WASHINGTON (AFP) - By February of this year, 58 per cent of the US population - more than 190 million people - had been infected with Covid-19, according to an antibody survey by the Centres for Disease Control and Prevention (CDC) published Tuesday (April 26).
The figure is far higher than the 80 million officially recorded cases, with the majority of infections undiagnosed, asymptomatic or unreported.
Roughly 75 per cent 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 the winter Omicron wave, particularly among children.
Each month from September 2021 to January 2022, the study examined some 75,000 blood specimens taken from across the country, 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 weight by age, sex and metropolitan status.
"Having infection-induced antibodies does not necessarily mean you are protected against future infections," said Dr 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 hospitalisation - and vaccination, either before or after infection, provides additional protection," she added.
Since the duration of infection-conferred immunity is unknown, it remains vital to stay up to date with Covid-19 vaccination, she stressed.
The United States is currently offering fourth shots to people 50 and over, and third shots to people under that age.
Pfizer on Tuesday said it had submitted an application to the Food and Drug Administration to authorise a third shot to children ages five to 11, following strong immune response data.
Children five and under are the only group not yet eligible for vaccination.
"The best way to protect them is to make sure that they are surrounded by people who are taking preventive measures, like staying up to date with our vaccines," said Dr Clarke.
President Joe Biden's administration meanwhile announced Tuesday it is doubling the number of outlets where at-risk Americans can obtain free Covid-19 therapeutic pills.
Oral therapeutics such as Pfizer's Paxlovid tablet are seen as an important new weapon in the struggle to knock out a virus that at its peak a year ago killed more than 3,000 people per day in the US alone.
White House Chief of Staff Ron Klain called Paxlovid a "miracle pill" and tweeted that "almost all Covid deaths can be prevented if people who test positive take" it.
With 20 million pill packs ordered for government purchase, they are now "in ample supply" and distribution will be ramped up from the current 20,000 locations to close to 40,000, according to a White House official.
Nationwide, cases are ticking up with infections caused by the BA.2 and BA.2.12.1 Omicron subvariants - including a rash of cases sweeping through the Washington elite.
Vice President Kamala Harris tested positive Tuesday, but is asymptomatic and not considered a current close contact of Mr Biden, the White House said.
Upstate New York and the Northeast region are also experiencing an increase in hospitalisations, and the CDC recommends indoor masking in those areas, though mandates have long ceased to be in effect there.
Even with rising hospitalisations, there has been a sharp reduction in fatalities, mainly as a result of rising population immunity, but also because the newer variants are intrinsically less severe than those that preceded them.
Daily deaths stand at a little over 300 per day. The country is expected to reach the grim milestone of one million deaths in the coming weeks.
