In just six months, nearly 8 million people worldwide have been stricken with confirmed cases of Covid-19, and at least 434,000 have died. But those deaths have not been distributed evenly; among the most vulnerable are people with underlying health conditions, such as diabetes and diseases that affect the heart and lungs. According to a new modeling study, roughly 1.7 billion people around the world — 22 percent of the global population — fall into that category.
That estimate, published today in The Lancet Global Health, excluded healthy older individuals without underlying health conditions, a group also known to be at risk because of their age. It also did not take into account risk factors like poverty and obesity, which can influence a person’s susceptibility to disease and access to treatment.
But such data could help health officials focus containment efforts on people vulnerable to the virus’s most dangerous effects and, perhaps, eventually prioritize them for vaccination, said Andrew Clark of the London School of Hygiene and Tropical Medicine, the study’s first author. Since the early days of the pandemic, researchers have known that chronic conditions can exacerbate disease. Now, there’s a better “understanding of the numbers involved,” Dr. Clark said.
The researchers compiled 11 categories of underlying conditions that may raise the risk of severe Covid-19 — a form of symptomatic disease that warrants hospitalization — using information from the World Health Organization and health agencies in the United States and Britain.
The list includes patients who regularly take immunosuppressive drugs, such as those with autoimmune disorders, or are undergoing immunity-weakening treatments like chemotherapy.
Not all of the conditions have been well studied in the context of Covid-19, cautioned Dr. Lona Mody, an epidemiologist at the University of Michigan who was not involved in the study. Little is known, for instance, about the risks for people with H.I.V., listed in the study as a potential complicating factor.
And Nina Schwalbe, a public health researcher at Columbia University and the author of a commentary accompanying the new study, reiterated that anyone infected with the coronavirus is at some risk of severe Covid-19, its effect dependent on a bevy of health-related factors.
The researchers mined data from the Global Burden of Disease Study, a comprehensive worldwide epidemiological survey last updated in 2017, to identify the number of individuals worldwide with at least one of these high-risk conditions. Nearly 200 countries were included in the final tally, with some, such as those in Europe, harder hit than others. In total, the analysis found, more than one-fifth of the world’s population may be at increased risk of more severe disease.
But increased risk is not equivalent to high risk, a category that refers to only a small subset of patients, Dr. Clark emphasized. Even if they are infected with the coronavirus, “not everyone with a condition will progress to a hospital,” he said.
The study also estimated that about 4 percent of the world’s population, around 349 million people, would require hospitalization if they became infected. That number includes patients without underlying medical conditions, such as healthy, older adults; and the risk of hospitalization increases with age.
Many of the other factors that can send a patient on a path toward severe disease are still unclear. Several are likely inextricably linked to so-called social determinants of health — conditions like poverty and health care access, which are not measured by the Global Burden of Disease Study, said Alex Ortega, an epidemiologist at Drexel University who was not involved in the study. The research also did not look at obesity, which has emerged as another prominent complicating risk factor. (A similar but separate analysis that included severe obesity as a complicating factor, published today by the Kaiser Family Foundation, found that nearly one-quarter of American adult workers are at risk of severe Covid-19.)
Moreover, the underlying conditions that increase the severity of Covid-19 in one country won’t necessarily do so in another, said Aditya Khanna, an epidemiologist at the University of Chicago who was not involved in the study. The local environment and people’s social habits also factor into health — and studies that apply the same metrics across different parts of the world “need to be taken with a grain of salt,” Dr. Khanna said.
Still, the study was greeted as a step in the right direction. “This begins to move us past the one-size-fits-all approach,” Ms. Schwalbe said. “The more precision we can get at who’s at risk of hospitalization, the better we can work with those people and tailor our response appropriately.”