Omicron variant may not be 'milder' itself, US scientists caution

The "milder" outcomes of infection from the Omicron variant of SARS-CoV-2 are likely due to more population immunity rather than the virus' properties, according to researchers in the US.

The Omicron variant was first documented in Botswana and South Africa in late November 2021.

Compared with earlier variants, Omicron resulted in notably lower hospitalisation and death rates, leading some to conclude that the variant causes less severe outcomes or is less virulent than previous variants.

In a perspective article, published in the New England Journal of Medicine, researchers noted that the perceived lower severity of Omicron infections is most likely due to factors related to the level of immunity in infected people.

By the fall of 2021, much of the South African population had been vaccinated or probably infected by another variant during earlier waves of the pandemic.

The researchers from Massachusetts General Hospital and Harvard Medical School in the US argue that previous exposure would likely have reduced the severity of a subsequent Omicron infection.

The milder symptoms may also be due in part to Omicron's ability to cause breakthrough infections and reinfections, including in people with stronger immune systems who are better equipped to fight off an infection, they said.

According to the researchers, the situation in South Africa is intrinsically different than that of other countries -- especially the young age of its population -- meaning that Omicron could progress differently in other populations around the world.

Based on their analysis, the researchers stress that as many people as possible globally should be vaccinated, and those most vulnerable to disease should receive a third booster shot.

"There must be a renewed push to vaccinate and boost those not yet protected, because Omicron is not necessarily intrinsically milder," said William Hanage, associate professor at Harvard T.H. Chan School of Public Health.

"This is especially true for those struggling to access vaccines, whether in the United States or elsewhere in the world," Hanage added.