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As the world works to understand how to best treat long COVID, it's crucial to answer a fundamental question — exactly what is it? Arriving at a definition of the condition is a key step in conceptualizing treatment and directing research.

Two major institutions are now offering important perspectives. The U.S. Centers for Disease Control and Prevention issued its definition earlier this year, and, recently, the World Health Organization produced its own clinical case definition of the condition, which both organizations call "post-COVID-19 conditions." (Not sure what is meant by "clinical case definition"? Here's an explanation from CDC.)

So how do the two definitions differ, and where do they overlap? Heidi Kosakowski, PT, DPT, PhD, APTA senior practice specialist, has been tracking the development of both. She says that while the two definitions are largely alike, the WHO definition offers some insight that goes a bit further than CDC's.

What Are the Definitions?

Both organizations readily acknowledge that their definitions are likely to change as understanding of post-COVID conditions increases, but for now, here's where they landed:

CDC: "While most persons with COVID-19 recover and return to normal health, some patients can have symptoms that can last for weeks or even months after recovery from acute illness. Even people who are not hospitalized and who have mild illness can experience persistent or late symptoms."

WHO: "Post COVID-19 condition occurs in individuals with a history of probable or confirmed SARS-CoV-2 infection, usually 3 months from the onset of COVID-19 with symptoms that last for at least 2 months and cannot be explained by an alternative diagnosis. Common symptoms include fatigue, shortness of breath, cognitive dysfunction but also others and generally have an impact on everyday functioning. Symptoms may be new onset following initial recovery from an acute COVID-19 episode or persist from the initial illness. Symptoms may also fluctuate or relapse over time."

Similar Results, Different Methods

Kosakowski says that the overlap between the two definitions is a good indication that there are some solid foundational understandings of post-COVID-19 conditions, an idea that's reinforced by the fact that the organizations took different paths to arrive at similar destinations.

"Both definitions stress the potentially lengthy duration of post-COIVD-19 conditions, as well as the idea that the conditions could include a wide range of symptoms, which they list elsewhere," Kosakowski said. "What's interesting is that the CDC's approach was based on input from a panel of provider and researcher experts, whereas the WHO used a Delphi approach that incorporated multiple participants, including patients and policymakers, as well as researchers and health care providers, to arrive at a consensus. It's encouraging to see that both methods resulted in those fundamental similarities."

The WHO Definition: More Detail

The real difference between the definitions, according to Kosakowski, is WHO's attempt to capture the long-term effects of long COVID-19 on daily function, as well as its explicit recognition of the episodic, ebb-and-flow pattern of symptoms.

"For the physical therapy profession, I think the most important element in the WHO definition is its specific mention of 'everyday functioning,'" she said. "This helps to underscore the important role of rehabilitation professionals in treating long COVID-19 conditions."

Also worth noting in the WHO definition is its acknowledgement that symptoms may fluctuate over time, Kosakowski added.

"The episodic nature of many symptoms is one of the most important considerations not only in treatment but in terms of what payers need to understand about the condition," she said.

The WHO Definition: Built On a Wider Reach

Given the global mission of WHO compared with CDC's more intensive U.S. focus, it's not surprising that the WHO definition is designed to be responsive to the experiences of low- and middle-income countries. Kosakowski says that although those considerations aren't explicit in the definition, the report on its creation makes several important statements about the demographics related to post-COVID-19 conditions.

"The WHO's global approach incorporates presentations of these conditions in countries with fewer resources, which is key to developing a robust definition," Kosakowski said. "The report also acknowledges that the effects of these conditions can occur irrespective of initial infection severity, and that long COVID occurs more frequently in women, among individuals who are middle-aged, and in people who experienced more symptoms initially."

Change Is Certain

Both WHO and CDC acknowledge that there's much still to learn about post-COVID-19 conditions, which means that their definitions are sure to change over time.

"This proposal of a clinical case definition is likely temporary, as new data continue to emerge," the WHO report states. "Initial reports describing post COVID-19 conditions were from small patient samples, with an inherent short follow-up, and likely subject to bias, and will be unraveled in ongoing meta-analyses. New research is exploring the use of electronic health records from representative samples of patients identified in primary care and elsewhere."

Similarly, CDC writes that "clinicians and researchers are still in the early stages of understanding post-COVID conditions," adding that "ways in which SARS-CoV-2 infection leads to reported symptoms are still being evaluated."

In other words, stay tuned.

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