Houston resident Julie Joe suspects she may have caught COVID-19, although she didn't have a positive test to prove it. Her middle-school daughter has had flu-like symptoms a couple of times since the spring semester began and she herself felt somewhat sick following her daughter's episodes.

She might be right to assume so because findings released on Tuesday by the US Centers for Disease Control and Prevention suggest that 6 out of 10 adults in the United States－almost 200 million－and 3 out of 4 children have had COVID-19. That far exceeds the 80.8 million cases officially tallied by the CDC as of Tuesday.

The rate of previous infection was even higher among children, at 75 percent among those 11 years old and under. The level was much lower, around 33 percent, among those over 65.

The study's authors estimated "seroprevalence" caused by a prior infection－the presence of antibodies against the virus－based on samples gathered from thousands of routine blood draws from around the country.

"We know that the reported cases are just the tip of the iceberg," Kristie Clarke, co-lead for the CDC's COVID-19 Epidemiology and Surveillance Taskforce Seroprevalence Team, said in a news briefing on Tuesday.

Cases can go undetected if people don't develop symptoms, don't get tested, or use at-home rapid tests that aren't reported to public health officials. Forthcoming CDC research indicates there may be three people infected for every case officially reported during the wave of infections triggered by the Omicron variant of the coronavirus, said Clarke, adding that millions of cases were missed as the highly contagious variant spread.

"These findings illustrate a high infection rate for the Omicron variant, especially among children," the CDC said.

Study limitations

The agency cautioned that the findings in its report are subject to some limitations. They may underestimate the cumulative number of coronavirus infections because those after vaccination might result in lower anti-nucleocapsid titers that cannot account for reinfections.

The findings come as infections and hospitalizations are rising once again across the country.

Nearly a third of the country registered substantial or high levels of COVID-19 transmission in the prior seven days, including most San Francisco Bay Area counties at the high level, The Mercury News reported on Sunday, citing the CDC.

However, millions of unused COVID-19 doses in the country are set to go wasted.

Since the emergency use authorizations for the first COVID-19 vaccines in the country last winter, federal data shows that US states received 720 million doses, and more than 570 million of those shots have been administered, according to a report by ABC News.

But state-provided data found that millions of those shots have not ended up in arms, largely due to a significant decline in the number of people willing to get vaccinated.

Millions of COVID-19 vaccine doses have either gone to waste, remain unused, or will expire in the coming weeks and months, according to the ABC News report.

Also on Tuesday, US Vice-President Kamala Harris tested positive for COVID-19, a spokesperson said in a statement.

Xinhua contributed to this story.

mayzhou@chinadailyusa.com