United States is seeing a rising number of COVID-19 cases driven by the more infectious Omicron subvariant BA.2.12.1, and health experts worry that "substantial" undercounted cases may bring difficulty in monitoring public health.

The U.S. Centers for Disease Control and Prevention (CDC) said that the new subvariant made up about 58 percent of all new U.S. cases for the week ending May 21. 

However, more people observe a "flood of cases in their social circles," and health experts warn that there might be a "hidden" wave, ABC News reported on Wednesday.

"We know that the number of infections is actually substantially higher than that. It's hard to know exactly how many but we know that a lot of people are getting diagnosed using home tests," said Dr. Ashish Jha, U.S. President Joe Biden's new coronavirus response coordinator.

In the past few weeks, almost every state has reported an increase in COVID-19 infections and hospitalizations, averaging 3,000 new COVID-19 hospitalizations each day.

As the country's Memorial Day holiday weekend approaches, its seven-day average now exceeds the historical record of 100,000 new COVID-19 cases in February, according to data compiled by Johns Hopkins University. New data suggests it has reached 118,954 cases.

The CDC claims that the reported daily case average consists largely of PCR tests. 

The Northeast remains the nation's most notable known COVID-19 hotspot with New York City hit the hardest by BA.2.12.1, up to 83.2 percent of all COVID-19 cases in the region.

The CDC has advised the public to wear masks on public transportation and indoor public spaces, but ended its mask mandate in early May. 

Vaccinations have stagnated and elected officials nationwide seem averse to impose new restrictions on a public that's ready to move on even as the U.S. death has surpassed one million.

Changing metrics

The Department of Health and Human Services has abandoned the requirement for hospitals to report several key COVID-19 metrics earlier this year. Some states even completely stopped reporting data such as those for hospital bed usage and availability, COVID-19 specific hospital metrics and ventilator use.

On top of that, a lot of people taking at-home tests do not report to health departments. 

Earlier this month, an influential modeling group at the University of Washington in Seattle estimated that only 13 percent of cases are being reported to health authorities in the U.S. – which would mean an undercount of more than a half million new infections every day.

"While home testing has created broader population access to infection status and hopefully better access to treatment, it has also created a major blind spot in our public health surveillance efforts," John Brownstein, an epidemiologist at Boston Children's Hospital told ABC News. "The latest BA.2 surge was masked by the shift in testing behavior and likely delayed decisions on public health interventions." 

During the first Omicron wave in winter, the CDC released a map in its data tracker that showed the risk at community level.

Brownstein warns that some Americans may not be fully aware of the continued dangers of the virus, as a county marked as low-transmission does not mean that the virus simply disappears.

"While the CDC map reflects decreased risk of severe complications from COVID infection, it unfortunately hides places with high community transmission," Brownstein said. "This masking of risk may provide a false sense of security especially to those at high risk of hospitalization and death."

(With input from AP)