LOS ANGELES - Experts warned that incomplete data likely masked an upward trend of COVID-19 infections in the United States despite a consistent average of around 30,000 cases per day in the last two weeks, NBC reported Sunday.

The BA.2 Omicron subvariant, which now accounts for about 72 percent of US cases and is more contagious than the original Omicron variant, is fueling that spread, Zeke Emanuel, vice provost of global initiatives at the University of Pennsylvania, told NBC.

"It's much more transmissible. It's around. We just don't have a lot of case counts," he said, adding a lack of testing as the primary reason cases go underreported.

At the height of the Omicron wave in January, the US administered more than 2 million tests per day, but the number had dropped to an average of about 530,000 as of Monday, according to Centers for Disease Control and Prevention (CDC).

David Dowdy, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, was quoted as saying that since the patients infected by BA.2 Omicron subvariant had milder symptoms, people less likely to test or reported the results to state health departments or the CDC.

The report said some local data revealed recent spikes, for example, average infection cases had risen nearly 80 percent in Nebraska, 75 percent in Arizona, 58 percent in New York and 55 percent in Massachusetts over the past two weeks.

Wastewater surveillance similarly suggested that infections were rising in Colorado, Ohio and Washington, among other states.

Data collected through March 31 showed that virus concentrations in the wastewater of Bangor, a city in the US state of Maine, had increased to levels similar to those reported around mid-February, when record high of over 10,000 daily COVID-19 cases were recorded, according to a report released by the CDC last week.

The experts also warned that the people most at risk because of the artificially low case counts are the immunocompromised and children under 5, who are not yet eligible for vaccines.

Those groups should still have a way to accurately gauge transmission in their communities, they noted.