Houston resident Julie Joe has been suspecting that she might have already had 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 new findings released Tuesday by the Centers for Disease Control and Prevention (CDC) suggest that 6 out of 10 adult Americans — almost 200 million — and 3 in 4 children have had COVID-19 infections. 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," said Dr Kristie Clarke, co-lead for the CDC's COVID-19 Epidemiology and Surveillance Taskforce Seroprevalence Team, during a press 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 estimates there actually may be three people infected for every case officially reported during the Omicron variant wave, Clarke added, suggesting that millions of cases were missed as the highly contagious variant spread.

The seroprevalence study by CDC is a repeated and cross-sectional national survey that estimates the proportion of the population in 50 states, the District of Columbia and Puerto Rico that has infection-induced antibodies to COVID-19 between September 2021 and February 2022.

More than 200,000 blood samples were analyzed for virus-fighting antibodies anti-nucleocapsid (anti-N) made from infections, not the vaccines, during the study. It found out that prior to December last year, the seroprevalence increase rate was less than 2 percent every four weeks.

Between December 2021 to February 2022, when Omicron was spreading, seroprevalence increased from around 45 percent to 75 percent among children age o-17. That means 30 percent of the young population was infected in three months.

Seroprevalence from December to February increased from 36.5 percent to 63.7 percent among adults aged 18-49 years old. That ratio is lower for older people — it reached roughly 50 percent among people ages 50-64 and 33 percent among those age 65 or older.

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

The CDC cautioned that the findings in its report are subject to some limitations. All samples were obtained for clinical testing and might overrepresent persons with greater healthcare access or who more frequently seek care.

The findings might underestimate the cumulative number of coronavirus infections because those after vaccination might result in lower anti-N titers that cannot account for reinfections.

CDC officials warned that having previously been infected doesn't necessarily mean that someone is protected going forward, given that immunity can wane over time. The CDC says vaccination remains the safest strategy for preventing COVID-19 complications and recommends vaccination for all eligible persons, including those with previous infections.

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

Harris has exhibited no symptoms and has not been a close contact to President Joe Biden, Harris' press secretary Kirsten Allen said.

She will isolate and return to the White House when she tests negative, Allen said.