The United States’ already confronting COVID-19 case numbers could actually be 16 million larger than officially stated, sparking fears over how widespread the pandemic is.

The figure comes from two US researchers, who are warning of massive under-reporting of COVID cases after conducting a peer-reviewed study that uncovered antibodies against the virus in 4094 people who were not showing symptoms – and had no reason to suspect they had been infected.

There are estimated to have been some 29.5 million official coronavirus cases in the US, and 535,000 deaths, making it one of the worst-hit nations.

Robert Stout and Steven Rigatti went looking for antibodies in 61,910 supposedly healthy people who had not been diagnosed with COVID-19.

More than 6 per cent of research participants were found to have antibodies against COVID-19.

Dr Stout and Dr Rigatti’s study, published on Wednesday (Australian time), has underscored the concern among experts that public health data from the Centres for Disease Control and Prevention cannot accurately tell us how widespread the coronavirus really is.

The 4094 people who were found to have had the virus without realising it raise further concerns about the number of people they infected because they weren’t diagnosed with COVID-19 while asymptomatic.

The researchers subsequently estimated that there may have been as many as 16 million Americans with asymptomatic or undiagnosed COVID-19 infections as of September 30, which is when they began evaluating the presence of antibodies in 61,910 people from across the US.

That is “substantially higher than estimates based on public health case reporting” to the CDC, the researchers wrote in their study.

The coronavirus has killed nearly 227,000 people in the United States. Photo: Reuters

Vaccinations are well under way in the US, with hopes it will stem the virus’ spread. Photo: Getty

Their findings came after America’s top infectious disease expert, Dr Anthony Fauci, warned states against easing coronavirus restrictions.

In Texas, businesses were recently given the green light to start operating at full capacity while Governor Greg Abbott lifted the state’s eight-month-old mask mandate.

Dr Fauci spoke to Fox News about how European countries “always seem to be a few weeks ahead of us in the dynamics of the outbreak”.

“They thought that they were home-free and they weren’t. And now they’re seeing an increase,” he said.

Dr Anthony Fauci, adviser to President Joe Biden, wishes Donald Trump would encourage his supporters to get the jab. Photo: AAP

Dr Fauci wishes former president Donald Trump would use his popularity to persuade his followers to get vaccinated against COVID-19, saying “I think it would make all the difference in the world” given he still has “incredible influence” among Republicans.

His comments came after a poll revealed about half of US men who identified themselves as Republicans said they had no plans to get the vaccine.

About one in five Americans have received at least one dose, with about one in nine fully vaccinated, according to the CDC.

Coronavirus deaths and newly confirmed infections have tumbled over the past two months in the US.

But cases are running at a still-troubling average of about 55,000 a day.