immunity is related to immune evasion, intrinsic increased transmissibility, or a combination of both remains uncertain.

Less severe symptoms, low vaccine efficacy

It said there are still limited data on the clinical severity of omicron.

“While preliminary findings from South Africa suggest it may be less severe than delta, and all cases reported in the EU/EEA to date have been mild or asymptomatic, it remains unclear to what extent omicron may be inherently less virulent,” said the WHO.

There have been limited available data and no peer-reviewed evidence on vaccine efficacy so far for omicron.

“Preliminary evidence, and the considerably altered antigenic profile of the omicron spike protein, suggests a reduction in vaccine efficacy against infection and transmission associated with omicron,” said the health body.

“In addition, preliminary evidence from a few studies of limited sample size has shown that sera obtained from vaccinated and previously infected individuals had lower neutralization activity than with any other circulating variants of concern of SARS-CoV-2 and the ancestral strain.”/aa