Washington, Sep 9 : The true number of US Covid-19 cases by the end of 2020 was likely more than 100 million, said a new study by researchers at Columbia University. "At the end of last year, you may recall hearing news reports that the number of Covid-19 cases in the US had topped 20 million. While that number came as truly sobering news, it also likely was an underestimate," wrote National Institutes of Health Director Francis Collins in a blog post on Tuesday about the study published by science journal Nature. In the study, researchers at Columbia University estimated the percentage of people who had been infected with SARS-CoV-2 in communities across the country through December 2020, the Xinhua news agency reported. They started with existing national data on the number of coronavirus cases both detected and undetected in 3,142 US counties and major metropolitan areas. They then factored in data from the Centers for Disease Control and Prevention (CDC) on the number of people who tested positive for antibodies against SARS-CoV-2. These CDC data are useful for picking up on past infections, including those that went undetected. From these data, the researchers calculated that only about 11 per cent of all Covid-19 cases were confirmed by a positive test result in March 2020. By the end of the year, with testing improvements and heightened public awareness of Covid-19, the ascertainment rate, or the number of infections that were known versus unknown, rose to about 25 per cent on average. "Many cases went undetected due to limited testing early in the year and a large number of infections that produced mild or no symptoms," Collins said. Collins said the revised number, which is nearly a third of the US population of 328 million, illustrates "just how rapidly this novel coronavirus spread through the country last year." The US surpassed 20 million confirmed Covid-19 cases on New Year's Day, according to data from the Johns Hopkins University (JHU). As of Wednesday afternoon, the country's caseload was over 40,412,000 with a death toll of 652,175, according to a tally by JHU. /IANS