Women and people who needed to be hospitalized with Covid are much more likely to develop long Covid, according to a new peer reviewed study published in JAMA on Monday, offering new insight into the persistent and sometimes disabling condition as researchers push to develop new treatments and cures.
An estimated 6% of people with symptomatic Covid infections develop persistent symptoms months after the initial infection, according to an analysis of published studies and medical records with data on 1.2 million Covid patients from 22 countries.
The researchers found that long Covid symptoms—which are grouped into three clusters of ongoing respiratory symptoms, persistent fatigue with bodily pain or mood swings and cognitive problems—were much more common among patients who needed to be hospitalized with Covid than those who did not, particularly among those who needed ICU care.
An estimated 43% of patients admitted to the ICU experienced at least one of the symptom clusters three months after infection, the study found, compared to 28% of those admitted to general hospital wards and just 6% for those who weren’t hospitalized.
The researchers also found significant differences in the risks of developing long Covid according to age and sex, with women and adults over the age of 20 at greater risk of the condition.
An estimated 63% of long Covid cases are in women worldwide, the researchers said, and nearly one in 10 of women over the age of 20 who had symptomatic Covid that didn’t need hospitalization developed persistent symptoms, compared to just 5% of men.
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Among the Covid survivors who developed persistent symptoms, the researchers estimate that 15% continued to have symptoms 12 months after infection. It is not known why or how some people experience symptoms for longer and others do not. The researchers said only time and further research will reveal how long symptoms persist in some people.
There have been more than 96.4 million Covid cases in the U.S. since the start of the pandemic, according to CDC data. More than 1 million people have died with Covid in this time and a good proportion of these cases are likely to be repeat infections. Though not all will have been symptomatic infections—which was the study’s focus—and many more cases have likely gone unreported, the study’s 6% prevalence means an estimated 5.8 million people experienced long Covid in the U.S. Around 870,000 would have had symptoms persisting for a year.
Despite appearing to be relatively common after infection, long Covid remains poorly understood several years into the coronavirus pandemic. Its underlying causes are not known—leading theories include issues with the immune system, the virus lingering in the body after initial infection and problems with blood clotting—there are no proven treatments and its precise prevalence is unclear. The condition appears to be similar to syndromes like chronic fatigue syndrome (ME/CFS), which have been neglected for years but are now receiving renewed attention owing to the similarities. Though it emerged as a clear problem fairly early on in the pandemic, as well as imposing a huge human and economic burden, long Covid has received relatively little attention compared to other aspects of the global outbreak and particularly when developing public health policy to contain the virus.