In the weeks and months after infection with COVID, some people develop a variety of symptoms, commonly called 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.
We still don’t understand why some people experience ongoing symptoms after COVID. And like the varying nature of long-term symptoms of COVID, the duration and intensity of symptoms varies from person to person.
It should be noted that we also had difficulty determining the exact prevalence of long-term COVID (ie, what proportion of people it affects). This has been the subject of considerable debate.
But a recent study published in The Lancet was touted as providing the most reliable estimates yet of the spread of the long-running COVID.
Estimates of the incidence of COVID generally range from about 5% to 50% of COVID cases, depending on which study you look at. Patients requiring hospital admission for COVID tend to sit at the upper end of the spectrum.
Much of the variability in prevalence estimates earlier in the pandemic was related to inconsistent definitions of long COVID. To clarify this and make diagnosis easier, public health authorities, including the UK’s National Institute for Health and Care Excellence (NICE), have introduced standardized definitions of persistent COVID.
According to NICE, the term prolonged COVID can be used to describe signs and symptoms that persist or develop beyond four weeks after infection with COVID. This is further divided into ‘persistent symptomatic COVID-19’, when symptoms persist for more than four weeks but less than 12 weeks, and ‘post-COVID syndrome’, when symptoms persist beyond 12 weeks of infection.
But despite these efforts, significant differences in prevalence estimates remain.
Read more: Long COVID: With no treatment options, it’s no wonder people are looking to unproven therapies like ‘blood washing’
What else causes this variation?
Several other factors also likely influence the reported prevalence figures in different studies. For example, the results may be affected by the lack of consistent screening tools or questionnaires 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 rather than other illnesses.
Meanwhile, vaccination status, treatments that reduce the risk of severe illness in patients with COVID (such as antiviral drugs), and the variant that caused the initial infection can affect a person’s risk of ongoing COVID.
Finally, the timing of the assessment also seems relevant. Recent figures from the UK’s Office for National Statistics show that although four in five patients with long-term COVID have symptoms lasting at least 12 weeks, only one in five are still symptomatic after two years.
Symptoms of prolonged COVID can be different for different people. UfaBizPhoto/Shutterstock
One in eight
The recent study, published in The Lancet, included 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 earlier efforts to quantify the long spread of COVID.
They used questionnaires that asked about a range of symptoms and gave them to participants at different points in time before, during and after contracting COVID. They also compared participants with a control group who had not been diagnosed with COVID and were similar in age and gender.
Of the participants who had COVID, 21.4% had at least one new symptom or a symptom that was significantly worse than before they had COVID, three to five months after infection. About 8.7% of uninfected people followed during the same time period reported symptoms. Based on this, the authors suggest that 12.7% of people who contract COVID, or one in eight, develop long-term COVID.
The authors also identified a list of major symptoms associated with prolonged COVID, including difficulty breathing, chest pain, heavy arms and legs, loss of smell, feeling hot and cold, numbness in the extremities, muscle pain, and fatigue.
Some disadvantages
Although this study has advanced our understanding of the real-world spread of long-term COVID, there are some important limitations. The majority of patients were unvaccinated, as most of the data were collected before the start of the vaccine rollout in the Netherlands. According to recent research, long-term COVID appears to be less common among people who are vaccinated.
Meanwhile, most participants were infected with the alpha variant. Studies have also reported a lower prevalence of persistent COVID among people recovering from infection caused by the currently dominant omicron variant compared to the alpha and delta variants.
So in the current context, the prevalence of prolonged COVID in the general population may be less than one in eight.
Read more: Long COVID: female gender, older age and pre-existing health problems increase risk – new study
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 condition will improve the outlook for the millions of people affected by prolonged COVID around the world.
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