Our understanding of why some people experience persistent symptoms after COVID is still poor. Like the varying nature of long-term symptoms of COVID, the duration and intensity of symptoms varies from person to person. Notably, we also struggled to pinpoint the exact prevalence of long-term COVID (ie, the percentage of people it affects). This has been the subject of much debate. However, recently, a study published in The Lancet has been touted as providing the most reliable estimates yet of the prevalence of long-term COVID. Estimates of how often COVID occurs generally range from about 5% to 50% of COVID cases, depending on which study you look at. Patients requiring hospitalization for COVID tend to sit at the higher end of the spectrum. Much of the variation in prevalence estimates earlier in the pandemic was related to inconsistent definitions of long-term COVID. To clarify this and 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 being infected with COVID. This is further divided into “ongoing symptomatic COVID-19”, when symptoms last more than four weeks but less than 12 weeks, and “post-COVID-19 syndrome”, when symptoms continue beyond 12 weeks after infection. However, despite these efforts, considerable variation in prevalence estimates has remained. Read more: Long-term COVID: with no treatment options, no wonder people are turning to unproven treatments like ‘blood washing’
What else causes this variation?
Several other factors are also likely to influence the reported prevalence figures in different studies. For example, results may be affected by a lack of consistent instruments or questionnaires to screen for long-term COVID. In addition, the lack of a matched control group (comparison group that did not have COVID) or information on patients’ symptoms prior to COVID, may be a limitation. This information allows researchers to reliably link the new symptoms to COVID and not to other diseases. Meanwhile, vaccination status, treatments that reduce the risk of severe disease in COVID patients (such as antivirals), and the variant that caused the initial infection can all affect a person’s risk of long-term COVID. Finally, the time of assessment also appears to be relevant. Recent data from the UK Office for National Statistics suggests that although four in five patients with long-term COVID have symptoms lasting at least 12 weeks, only one in five continue to be 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 long-term prevalence of COVID. They used questionnaires that asked about a range of symptoms and gave them to participants at various time points before, during and after being infected with COVID. They also compared the participants to a control group who had not been diagnosed with COVID and were similar in age and gender. Of the participants who had COVID-19, 21.4% developed at least one new symptom or a symptom that was significantly worse than before 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 infected with COVID, or one in eight, develop long-term COVID. The authors also found a list of key symptoms associated with long-term COVID-19, including shortness of breath, chest pain, heavy arms and legs, loss of smell, feeling hot and cold, tingling in the extremities, muscle pain and fatigue.
A few cons
While this study has improved our understanding of the prevalence of long-term COVID in the real world, there are some important limitations. The majority of patients were not vaccinated because most of the data were collected before the vaccine was launched in the Netherlands. According to recent research, long-term COVID appears to be less common among people who have been vaccinated. Meanwhile, most participants were infected with the alpha variant. Studies have also reported a lower prevalence of long-term COVID among people recovering from infection caused by the currently dominant omicron variant compared to the alpha and delta variants. Thus, 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 additional research, dedicated funding and greater recognition of the condition will improve the outlook for the millions of people affected by prolonged COVID around the world.
