When it comes to figuring out the impact the global pandemic has had on the human population, there's a big area of uncertainty: how many people have been infected with SARS-CoV-2 without realizing it?

We know that people who've caught the coronavirus don't necessarily show any signs of it – around 40% of cases are thought to be asymptomatic – and that puts a question mark over the official numbers that have been gathered so far.

To try and establish how many unreported infections we might have missed, researchers looked at the blood test results of 61,910 people who didn't think they'd ever caught the virus, to see if any antibodies were linked to fighting SARS-CoV-2, the virus that causes the COVID-19 illness.

Across the sample, 4,094 people had antibodies related to a SARS-CoV-2 infection, around 6 or 7 individuals in every 100 people. Extrapolate that fraction across the whole population, and the US may have had around 16 million asymptomatic or undiagnosed cases up to 30 September 2020, when the blood tests were done.

To put that number another way: double the cases that were actually recorded in the US before October 2020.

"Counts of officially reported cases may substantially underestimate the overall burden of infection in the United States," write the researchers in their paper. "Viral serologic testing may provide a more accurate estimate of cumulative disease prevalence."

The team does note certain limitations in their work, including the fact that participants self-reported their health status, and the sample was "an imbalanced representation of the US population by age, sex, and residence location". 

However, their results highlight the need to continue disease surveillance across the entire population, and it makes sense, given the virus doesn't always reveal itself even in people who have it. Another antibody study published last year showed that the presence of SARS-CoV-2 antibodies varies widely across US jurisdictions, ranging from less than 1% to 23% of the region's population.

That variation happens across age groups and genders as well as geographical location, further complicating the task of trying to get a handle on just how many people have been hit with a SARS-CoV-2 infection.

Another question that could use a more definitive answer is just how many antibodies are produced in asymptomatic cases. It's possible that in these people the body doesn't need to fight as hard to battle the virus, and so produces fewer of the antibodies needed – or at least loses them more quickly.

Not having any symptoms of COVID-19 may be better for the person who actually has the virus, but it makes tracking and containing the coronavirus more difficult. Asymptomatic carriers can still pass on the virus, almost undetected.

"Understanding... the actual prevalence of antibodies in the community would be helpful in understanding the likelihood of us continuing to have outbreaks," infectious disease physician Sara Keller, from the Johns Hopkins University School of Medicine and not involved in the study, told the Los Angeles Times.

Ultimately the more data we have on this damaging coronavirus – from case numbers to transmission rates – the better we can get it under control, and this latest study is going to help with that as we look to get life back to normal.

The research has been published in JAMA Network Open.