NEW YORK (PIX11) — While many people recover from COVID-19 within a matter of weeks, two new studies by the Centers for Disease Control and Prevention and National Center for Health Statistics found that last year nearly 18 million Americans reported having lingering effects, half claiming they still had the condition.

Diagnosed as long COVID-19, the condition occurs when patients still have symptoms at least four weeks after they have cleared the infection. In some instances, the symptoms can persist for months, even years.

Infectious disease specialist Dr. Anthony Santella noted that the symptoms can vary.

“Symptoms can range from difficulty breathing, muscle pain, loss of smell and taste, respiratory distress,” Santella said.

Santella stressed that it’s important to know that the symptoms aren’t necessarily unique and could be a sign of another ailment.

Roughly one million children have had long COVID, with more than a quarter of a million still reporting symptoms, according to the federal studies.

One report found that women were more likely to suffer, with nearly twice as many (4.4%) saying they had long COVID compared to men (2.3%).

When it came to age, adults between the ages of 35 and 49 were most likely to have the lingering effects. People aged 65 and older were least likely.

Among racial and ethnic groups, Hispanic adults were most likely to have had long COVID (8.3%), followed by white adults (7.1%), a lower percentage for Black adults (5.4%), and even lower for Asian adults (2.6%).

By age group, children between 12 and 17 years old were most likely to have had or still have long COVID.

Santella explained why some people struggle with COVID longer than others.

“There are a number of hypotheses they’re testing,” Santella said. “The most common ones we’re hearing about is the inflammation that the infection causes and how we as individuals feel that inflammation weeks and months after it clears.”

One concern of the new survey is that all of the respondents may not even know they had long COVID. The reports could help in development of new methods to treat the illness.

“The significance of these studies shows that we need to continue to produce good, high-quality science because the clinical trials and data produced today will produce science and medicine of tomorrow,” Santella said.

Accurate numbers are uncertain, but the United States Department of Health and Human Services estimates that as many as 23 million people in the U.S. have had long COVID.

Earlier this year, the Biden administration said it was forming a new office of COVID research to study the condition and help those who have been diagnosed with it.