The United States may have experienced over 6.4 million cases of COVID-19 by 18 April 2020, according to a probability analysis published in Nature Communications. In the same period, there were 721,245 confirmed cases.
The first known case of COVID-19 in the US was confirmed on 21 January 2020. For the first few months of the pandemic, the US Centre for Disease Control recommended that testing be prioritized for patients in hospital who tended to present moderate or severe symptoms. However, studies suggest that 30–70% of individuals who test positive for the virus present with mild symptoms or may have none at all.
Jade Benjamin-Chung, Sean Wu and colleagues estimated the total number of SARS-CoV-2 infections in each US state from 28 February to 18 April 2020 using a probabilistic bias analysis to account for incomplete testing and less than 100% test accuracy. The authors estimate that there were 6,454,951 cases of SARS-CoV-2 infection (19 per 1,000 people). This estimate is about 9 times larger than the number of confirmed cases during the same period (2 per 1,000 people) and suggests that 89% of infections were undocumented. The majority of this difference (approximately 86%) was due to incomplete testing, with the remainder due to limited test accuracy.
The authors found that COVID-19 incidence was highest in the Northeast, Midwest and Louisiana when using confirmed case counts or the estimated number of infections. Underestimation of the number of cases was more common in Puerto Rico, California and some southern states. In 33 states, the estimated number of infections was at least 10 times higher than the number of confirmed cases.
The authors note that their methodology does not incorporate a transmission model and so they are unable to make forecasts about the spread of the virus. However, they argue that their method provides a more realistic picture of infection burden at a given point in time.
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