The Omicron COVID-19 variant is less likely to cause severe symptoms because it affects the upper airways rather than the lungs, but that makes it more transmissible because it is more likely to spread in the air, recent studies and clinical data show.

Previous research on Omicron from South Africa, where it was first identified in November, indicated that the new variant led to milder symptoms among infected patients. At the time, scientists associated the cause to the higher population immunity in South Africa.

However, according to a dozen of studies that have surfaced in recent weeks, the variant is more likely to infect the upper respiratory system, including the nose and throat, rather than the lungs, where previous versions of the variant have caused serious breathing problems.

The studies, which have not been peer-reviewed, came into the spotlight as Omicron continued to infect people with and without vaccination globally, causing outbreaks in more than 100 countries and overwhelming hospitals in some countries.

Among the studies is research done by a team led by professor Ravi Gupta at the University of Cambridge that found the Omicron spike protein is less efficient than the Delta variant spike at entering lung cells.

"We speculate that the more efficient the virus is at infecting our cells, the more severe the disease might be," Gupta told University of Cambridge Research. "The fact that Omicron is not so good at entering lung cells and that it causes fewer fused cells with lower infection levels in the lab suggests this new variant may cause less severe lung associated disease."

The team's research also indicated that the new variant "may be significantly better" than its predecessors at evading vaccine-induced antibodies.

By testing "pseudo viruses "against blood samples of vaccinated individuals who had received at least two doses of either AstraZeneca or Pfizer research vaccines, Gupta and his team found that Omicron required 10 times more serum antibody than Delta to neutralize the virus.

Double-edged sword

"While further work is needed to corroborate these findings, overall, it suggests that Omicron's mutations present the virus with a double-edged sword," Gupta said. "It's got better at evading the immune system, but it might have lost some of its ability to cause severe disease."

A group of researchers at the University of Hong Kong found similar results in findings posted on Dec 22.They found that Omicron can replicate faster in the bronchi, two large tubes that carry air from the windpipe to the lungs, but it grows more slowly than Delta and other variants in the lungs.

Studies done on animals produced similar results. According to recent research completed by a group of Japanese and US scientists on hamsters and mice, animals that have been infected with Omicron on average experienced milder symptoms than those infected with other variants. They experience less severe lung damage, did not lose as much weight and were less likely to die, the scientists found.

"This was surprising, since every other variant has robustly infected these hamsters," The New York Times quoted Michael Diamond, a virologist at Washington University and a coauthor of the study, as saying.

A study done by researchers of the Molecular Virology Research Group at the University of Liverpool also showed that mice infected with Omicron tend to lose less weight and suffer from less severe pneumonia.

These studies will have to be verified with further research, such as testing among humans infected with Omicron, to explain why people who tested positive for Omicron are less likely to need hospital treatment than those infected with the previous variants, scientists said.