How do you count the cost of a pandemic? Covid-19 has killed an estimated 15 million people since it emerged at the end of 2019, but its impact on health reaches much further.

For hundreds of millions of people around the world, infection with the coronavirus SARS-CoV-2 has brought a range of problems, from the acute effects of the illness to the lasting symptoms known as long Covid.

Working out the size of that health burden is challenging, but important – governments use such figures to plan how to spend health-care budgets. So, researchers are starting to tally the overall health impacts and trying to draw lessons from any patterns. They’re hoping, for example, to discern how different populations are affected and to provide evidence about the effects of vaccine roll-outs and new variants of the virus.

Even without a pandemic, there is no easy way to tally all the effects of various health conditions: good data can be hard to come by and decisions on how to measure burdens are inherently subjective.

“There are a whole lot of social value choices where there isn’t hard science,” says Theo Vos, an epidemiologist at the Institute for Health Metrics and Evaluation (IHME) at the University of Washington in Seattle, a research centre that aims to categorize the global health burden of diseases. “How do you value a year with asthma, a year without a leg, a year with depression?”

Those calculations are even harder when scientists are grappling with a new virus and a poorly characterised disease.

Research groups are exploring a number of ways to calculate the burden of Covid-19 and many are starting to report their results. Early data suggest that the impact is significant and varies by country. One study found that Covid-19 took a heavy toll across 16 European countries, but that the impacts on different nations varied owing to factors ranging from the population’s age structure to political responses to the pandemic.

Estimates produced by national teams provide more detail. In Scotland, Covid-19 was second only to ischaemic heart disease in terms of the impact it had on the population’s health in 2020. In the Netherlands that year, the burden was 16 times that of a typical influenza season, according to a preprint published last November.

With the pandemic still raging across many parts of the world, it’s too early to calculate the full toll. But some researchers think it has helped to change how they calculate the health effects of diseases. “The pandemic has strengthened collaborations in the field of disease burden,” says Sara Monteiro Pires, an epidemiologist at the Technical University of Denmark.

Researchers are now harmonizing the processes they use to estimate disease burden, and tailoring the models to the data available in each location. They hope this will make the results more precise.

The UK National Health Service lists a dozen Covid-19 symptoms for adults, from loss of smell to a high temperature. Even people who have a relatively mild case and ride out the symptoms at home can see lasting health effects, such as fatigue or shortness of breath. If symptoms continue beyond a couple of months, people can be diagnosed with an illness widely known as long Covid.

To quantify how a disease affects an entire population, scientists combine data on individual experiences. These include the number of people infected, the number who had certain symptoms, the length of illnesses, how many needed hospital treatments or died and patients’ ages, among other things. They then use them to work out how many years of life have been lost to the disease and how many years are lived with disabling symptoms.

Researchers can use the average life expectancies in a country to work out how many years of life have been lost owing to premature death. Losses due to disability are harder to calculate, however. To quantify those, researchers use data about the number of people affected by a certain illness, the length of time they have it and a value for the illness known as a disability weight.

The IHME’s Global Burden of Disease group maintains a standardised list of disability weights; the latest version available, published in 2019, gives a mild earache a disability weight of 0.013 and severe multiple sclerosis 0.719 (a weight of 0 is perfect health; a weight of 1 is death).

Currently, there is no standardised disability weight for Covid-19. Instead, researchers use the disability weights associated with other infectious diseases and similar health conditions.

Totalling the years of life lost due to illness, disability or premature death gives an estimate of the burden in a unit known as disability-adjusted life years, or DALYs. It is the keystone of research into the burden of disease.

The data that go into DALYs come from a variety of sources. Many are routinely collected by national health authorities. For Covid-19, some data have been gathered through targeted surveillance efforts such as the REACT study (Real-time Assessment of Community Transmission), a massive sampling exercise that began in 2020 and has chronicled how SARS-CoV-2 is moving through England and what symptoms people are experiencing.

Data from the REACT study suggest that Covid-19’s health effects can linger. A preprint posted on the medRxiv server last July suggested that 19 per cent of the English population had had Covid-19, and that around one-third of those – more than 2 million adults – had experienced one or more symptoms for at least 12 weeks. “That’s six per cent of the population,” says Paul Elliot, an epidemiologist at Imperial College London, who leads the REACT study and co-authored the study.

• A Nature report