Following the development of vaccines and treatments for acute COVID-19 in record time, researchers and drugmakers are now focusing on finding a cure for long COVID, a more elusive target marked by hundreds of different symptoms afflicting millions of people.

Leading pharmaceutical companies, including those that have launched antiviral pills and monoclonal antibodies for COVID-19, are holding preliminary discussions with researchers about how to target the disease, according to five scientists in the United States and the United Kingdom.

Companies such as GlaxoSmithKline, Vir Biotechnology, and Humanigen confirmed speaking with researchers about trials using their current treatments against long COVID. Others, including Pfizer and Roche, expressed interest but declined to comment on plans.

Major pharmaceutical companies, according to researchers, biotech companies, and public health experts, are critical to developing a proven treatment for the disease, which currently affects more than 100 million people, according to the World Health Organization.

“When you look at the numbers for heart failure, for diabetes, etc, that is the ballpark we are talking about,” said Amitava Banerjee, a leading researcher on a long COVID trial.

Long COVID, with some 200 reported afflictions that include fatigue, chest pain and brain fog, is defined by symptoms that last longer than 3 months. It sidelines people who have had both mild and severe COVID-19, including children. In the United States, it is estimated to have affected 1-in-7 working age adults.

Sandi Zack, 53, a former elementary school teacher from the Atlanta area who can no longer work, described symptoms including extreme fatigue, dizziness, pain and heart palpitations since contracting COVID in December 2020.

She has sought help from a range of specialists and tried a variety of drugs to ease her symptoms including steroids and the antidepressant fluvoxamine.

“We’re all still out here,” she said. “Hoping, and waiting.”

There are fewer than 20 clinical trials underway testing drugs, a handful of which have moved beyond early stages, according to interviews with more than a dozen independent and government-backed scientists and a Reuters review of a global clinical trials database.

Scientists hope their research will uncover the causes of long COVID, a major hurdle in finding targets for new drugs or identifying existing medicines that may work as treatments.

“We are getting to the stage where we are getting traction, and for people suffering, we are getting treatments tested,” said David Strain, a University of Exeter Medical School lecturer whose research has informed which treatments will be tested in a major British trial. “Hopefully we will have things we can we offer them to get their lives back to normal in the near future.”

Big pharmaceutical companies are looking for disease-specific biomarkers that would allow them to assess the value of tested medicines, experts say.

“What they’re struggling with is a case definition for long COVID,” said Dr. Amy Proal, an expert in post-viral diseases at the PolyBio Research Foundation in Mercer Island, Washington. She said she has held confidential meetings with two venture capital groups and one major pharmaceutical company.

Possible underlying causes researchers are studying include damage from the original infection, lingering reservoirs of virus in the body, an autoimmune response, in which the immune system attacks its own cells, and a dysregulated immune response causing excess inflammation that damages small blood vessels or nerves. It could be a combination of those or other factors, they say.