Seventeen Million Undiagnosed Covid-19 Cases In US Last Summer, Researchers Estimate

The National Institutes of Health surveyed more than 11,000 people who hadn't been diagnosed with the disease between May and July 2020, and found that 4.6 percent of participants had Covid-19 antibodies.

This equates to 4.8 million undiagnosed case for every undiagnosed case last summer, totalling nearly 17 million undetected cases by mid-July.

While some participants reported being sick at some point during 2020, the disease spread further than was immediately apparent due to the prevalence of asymptomatic cases.

The study reads: "This study demonstrates that spread of the SARS-CoV-2 virus in the US during the first six months of the pandemic was more widespread than has been suggested by data reporting diagnostic test-confirmed cases.

"Similar to responses to other respiratory viruses, such as influenza, many individuals develop asymptomatic or mild disease that is not medically attended and therefore never diagnosed.

"Our findings indicate that there are nearly five individuals with a previous asymptomatic infection for every diagnosed case."

The upside is that more people than previously thought might have immunity to Covid-19 - although there is uncertainty about whether asymptomatic people develop a sufficient adaptive immune response, and how long that immunity might last.

The study concludes: "These data suggest a higher level of infection-induced immunity exists in the population and the size of those with this immunity is even greater now as the virus continued to spread in the months since this study was performed.

"Further long-term studies of immunity in the population will be necessary to further understand durability of response to the vaccine versus infection, how infection-induced immunity impacts vaccine response and performance, and if herd immunity can play a role in controlling SARS-CoV-2 spread."

Of course, the development of vaccines has been a huge step forward since last summer. As well as providing protection for the recipient, a recent study has indicated that the Oxford-AstraZeneca jab could stop inoculated people from passing the disease onto others.

The UK is currently aiming to offer all over-50s a vaccination jab by May.