Portugal Colonial - Nearly 60% of US population had Covid by February: CDC

The figure is far higher than the 80 million officially recorded cases, with the majority of infections undiagnosed, asymptomatic or unreported.

Roughly 75 percent of people under 18 had been infected, according to a paper based on a nationally representative study of antibody levels.

There was a huge surge during last winter's Omicron wave, particularly among children.

Each month from September 2021 to January 2022, the study examined some 75,000 blood specimens across 18 states, as well as 45,000 samples in February.

The study examined only antibodies created in response to prior infection, not vaccination.

National estimates were then produced using statistical methods to weigh age, sex and metropolitan status.

"We did not look at whether people had a level of antibodies from prior infection that provides protection against reinfection or severe disease," said Kristie Clarke, co-lead for the national Covid-19 serology task force, on a call with reporters.

"Previous infection has been shown to provide some protection against severe disease and hospitalization -- and vaccination, either before or after infection, provides additional protection," she added.

Since it is not known how long ago people were infected, it remains vital to stay up to date with Covid vaccination, she stressed.

The US is currently offering fourth shots to people 50 and over, and third shots to people under that age.

Children five and under are the only group not yet eligible for vaccination.

Nationwide, cases are ticking up with infections caused by the BA.2 and BA.2.12.1 Omicron subvariants.

Upstate New York and the Northeast region are also experiencing an increase in hospitalizations, and the CDC recommends indoor masking in those areas, though mandates have long ceased to be in effect there.

P.Mira--PC