Reports of mostly mild illness from COVID-19 infections caused by omicron need to be interpreted with caution because they may not reflect the new variant’s severity across a broad range of people.

Fatigue, head and body aches and occasional sore throats and coughs are among the typical symptoms experienced by omicron patients, according to Angelique Coetzee, the doctor in South Africa whose observations helped scientists identify the worrisome strain. The manifestations contrast with the rapid heartbeat, low blood-oxygen levels and a loss of smell and taste often seen in COVID-19 patients sickened by the delta variant, she said.

While such anecdotes are encouraging, they may represent only a subset of cases and a fraction of the risk that could emerge if it spreads widely across the globe. Studies of disease patterns are needed to gauge omicron’s virulence across diverse patient groups, public health experts said.

Some reasons experts are cautious about omicron cases described so far:

They may be occurring mainly in young people and those at lower-risk now, but that could change as it spreads. It’s early days, and more serious symptoms may develop in the second week of infection. Vaccinations and prior infections may provide current patients with protection that others don’t yet have. COVID-19 causes a spectrum of disease and is more likely to be life-threatening in people older than 65. About 80% of cases have mild or no symptoms, 15% are severe and require oxygen, and 5% are critical, leading to ventilation, according to the World Health Organization.

“It’s premature to make any statements on severity,” said Raina MacIntyre, professor of global biosecurity at the University of New South Wales in Sydney. “Only 20% of COVID cases are severe, so we need epidemiologic studies. Also hospitalizations and intensive care unit admissions lag behind cases, and people usually experience one week of mild symptoms before getting sick.”

The WHO has warned of the potential for COVID-19 surges with “severe consequences” fueled by omicron, whose constellation of genetic changes suggest it may be both more transmissible and capable of evading the immunity provided by vaccination or a prior infection.

Although only 36% of adults in South Africa are fully vaccinated, the country has experienced three COVID-19 epidemics and at least 3 million people have been infected. A 2020 study found more than a quarter of patients in public health facilities in Gauteng, the province including Johannesburg where cases are on the rise, had antibodies to the virus from infections during the first wave.

Partial immunity

A previous case of COVID-19 may provide some level of protection against omicron, resulting in a milder illness, said Sanjaya Senanayake, an infectious diseases physician and associate professor of medicine at the Australian National University in Canberra. Researchers in Qatar reported this month that reinfections were 90% less likely to result in hospitalization or death than primary infections.

“We can’t definitively say that that means, throughout the world, omicron’s going to be mild for everyone,” he said. “It still could be quite a virulent, nasty strain.”

Vast swaths of the globe remain unprotected, having not been vaccinated or previously infected. And breakthrough infections in fully vaccinated people are known to occur, especially with the delta variant, Senanayake said.

“The question is, to what degree is the vaccine less efficacious” at preventing omicron infections than those caused by delta, he said. “We’ll probably get better epidemiologic data from developed countries that have omicron cases and have very good reporting and surveillance systems.”

Health authorities in the U.S. are recommending booster shots to bolster immune protection against a winter COVID-19 wave, including the potential spread of omicron there. An additional dose of vaccine is likely to trigger an increase in antibody levels that may shield against severe illness from omicron, said Catherine Bennett, chair in epidemiology at Melbourne’s Deakin University.

Antiviral pills being developed by companies such as Merck & Co. and Pfizer Inc. could also mitigate the threat of omicron and other worrisome variants, she said.

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