Study reveals how long COVID stems from mild cases in most people

Even mild COVID-19 cases can have major and long-lasting effects on people’s health. That is one of the key findings from our recent multicountry study on long COVID-19 – or long COVID – recently published in the Journal of the American Medical Association.

Long COVID is defined as the continuation or development of symptoms three months after the initial infection from SARS-CoV-2, the virus that causes COVID-19. These symptoms last for at least two months after onset with no other explanation.

We found that a staggering 90% of people living with long COVID initially experienced only mild illness with COVID-19. After developing long COVID, however, the typical person experienced symptoms including fatigue, shortness of breath and cognitive problems such as brain fog – or a combination of these – that affected daily functioning. These symptoms had an impact on health as severe as the long-term effects of traumatic brain injury. Our study also found that women have twice the risk of men and four times the risk of children for developing long COVID.

We analyzed data from 54 studies reporting on over 1 million people from 22 countries who had experienced symptoms of COVID-19. We counted how many people with COVID-19 developed clusters of new long-COVID symptoms and determined how their risk of developing the disease varied based on their age, sex and whether they were hospitalized for COVID-19.

We found that patients who were hospitalized for COVID-19 had a greater risk of developing long COVID – and of having longer-lasting symptoms – compared with people who had not been hospitalized. However, because the vast majority of COVID-19 cases do not require hospitalization, many more cases of long COVID have arisen from these milder cases despite their lower risk. Among all people with long COVID, our study found that nearly one out of every seven were still experiencing these symptoms a year later, and researchers don’t yet know how many of these cases may become chronic.

Compared with COVID-19, relatively little is known about long COVID.

Our systematic, multicountry analysis of this condition delivered findings that illuminate the potentially steep human and economic costs of long COVID around the world. Many people who are living with the condition are working-age adults. Being unable to work for many months could cause people to lose their income, their livelihoods and their housing. For parents or caregivers living with long COVID, the condition may make them unable to care for their loved ones.

We think, based on the pervasiveness and severity of long COVID, that it is keeping people from working and therefore contributing to labor shortages. Long COVID could also be a factor in how people losing their jobs has disproportionately affected women.

We believe that finding effective and affordable treatments for people living with long COVID should be a priority for researchers and research funders. Long COVID clinics have opened to provide specialized care, but the treatments they offer are limited, inconsistent and may be costly.

Long COVID is a complex and dynamic condition – some symptoms disappear, then return, and new symptoms appear. But researchers don’t yet know why.

While our study focused on the three most common symptoms associated with long COVID that affect daily functioning, the condition can also include symptoms like loss of smell and taste, insomnia, gastrointestinal problems and headaches, among others. But in most cases these additional symptoms occur together with the main symptoms we made estimates for.

There are many unanswered questions about what predisposes people to long COVID. For example, how do different risk factors, including smoking and high body-mass index, influence people’s likelihood of developing the condition? Does getting reinfected with SARS-CoV-2 change the risk for long COVID? Also, it is unclear how protection against long COVID changes over time after a person has been vaccinated or boosted against COVID-19.

COVID-19 variants also present new puzzles. Researchers know that the omicron variant is less deadly than previous strains. Initial evidence shows lower risk of long COVID from omicron compared with earlier strains, but far more data is needed.

Most of the people we studied were infected with the deadlier variants that were circulating before omicron became dominant. We will continue to build on our research on long COVID as part of the Global Burden of Disease study – which makes estimates of deaths and disability due to all diseases and injuries in every country in the world – in order to to get a clearer picture of how COVID-19’s long-term toll shifted once omicron arrived.