A new US study is the latest to identify several factors that make some people more susceptible to a long Covid than others. Yet with millions worldwide experiencing debilitating symptoms weeks or months after first infection, the medical establishment still doesn’t understand why.
A new study from the University of Southern California (USC) has found that patients who have been obese at some point before infection with Covid-19 have a higher risk of developing Covid in the long term.
The researchers also found links between specific symptoms experienced during the initial infection and the likelihood of developing long-lasting Covid, with sore throats, headaches and hair loss more likely to indicate that symptoms would persist months later.
The World Health Organization (WHO) defines long-term Covid as the presence of symptoms that last at least two months and cannot be explained by another diagnosis after the coronavirus infection. The WHO says common symptoms include fatigue, shortness of breath and cognitive dysfunction, noting that symptoms can also fluctuate or reappear over time.
Since medical professionals first became aware of the long-running Covid phenomenon in 2020, scientists and researchers have been puzzled by the different profiles of susceptible patients, and researchers have struggled to provide conclusive answers.
Studies point to a list of possible predictors, including repeated Covid infections, high viral load infections, the presence of latent Epstein-Barr virus, the presence of autoimmune antibodies, and lack of vaccination.
Some studies also contradict each other. The USC study found no links between long Covid and age, race or gender, while a June 2022 study funded by Johnson & Johnson found that the likelihood of having long Covid syndrome was “significantly greater” among women
“At the start of 2020, we didn’t know anything,” says Rebecca Livingston, a lead clinical physiotherapist in the post-Covid service at University College London Hospital.
“Our thinking and understanding around long-term Covid has definitely progressed and research is helping us put some of these pieces of the puzzle together. But the more we find out, the more we realize we still don’t know the whole thing.”
“People don’t think I have it”
Meanwhile, the long Covid is affecting millions around the world.
A UK study found that an estimated 2 million people were living with prolonged Covid in June 2022. The USC study found that almost one in four people who had coronavirus infections still had symptoms until to 12 weeks later. In Wuhan, China, a May 2022 study found that among people hospitalized with Covid, half still had at least one symptom two years after infection.
The numbers are so high, in part, because the Covid-19 virus is highly contagious. In the past two years, more people have caught Covid than the common cold or the seasonal flu.
But these figures are also likely to be underestimates. “People don’t think they have it,” says Ruth Ainley, respiratory physiotherapist and Covid specialist. “They think they’re tired all the time because they’re down with the virus, so they don’t put two and two together.”
Even when people think they are unwell, some people are more likely than others to seek medical help. “The data we have would tell us that most people who have had Covid for a long time are female, middle-aged and white,” says Livingston.
“This data also reflects the people we see in the clinic, and we know there are significant inequities in access … to health care, so it’s probably not fully representative of the full picture.”
News articles have often focused on the shock of Covid leaving young, healthy and athletic people with debilitating symptoms. But people with very active lifestyles may also be more likely to notice symptoms like fatigue and take those symptoms seriously than older people.
“Older people are very underdiagnosed,” says Ainley. “It’s been canceled because they haven’t recovered from Covid, or they’re a bit tired and that’s to be expected at their age.”
A “difficult picture to piece together”
Even among patients known to have a long Covid, the complicated nature of the disease makes analysis difficult. There are more than 200 recognized symptoms of long-term Covid, according to the American Medical Association, which estimates that 20 to 30% of patients are affected, even after a mild initial illness.
And there are few recognizable patterns of when symptoms may appear or how long they may last.
“Our expectation, when we started working with post-Covid people, was that we would systematically assess them and that they would fit into categories. But the reality is that it’s a lot fuzzier,” says Livingston. “People will have symptoms that affect many different systems, and some people will have some symptoms and not others. It makes it a very difficult picture to put together.”
The impact of the long Covid on the lives of many patients is significant. In addition to physical symptoms, a 2022 National Institutes of Health study found that a “substantial” number of patients, more than a third, experienced PTSD, anxiety or depression three months after the onset of symptoms .
“You see really heartbreaking cases every day,” says Livingston. “It’s a very difficult condition to live with.”
Waiting for an “aha moment”
Looking ahead, there is some hope.
Ainley compares the struggle to understand the Covid-19 virus, and especially the long Covid, to the early experiences with HIV, when little was known about how to treat the virus or how it spread. “Now, HIV is not a death sentence like it used to be, but that took 30 to 40 years. The problem with the long Covid is that we’re two years into it and we don’t understand enough about the mechanisms of how it works.”
Livingston hopes that as more representative data emerges about who has had Covid for a long time, more patterns will emerge to shed new light on who is susceptible.
“All the research is coming apart and it’s helping to develop our understanding,” says Livingston. “I’d like to think there will be an aha moment sometime in the future. You have to expect that when you’re a doctor or a patient.”
As cases continue to rise in Europe and the United States, taking steps to prevent Covid infection in the first place remains the best line of defense.
And for those who already have the disease, research may soon provide much-needed answers. “There’s research to look at why people are susceptible to things, but there’s also research that needs to look at how we treat people and how we help people recover,” says Livingston.
“Long Covid is something we will have to think about and deal with for a long time. But we do know that people are recovering and that there are rehabilitation approaches that can help people.”