The definition is based on 12 symptoms that most separate people with long Covid six months or more after their infection from people who did not have the coronavirus.
Since the very early days of the pandemic, many people have suffered myriad sometimes debilitating symptoms that persist long after they were infected with Covid-19.
Patients adopted the name long Covid. Scientists call the condition post-acute sequelae, or PASC.
But there still is no systematic, universally accepted definition of long Covid for research, and which could serve as the foundation for future tools to diagnose the condition.
“It’s really attempting to come up with a concrete, replicable specific definition for long Covid,” said Dr. Leora Horwitz, author of the study and a professor at NYU Grossman School of Medicine.
The study funded by the National Institutes of Health examined nearly 10,000 participants across 85 hospitals, health centers and community centers in 33 states.
More than 8,600 patients who had Covid were compared with more than 1,100 who did not have the virus.
The research is part of the NIH’s massive $1.15 billion RECOVER research initiative that aims to define long Covid, understand what causes the condition and develop treatments for it. RECOVER is an acronym for Researching COVID to Enhance Recovery.
The symptoms that stood out the most among participants with long Covid included loss of smell and taste, post-exertional malaise, chronic cough, brain fog, thirst, palpitations, chest pain, fatigue, changes in sexual desire, dizziness, gastrointestinal issues, abnormal movements and hair loss.
The scientists assigned points based on how much each symptom distinguished participants with long Covid from those who did not catch the virus.
A participant who has 12 points or more is considered likely to have long Covid.
Loss of smell and taste and post-exertional malaise, for example, stood out more than other symptoms, and had scores of 8 and 7 points, respectively. Palpitations and dizziness, which are characteristic of long Covid, but which are also common symptoms in many other conditions, scored 2 points and 1 point, respectively.
Future clinical use
Horwitz, author of the study, said the proposed definition of long Covid could help develop a method for doctors to diagnose patients.
But Horwitz said the definition presented in the study is an early working one, still needs to be refined and is not yet ready for clinical use.
In the absence of a universally accepted definition, many long Covid patients have struggled to get appropriate health care, particularly early in the pandemic, because some symptoms are common to other conditions, which can make a diagnosis difficult.
There are no tests that can diagnose long Covid based on markers in the blood. Scientists participating in RECOVER are trying to understand the underlying biology that causes long Covid, which could potentially lead to such tests in the future.
Horwitz said the proposed definition could help create a rubric to diagnose patients with long Covid in a manner similar to Lupus. There’s no single blood test that can diagnose Lupus, so physicians also rely on a collection of common symptoms to determine whether a patient has the disease.
Horwitz said the goal is to give researchers a more systematic definition that can be used to answer questions about risk factors and how likely long Covid is after repeat infection and between different variants of the virus, among other issues.
Biological samples from the patients who developed long Covid during the study could be used to investigate what causes the condition and potentially help find treatments and guide enrollment in future clinical trials, according to the study.
Long Covid more common before omicron
The study also found that long Covid was more common among people infected before the omicron variant swept the U.S. in December 2021.
About 17% of patients who enrolled more than 30 days after their infection during omicron developed long Covid. By contrast, about 35% of those infected before the omicron era developed long Covid.
But patients who were reinfected during omicron were more likely to develop long Covid than those who reported one infection when the variant was surging. About 21% of those with repeat infections who enrolled after 30 days developed long Covid compared with 16% who caught Covid once.
People who were fully vaccinated were less likely to develop long Covid regardless of when they were infected.
About 16% of participants up to date on their shots who got infected during omicron developed long Covid, compared with 22% who did not receive their shots. Before omicron, 31% of people up to date on their shots who got infected developed long Covid, compared with 37% who were not vaccinated.