In the weeks and months following a COVID infection, some people develop a variety of symptoms, commonly known as prolonged COVID. Fatigue is the most common, affecting just over half of people with ongoing symptoms. Other common symptoms include shortness of breath, loss of smell, muscle pain, and brain fog.
Our understanding of why some people face continued symptoms after COVID is still poor. And just like the variable nature of long-term symptoms of COVID, the duration and intensity of symptoms differ from person to person.
In particular, we have also had difficulty determining the precise prevalence of long-term COVID (ie, what proportion of people it affects). This has been a subject of considerable debate.
But recently, a study published in The Lancet has been announced that provides the most reliable estimates yet of the prevalence of long-term COVID.
Estimates of how often COVID occurs in general range from 5% to 50% of COVID cases, depending on which study you look at. Patients requiring hospital admission for COVID have tended to sit at the higher end of the spectrum.
Much of the variability in pre-pandemic prevalence estimates was related to inconsistent definitions of long-term COVID. To clarify this and to facilitate diagnosis, public health authorities, including the UK’s National Institute for Health and Care Excellence (NICE), have introduced standardized definitions for long-term COVID.
According to NICE, the term prolonged COVID can be used to describe signs and symptoms that continue or develop beyond four weeks after a COVID infection. This is further divided into ‘ongoing symptomatic COVID-19’, when symptoms last more than four weeks but less than 12 weeks, and ‘post-COVID syndrome’, when symptoms continue beyond 12 weeks from infection
But despite these efforts, considerable variation in prevalence estimates has remained.
Read more: Long COVID: With no treatment options, it’s no wonder people are turning to unproven therapies like ‘blood washing’
What else is causing this variation?
Several other factors also likely affect the prevalence figures reported in different studies. For example, the results may be affected by the lack of consistent instruments or questionnaires to detect long-term COVID.
Also, not having a matched control group (a comparison group that did not have COVID) or information about patients’ symptoms before COVID, may be limiting. This information allows researchers to reliably link new symptoms to COVID rather than other illnesses.
Meanwhile, vaccination status, treatments that reduce the risk of serious illness in COVID patients (such as antivirals), and the variant that caused the initial infection can affect a person’s long-term risk of COVID .
Finally, the timing of the assessment also seems relevant. Recent data from the UK’s Office for National Statistics suggests that while four out of five long-term COVID patients have symptoms lasting at least 12 weeks, only one in five are still symptomatic at two years.
Symptoms of long-term COVID can be different for different people. UfaBizPhoto/Shutterstock
One in eight
The recent study published in The Lancet includes data from more than 76,000 people in the Netherlands. The authors have made one of the first attempts to mitigate the various biases that have plagued previous efforts to quantify the long-term prevalence of COVID.
They used questionnaires that asked about a range of symptoms and gave them to participants at various times before, during, and after infection with COVID. They also compared the participants to a control group who had not been diagnosed with COVID and were similar in age and sex.
Of the participants who had COVID, 21.4% experienced at least one new symptom, or a symptom that had gotten significantly worse since before they had COVID, three to five months after infection. About 8.7% of uninfected people followed over the same time period reported symptoms. On this basis, the authors suggest that 12.7% of people who contract COVID, or one in eight, develop long-term COVID.
The authors also checked a list of core symptoms associated with long-term COVID, including shortness of breath, chest pain, heavy arms and legs, loss of sense of smell, feeling hot and cold, tingling in the extremities, muscle pain and tiredness
Some drawbacks
Although this study has advanced our understanding of the real-world prevalence of long-term COVID, there are some important limitations. The majority of patients were unvaccinated because most of the data were collected before the vaccine rollout began in the Netherlands. According to recent research, long-term COVID appears to be less common among vaccinated people.
Meanwhile, most of the participants were infected with the alpha variant. Studies have also reported a lower prevalence of long-lasting COVID among people who recover from infection caused by the currently dominant omicron variant compared to the alpha and delta variants.
Therefore, in the current context, the prevalence of long-term COVID in the general population may be less than one in eight.
Read more: Long COVID: Female sex, older age and existing health problems increase risk: new research
Importantly, our understanding of the mechanisms that cause persistent symptoms and what makes people vulnerable remains quite limited. Only further research, dedicated funding support and greater recognition of the disease will improve the outlook for the millions of people affected by COVID for a long time worldwide.