How many people are affected?
This has been difficult to judge because not everyone who has had COVID-19 gets tested for it and there are no formal diagnostic criteria yet for long COVID. The CDC estimates that 19% of patients in the United States who have ever had COVID-19 have long COVID symptoms.
Some estimates go higher. A University of Oxford study in September 2021 found more than a third of patients had symptoms of long COVID between 3 months and 6 months after a COVID-19 diagnosis. As many as 55% of COVID-19 patients in one Chinese study had one or more lingering symptoms 2 years later, Lixue Huang, MD, of the China-Japan Friendship Hospital in Beijing, and colleagues reported in the journal Lancet Respiratory Medicine in May.
According to the CDC, age is a factor. “Older adults are less likely to have long COVID than younger adults. Nearly three times as many adults ages 50-59 currently have long COVID than those age 80 and older,” the CDC says. Women and racial and ethnic minorities are more likely to be affected.
Many people are experiencing neurological effects, such as the so-called brain fog, according to Ziyad Al-Aly, MD, of Washington University and the VA St. Louis Health Care System, and colleagues, whose report was published in Nature Medicine in September. They estimated that 6.6 million Americans have brain impairments associated with COVID infection.
“Some of the neurologic disorders reported here are serious chronic conditions that will impact some people for a lifetime,” they wrote. “Given the colossal scale of the pandemic, and even though the absolute numbers reported in this work are small, these may translate into a large number of affected individuals around the world – and this will likely contribute to a rise in the burden of neurologic diseases.”
Causes
It’s not clear what the underlying causes are, but most research points to a combination of factors. Suspects include ongoing inflammation, tiny blood clots, and reactivation of latent viruses. In May, Brent Palmer, PhD, of the University of Colorado, Denver, and colleagues found people with long COVID had persistent activation of T-cells that were specific for SARS-CoV-2.
COVID-19 itself can damage organs, and long COVID might be caused by ongoing damage. In August, Alexandros Rovas, MD, of University Hospital Munster in Germany, and colleagues found patients with long COVID had evidence of damage to their capillaries. “Whether, to what extent, and when the observed damage might be reversible remains unclear,” they wrote in the journal Angiogenesis.
People with long COVID have immune responses to other viruses, such as Epstein-Barr – evidence that COVID-19 might reactivate latent viruses. “Our data suggest the involvement of persistent antigen, reactivation of latent herpesviruses, and chronic inflammation,” immunobiologist Akiko Iwasaki, PhD, of Yale University, New Haven, Conn., and colleagues wrote in a study posted in August that had not yet been peer-reviewed for publication.
This might be causing an autoimmune response. “The infection may cause the immune system to start making autoantibodies that attack a person’s own organs and tissues,” the NIH says.
There could be other factors. A study by Harvard researchers found that people who felt stressed, depressed, or lonely before catching COVID-19 were more likely to develop long COVID afterward. “Distress was more strongly associated with developing long COVID than physical health risk factors such as obesity, asthma, and hypertension,” Siwen Wang, MD, a research fellow with Harvard University’s T.H. Chan School of Public Health, Boston, said in a statement. Plus, nearly 44% of those in the study developed COVID-19 infections after having been assessed for stress, Dr. Wang and colleagues reported in the journal JAMA Psychiatry.