A team of researchers at Hong Kong University's faculty of medicine found omicron replicates 70 times faster in human airways, but infection in the lungs appears less severe.

The study is under peer review.

Chan noted that a highly contagious virus like omicron may cause more severe disease and death simply by spreading much faster, even though the lung infection appears not as bad.

The omicron variant of Covid-19 replicates 70 times faster in human airways than delta, but infection in the lungs appears to be less severe compared to the original virus strain, according to a study published this week by researchers at the University of Hong Kong's Faculty of Medicine.

Dr. Michael Chan Chi-wai and a team of researchers found that the variant replicates much faster in the bronchus, which connects the windpipe to the lungs, 24 hours after infection. Yet it reproduces more than 10 times slower in the actual human lung tissue, they said.

The study is under peer review. Researchers have been publishing their findings before peer review is completed due to the urgent nature of the pandemic.

Omicron's rapid replication in the airway may explain why it transmits faster than previous variants of the virus, but lower infection in the lungs may indicate that it causes less severe disease, according to the study's findings.

"It is important to note that the severity of disease in humans is not determined only by virus replication but also by the host immune response to the infection," Chan said in a statement.

Chan noted that a highly contagious virus like omicron may cause more severe disease and death simply by spreading much faster, even though the lung infection appears not as bad.

"Therefore, taken together with our recent studies showing that the omicron variant can partially escape immunity from vaccines and past infection, the overall threat from Omicron variant is likely to be very significant," Chan cautioned.

White House chief medical advisor Dr. Anthony Fauci, citing data from South Africa, said Wednesday that omicron reduces the protection of Pfizer and BioNTech's two-dose vaccine against infection to 33%, compared to 80% protection before the variant emerged.

However, the two-dose vaccine is still 70% effective at preventing hospitalization, Fauci said. Pfizer's booster dose provides 75% protection against symptomatic disease, he said, citing data from the U.K. Health Security Agency.

Pfizer and BioNTech have also published preliminary data from their own lab study showing boosters are effective against omicron, but the initial two-dose series is significantly hit by omicron.

The World Health Organization on Tuesday said omicron spreading faster than any previous Covid variant. Maria Van Kerkhove, the WHO'S Covid-19 technical lead, cautioned against treating omicron as a mild strain.

"We know that people infected with omicron can have the full spectrum of disease, from asymptomatic infection to mild disease, all the way to severe disease to death," Van Kerkhove told the public during a question and answer session.

She warned that increased transmission will lead to more hospitalizations, burdening already strapped health-care systems.

"If a system is overburdened, then people will die," Van Kerkhove said. "We have to be really careful that there isn't a narrative out there that it's just a mild disease."