A map shows how many people had undiagnosed COVID-19 in the first 6 months of the pandemic, across 7 regions of the US

• During the first six months of the pandemic, 16.8 million coronavirus cases went undiagnosed in the US.
• That's according to a report that compares the prevalence of coronavirus antibodies in various regions.
• The Mid-Atlantic saw the most undiagnosed cases - nearly 9 out of every 100 people - by July 2020.

On paper, the US's summer and winter coronavirus surges look far more devastating than the first one in the spring of 2020.

But a new report published in the journal Science Translational Medicine offers the most robust look yet at how widespread the virus actually was during that initial wave. The results show that for every diagnosed case of COVID-19 in the US, nearly five others went undiagnosed during the first six months of the pandemic.

That amounts to roughly 16.8 million undiagnosed cases by mid-July 2020 - in addition to the 3 million cases officially reported during that time.

Kaitlyn Sadtler, an investigator at the National Institutes of Health who worked on the study, told Insider that her team spent several months making sure the figure was right.

"It was shocking to an extent of, 'Wow, that's a lot of people,' but at the same time, we knew that there was this big black box out there - the unknown," she said.

The estimates are based on a collection of blood samples, which the researchers gathered from around 9,000 people across the US from April 1 to August 4, 2020. None of the individuals sampled had ever been diagnosed with COVID-19, but nearly 5% of the samples came back positive for coronavirus antibodies. The researchers determined that these people had gotten undiagnosed infections.

Some regions were hit harder than others, they found. The Mid-Atlantic saw the highest prevalence of COVID-19 cases: Nearly 9 out of every 100 people in the region had an undiagnosed infection, according to the report.

The map below shows how that compares to other regions across the country.

Nearly 7% of people in the Northeast had an undiagnosed infection, compared with less than 2% in the West and Midwest, 3% in the South and Central US, and 4.5% in the Southwest.

Undiagnosed cases were most commonly found among Black Americans: Around 14% of the samples from the group came back positive for coronavirus antibodies. Meanwhile, around 2% of samples from white and Asian American people came back positive - the lowest of any racial group.

Undiagnosed infections were also more common among people ages 18 to 44 than among older age groups - 6% of those samples tested positive for antibodies.

If more young people got COVID-19 early in the pandemic than we thought, it could mean the US is closer to herd immunity than experts were estimating. (Vaccination rates are still relatively low among young Americans - just 38% of people ages 18 to 29 have gotten at least one dose compared to 80% of those ages 65 and older.)

However, Sadtler said people should assume they're still vulnerable to infection until they've gotten a vaccine.

"It definitely doesn't mean that younger folks should rely on thinking they might have had an infection," Sadtler said. "Everybody should go out and get vaccinated."