What causes long Covid? Scientists are zeroing in on the answer.

Even as the number of new Covid-19 cases in the US is dropping, hundreds of thousands of Americans are still testing positive every day. More than 28 million new cases have been reported since Omicron emerged in the US just two months ago, and the variant now drives 99.9 percent of cases, as of January 22, according to the Centers for Disease Control and Prevention.

Thanks to vaccines, boosters, and increasingly available treatments, most people who get infected today won’t end up in the hospital or die. A big question, however, looms over the survivors: What about long Covid?

Long Covid is a condition that arises after acute infection and often includes shortness of breath, fatigue, and “brain fog” but can also involve a wide range of debilitating problems in the heart, brain, lungs, gut, and other organs. According to the World Health Organization’s working definition, long Covid usually occurs three months after symptomatic Covid-19 begins and lasts for at least two months. Sometimes, the symptoms just never go away after the initial infection. Occasionally, they appear months after recovery or after an asymptomatic case. This means that if you’ve recovered from Covid-19, you’re not necessarily in the clear.

No one knows exactly how many people have or had long Covid. Estimates so far are “wildly disparate” in part because researchers define the condition differently and because the people seeking care may only be a small portion of those affected, said Nahid Bhadelia, an associate professor at Boston University School of Medicine. Studies on the conservative end have found that 10 to 20 percent of Covid-19 survivors get long Covid, while others report 50 percent.