How common is long covid?

IN THE FIRST year of the covid-19 pandemic the toll of the disease was measured by the number of people who died. Now, a second number is becoming prominent. This is the prevalence of “long covid”, the drawn-out form of the disease, which lasts for months or years. Long covid is poorly understood, often debilitating and is a looming burden on health-care systems even in countries where vaccination is slowing the rate of new infections. How many people with covid-19 go on to develop the long version?

The answer is complicated by the difficulty of diagnosing the condition, which is now formally called post-covid syndrome. Diagnosing any disease is rarely straightforward because the symptoms of various illnesses overlap and tests—if they exist—are not always conclusive. Treatment by trial and error is common practice, starting with drugs for what seems to be the most likely ailment. These challenges are magnified for long covid, and not only because it is a new disease. A recent study found 203 symptoms of long covid in ten of the body’s organ systems. Many experts agree that long covid is not a single disease but several symptoms caused by different impairments wrought by the virus.

Britain’s health service defines long covid as signs or symptoms that develop after a confirmed or suspected covid-19 infection, last for more than 12 weeks and are not explained by an alternative diagnosis. Some studies draw the line earlier, tallying symptoms that persist for four or five weeks.

Britain’s Office for National Statistics (ONS) has found that 21% of people testing positive for covid-19 continue to have at least one symptom five weeks after infection, and 14% still have symptoms at 12 weeks. Other studies have come up with estimates that are substantially higher or lower. The variation of results is probably caused by things such as how broad the questions about symptoms are and who is included in the studies (some, for example, are dominated by people who have been admitted to hospital). Approved medical tests for long covid to settle the matter do not yet exist. But a recent study of data from people who wear fitness trackers found that 14% of people testing positive for covid-19 developed an irregular heart rate that persisted for at least four months—which is consistent with the ONS estimates. An irregular heart rate can induce breathlessness and light-headedness, which are two of the most common long-covid symptoms.

How quickly, and even whether, people recover remains unclear. Several studies have found that the probability of having long covid drops off more rapidly in the first three months after infection (meaning that lots of people recover) and starts to plateau around six months after infection. In Britain, the ONS estimates that as of June 6th about 385,000 people had had self-reported long covid for more than a year.

Some are more debilitated than others by the illness. Two-thirds of Britons with long covid that drags on longer than four weeks say that their symptoms adversely affect their daily activities, including 18% who say their activities are “limited a lot”. In countries that have had big covid-19 epidemics, or will do so in the coming months because they are lagging on vaccination, long covid will be a burden on health-care systems for years to come.

The subject of this explainer was suggested by our readers. Is there a topic related to the covid-19 pandemic that you would like our writers to explain? Please send your suggestions and feedback to [email protected]

Dig deeper

All our stories relating to the pandemic and the vaccines can be found on our coronavirus hub. You can also find trackers showing the global roll-out of vaccines, excess deaths by country and the virus’s spread across Europe and America.