Around 65 million people in the U.S. had been infected with Covid-19 by early March, a new analysis shows.

The findings, published in the Proceedings of the National Academy of Sciences, suggest approximately 60% of virus cases have been undercounted by then - adding to growing evidence the pandemic's actual figure is far larger than official tallies indicate.

"There are all sorts of different data sources we can draw on to understand the Covid-19 pandemic," Adrian Raftery, a professor of sociology and statistics at the University of Washington and senior research author, said in quotes by The Guardian.

Based on the latest modeling by the University of Washington, Covid-19 has sickened almost 1 in 5 Americans, Bloomberg said.

The model, which aims to reduce biases in data gathering, estimates that around 65 million people, or almost 20% of Americans, had been infected as of March 7, according to the report.

"It's good to see people start estimating how far we potentially could be off," Jennifer Nuzzo, an epidemiologist at the Johns Hopkins Center for Health Security, said.

The study incorporated data on the number of fatalities, the series of clinical trials administered each day and the proportion that come back positive, The Guardian said.

"That is what we're seeing now with the Delta variant because it's highly contagious and it's ripping through communities of people who haven't been immunized yet," lead author Nicholas Irons said.

Monitoring cases of the virus and deaths is not easy.

The World Health Organization and Johns Hopkins University report different numbers and still underreport those numbers.

For instance, as of July 22, there have been anywhere from 191 million to 191.7 million confirmed cases of Covid-19 since the pandemic broke out, data from the organization and JHU showed, respectively.

In the same time period, there have been 4.1 million deaths, based on data from both the organization and JHU.

Meanwhile, a recent study from Massachusetts Institute of Technology estimated the actual number of infections is 12 times higher than official estimates.

The MIT study also estimated the actual number of deaths is 50% higher than official numbers.

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