US hits more than 100M COVID-19 cases. Experts say this is likely an undercount

The United States has officially recorded more than 100 million COVID-19 cases.

According to data from the Centers for Disease Control and Prevention updated late Thursday, the U.S. hit the milestone as of Dec. 21.

This makes America the first nation to report total cases in the nine-figure range, data from Johns Hopkins Coronavirus Resource Center shows.

Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital and an ABC News contributor, said that while the 100 million mark is momentous, it's also a severe undercount.

"Obviously it's a milestone that signifies the sheer amount of transmission that has occurred around this virus and the population burden that we have faced," he said. "At the same time, we recognize that reported cases are absolutely a massive undercount -- at the beginning of the pandemic where testing was nonexistent to the shift to home testing where a significant proportion of cases has gone unreported."

There are likely several reasons for underreported cases including people testing at home and not relaying their results to public health officials, not knowing where to get tested or people choosing not to test at all.

"I think we know that a large majority of the population has already been infected with COVID," Brownstein said. "And so, this number only represents a fraction of all the cases. [The milestone] was very likely hit many months ago."

In fact, between February 2020 and September 2021, the CDC estimates that only one in four COVID-19 infections were reported and there were likely about 146 million infections that occurred during this time.

Brownstein said he believes there are opportunities to collect home testing results as surveillance data to report the true burden of cases more accurately.

For example, the National Institutes of Health launched a website in late November by which people can anonymously report the results of their at-home COVID-19 tests, regardless of which brand they use.

"It's challenging because it's relying on individuals to put in that data, but you could also do nationally representative sampling," he said. "We've actually done this as part of our work to understand the relative proportion of people testing positive via home tests versus lab tests and if you can come up with an estimate of how people are behaving, you can adjust the official numbers."

"So, I think there's some level of opportunities around crowdsourcing and opportunities around surveying the population to try to capture that," Brownstein continued.

Brownstein said there is likely a surge that occurred after Thanksgiving that we don't know the true burden of yet, with a surge likely to occur after Christmas and New Year's.

Therefore, he stressed the importance of making sure Americans are up to date on their COVID-19 vaccines and boosters as well as their flu shots before gathering.

"There's absolutely still time to go and get vaccinated as we head into the deeper winter months, both for COVID and flu," he said. "And people just need to exercise caution. We know what we need to do. We know the pandemic playbook by heart at this point, so it's just about executing it."

ABC News' Youri Benadjaoud contributed to this report.