The notion that the coronavirus variant Omicron has only caused an "enhanced influenza" lacks scientific evidence, Zhang Wenhong, a Shanghai-based Chinese infectious disease specialist, said at an event in Shanghai on Saturday.

After studying medical images of those infected with the variant, Zhang warned that the variant will "bite people."

"Omicron had a battle with the human immune system and won," Zhang said, adding that countries and regions need to build a strong immunologic barrier and guarantee enough medical resources to defend against the variant.

First detected in South Africa in November 2021, Omicron has surged rapidly around the world, leading some countries to reimpose more restrictive measures including lockdowns. It has also made inroads on the Chinese mainland, where two local COVID-19 cases linked with Omicron variant were found in the northern city of Tianjin.

Zhang said some Western countries' COVID-19 policies are still relaxed in face of the Omicron onslaught because they are confident about their vaccination, which effect is however still unknown.

Data from the U.S. Centers for Disease Control and Prevention on Sunday shows that 62.5 percent of the U.S. population is fully vaccinated, among whom 36 percent have received booster shots.

He warned that the situation is rather dire for the groups with low immunological ability.

"We allow the argument about vaccination. But we should not underestimate the effect of vaccines in fighting COVID-19," he said.

Zhang said the world will be re-opened again only when a strong immunological barrier is built and a very low fatality rate is achieved.

The World Health Organization (WHO) has also warned about taking Omicron lightly.

"While Omicron does appear to be less severe compared to Delta, especially in those vaccinated, it does not mean it should be categorized as mild," WHO director-general Tedros Adhanom Ghebreyesus told media at a press conference on Thursday.

"Just like previous variants, Omicron is hospitalizing people, and it is killing people," he said. "In fact, the tsunami of cases is so huge and quick that it is overwhelming health systems around the world."

(With input from agencies)