First US Omicron cases mostly among inoculated

WASHINGTON • The first Omicron cases in the United States were detected mostly in vaccinated people who experienced mild illnesses, with only one hospitalisation and no deaths reported, the US Centres for Disease Control and Prevention (CDC) has said.

As at Wednesday, the US had investigated 43 cases of the Covid-19 variant that set the world on high alert last month because of mutations that may make it spread more easily.

About four-fifths of those cases occurred in people who had been fully vaccinated, the CDC said on Friday, including one who was in hospital for two days.

Common symptoms included cough, tiredness and congestion or runny nose, according to the agency's morbidity and mortality weekly report.

Scientists are still trying to understand how Omicron compares with the dominant Delta variant.

Among the big questions are how quickly Omicron spreads, how well it evades immune responses and how sick it makes people.

The CDC report, while early, is the first detailed look at Omicron cases in the US.

Many of the first reported cases appear to be mild although, as with all variants, a lag exists between infection and more severe outcomes, while vaccination and prior Covid-19 illness can mitigate disease severity, according to the report.

The authors cautioned against drawing broad conclusions from the early numbers in part because those who have travelled overseas recently might be more likely to be young and vaccinated.

The majority of the reported cases were among adults under the age of 40, who tend to have milder symptoms.

About a third of the early patients had also received another shot or booster dose, though several received that extra dose less than two weeks before becoming ill.

About 14 per cent of the cases, meanwhile, had evidence of a previous infection.

Omicron cases had been reported across 22 states as at Wednesday, according to the CDC.

Additional infections from late last month are likely to emerge soon, the authors said.

The earliest sign of the variant being detected in the US was a person who had travelled overseas and began feeling ill on Nov 15.

That was more than a week before South Africa alerted the World Health Organisation to it on Nov 24, suggesting Omicron was in the US well before it set off alarms and triggered fresh travel restrictions.

The findings are "very, very preliminary," said Dr Gregory Poland, director of the Vaccine Research Group at the Mayo Clinic in Rochester, Minnesota. He said he is wary of dismissing the variant as mild.

The mild illness seen could be a factor of the vaccinations and prior infections in the population exposed so far, he said.

It is also not clear whether vaccinated people who suffer breakthrough infections could experience the long-term symptoms known as long Covid-19.

"Your singularly best strategy to protect yourself from infection is a primary series and a booster and continuing to wear a mask properly indoors," he said.

New York Governor Kathy Hochul ordered businesses to require masking indoors if they do not already mandate Covid-19 vaccinations.

Still, the new variant "doesn't seem to be mowing down generally healthy people who are vaccinated", said Dr Nasia Safdar, an infectious disease physician at UW Health in Madison, Wisconsin.

"So that's reassuring."

How it affects more vulnerable populations, like those who are older or immunocompromised, is also yet to be seen, she said.

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