A spate of new studies on lab animals and human tissues are providing the first indication of why the omicron variant causes milder disease than previous versions of the coronavirus.

According to an article published in The New York Times, in studies on mice and hamsters, omicron produced less-damaging infections, often limited largely to the upper airway: the nose, throat and windpipe. The variant did much less harm to the lungs, where previous variants would often cause scarring and serious breathing difficulty.

In November, when the first report on the omicron variant came out of South Africa, all scientists knew was that it had a distinctive and alarming combination of more than 50 genetic mutations.

Previous research had shown that some of these mutations enabled coronaviruses to grab onto cells more tightly. Others allowed the virus to evade antibodies, which serve as an early line of defense against infection.

Over the past month, more than a dozen research groups have been observing the new pathogen in the lab.

But as global cases skyrocketed, hospitalizations increased only modestly.

More than a half-dozen experiments made public in recent days all pointed to the same conclusion: Omicron is milder than delta and other earlier versions of the virus.

On Wednesday, a large consortium of Japanese and American scientists released a report on hamsters and mice that had been infected with either omicron or one of several earlier variants. Those infected with omicron had less lung damage, lost less weight and were less likely to die, the study found.

Several other studies on mice and hamsters have reached the same conclusion. (Like most urgent omicron research, these studies have been posted online but have not yet been published in scientific journals.)

The reason that omicron is milder may be a matter of anatomy. Researchers found that the level of omicron in the noses of the hamsters was the same as in animals infected with an earlier form of the coronavirus. But omicron levels in the lungs were one-tenth or less of the level of other variants. A similar finding came from researchers at the University of Hong Kong.

These findings will have to be followed up with further studies, such as experiments with monkeys or examination of the airways of people infected with omicron. If the results hold up to scrutiny, they might explain why people infected with omicron seem less likely to be hospitalized than those with delta.


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