How common is the 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 becomes important. This is the prevalence of “long covid”, the prolonged form of the disease, which lasts for months or years. Long covid is poorly understood, often debilitating, and poses an imminent burden on health systems, even in countries where immunization slows the rate of new infections. How many people with covid-19 develop the long version?

The answer is complicated by the difficulty of diagnosing the disease, which is now formally referred to as post-covid syndrome. Diagnosis of a disease is rarely straightforward because the symptoms of various diseases overlap and tests, if they exist, are not always conclusive. Treatment by trial and error is standard practice, starting with medication for what appears to be the most likely disease. These challenges are magnified for the long covid, and not just 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 caused by the virus.

The UK Health Service defines long covid as signs or symptoms that develop after a confirmed or suspected infection with covid-19, last for more than 12 weeks, and cannot be explained by an alternative diagnosis. Some studies draw the line earlier, counting symptoms that persist for four or five weeks.

The UK Office for National Statistics (ONS) found that 21% of people who test 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 resulted in estimates that are significantly higher or lower. The variation in results is likely caused by factors such as the scope of the questions about symptoms and the people included in the studies (some, for example, are dominated by people who have been admitted to hospital). Medical tests approved for the long covid to settle the case do not yet exist. But a recent study of data from people who wear fitness trackers found that 14% of people who tested positive for covid-19 developed an irregular heart rate that persisted for at least four months, which is consistent with estimates by the ONS. An irregular heart rate can cause shortness of breath and dizziness, which are two of the most common symptoms of long covid.

How quickly, and even if, people recover remains unclear. Several studies have shown that the likelihood of having a long covid decreases more quickly in the first three months after infection (which means many people recover) and begins to stabilize around six months after infection. In Britain, the ONS estimates that as of June 6, around 385,000 people had self-declared for more than a year.

Some are more weakened than others by the disease. Two-thirds of Britons with a long covid lasting more than four weeks say their symptoms interfere with their daily activities, including 18% who say their activities are “very limited”. In countries that have experienced large epidemics of covid-19, or will do so in the coming months because they are behind on vaccination, the long covid will be a burden on health systems for years to come.