The coronavirus may have infected more than 100,000 people in the United States by the time the White House declared a national emergency on March 13 , according to a new study. Such a large reservoir of infections, mostly under the radar of health authorities, could have severely affected the outcome of nationwide efforts to control the disease. “[It] suggests that the United States was well past the possibility of containment by 12 March,” said the researchers led by Alex Perkins, associate professor at the University of Notre Dame, in a peer-reviewed paper published in the Proceedings of the National Academy of Sciences of the United States (PNAS) on Friday. Sars-CoV-2, the virus causing Covid-19, was first isolated and fully sequenced by Chinese scientists in early January, allowing many countries, including the United States, to develop test kits. But large-scale testing was not available in the US for more than two months after the earliest reported outbreak in Wuhan because of technical blunders and administrative hurdles. Robert Redfield, director of the US Centres for Disease Control and Prevention, said on Thursday that only about 10 to 20 per cent of those infected had received a test. That meant up to 60 million Americans may have had the virus by the time he made the comment. But according to Perkins’ study the actual situation could have been even worse than Redfield indicated. The US had 1,514 cases confirmed by March 12, just 1.3 per cent of the estimated actual cases according to their modelling. The estimated figure, based on several factors, could change according to different assumptions, but in almost all possible scenarios the “infections in the United States were severely under-reported”, Perkins said. The issue of under-reporting was not limited to the US but quantifying the gap between official figures and the actual number of infections remained a challenge. Some previous efforts involving serological surveys looked at the proportion of people carrying antibodies in a population. But the constraints on these surveys included that they were limited to a neighbourhood, conducted in later stages of the outbreak or were affected by how quickly the antibodies decreased after recovery. Republican voters say US Covid-19 death tally is ‘acceptable’, poll shows Mathematical modelling was a simpler approach, but building and testing a model required data. Much data related to the early outbreaks in the US is missing because of the lack of testing, according to the researchers. Fortunately, researchers in other parts of the world with more successful testing programmes, such as South Korea, Singapore and China, have collected a large amount of raw data that allowed the American team to calibrate some parameters and complete the modelling. A major advantage of the mathematical approach was that it allowed researchers to obtain daily estimates of the detection rate. “This revealed that detection of symptomatic infections decreased throughout February as exponential growth of infections outpaced increases in testing,” the report said. US President Donald Trump has said repeatedly that the US had more Covid-19 cases because it did more testing. The truth could be the opposite, according to the new study. Inadequate testing caused the US to miss “a crucial opportunity” to limit the impact of the pandemic, because “although the number of tests administered increased in March, so too did the number of infections and consequently, the demand for testing”, said the researchers. But they said their estimate should be treated with caution because of substantial uncertainties. In one extreme scenario, the estimated infected population reached 1 million by March 12. That would make one of the study’s core assumptions, that all the cases in the US originated from a few imported cases, questionable. Even at this stage, well into the pandemic and with over 5.5 million Covid-19 cases in the US, Perkins and colleagues believed large-scale mitigation measures such as social distancing were still necessary to “prevent even more cases and deaths”.