WASHINGTON (NYTIMES) - Over the past year, many scientific teams around the world have reported that obese people who contract the coronavirus are especially likely to become dangerously ill.

Now, a large new study, of nearly 150,000 adults at more than 200 hospitals across the United States, paints a more detailed picture of the connection between weight and Covid-19 outcomes.

The study, performed by a team of researchers at the US Centres for Disease Control and Prevention (CDC), has confirmed that obesity significantly increases the risk for hospitalisation and death among those who contract the virus. And among those who are obese, the risk increases as a patient's body mass index, or BMI - a ratio of weight to height - increases.

Patients with a BMI of 45 or higher, which corresponds to severe obesity, were 33 per cent more likely to be hospitalised and 61 per cent more likely to die than those who were at a healthy weight, the researchers found.

"The findings of the study highlight the serious clinical public health implications of elevated BMI, and they suggest the continued need for intensive management of Covid-19 illness, especially among patients affected by severe obesity," said the lead author Lyudmyla Kompaniyets, a health economist at the Division of Nutrition, Physical Activity and Obesity at the CDC.

But the relationship between weight and outcomes is nuanced. Covid-19 patients who were underweight were also more likely to be hospitalised than those who were at a healthy weight, although they were not more likely to be admitted to the intensive care unit or to die.

Ms Kompaniyets and her colleagues used a database of Covid-19 cases to identify 148,494 adults who received a diagnosis of the disease at American hospitals from last March to December. They calculated the BMI of each patient and looked for correlations between BMI and a variety of serious outcomes, including hospitalisation, ICU admission, mechanical ventilation and death.

They found that obesity, which is defined as a BMI of 30 or higher, increased the risk of both hospitalisation and death.

Patients with a BMI of 30 to 34.9 were just 7 per cent more likely to be hospitalised and 8 per cent more likely to die than people who were at a healthy weight, but the risks increased sharply as BMI rose.

Providing evidence for this kind of "dose response" relationship makes the study particularly compelling, said Dr Anne Dixon, director of pulmonary and critical care medicine at the University of Vermont Medical Centre, who was not involved in the research.

"What it shows is the more severe your obesity, the worse the effect is. And the fact that goes up stepwise with increasing levels of obesity, I think, adds sort of biological plausibility to the relationship between obesity and the outcome."

The connection between obesity and poor outcomes was strongest among patients under 65, but it held even for older adults. Previous, smaller studies have not found strong links between obesity and severity of Covid-19 in older adults.

"Potentially because they had more power from this large sample size, they've demonstrated that obesity remains an important risk factor for death in older adults as well," said Dr Michaela R. Anderson, an expert in pulmonary and critical care medicine at Columbia University Medical Centre, who was not involved in the study.

"It's a beautifully done study with a massive population."

Ms Kompaniyets and her colleagues also documented a linear relationship between BMI and the likelihood of needing mechanical ventilation; the higher the BMI, the more likely a patient was to require such intervention, which is invasive and can come with serious complications.

The study also found that patients who were underweight, with a BMI below 18.5, were 20 per cent more likely to be hospitalised than those who had a healthy weight. The reasons are not entirely clear, but may stem from the fact that some of these patients were malnourished or frail or had other diseases.

The BMI range associated with the best outcomes, the researchers found, was near the dividing line between what is considered healthy and overweight, consistent with some prior research suggesting that a few extra pounds might help protect people when they contract an infectious disease.

"Exactly why that association exists is currently unknown," said Dr Alyson Goodman, a paediatrician and medical public health researcher at the CDC and a co-author of the study. One possibility is that having a bit of extra fat may provide much needed energy reserves over the course of a long illness.

The findings highlight the importance of carefully managing the care of patients who are severely obese and of ensuring that people who are obese have access to vaccines and other preventive measures.

"This just provides further evidence for the recommendation to vaccinate those with a high BMI as early as feasible," said Sara Y. Tartof, an infectious disease public health researcher at the Department of Research & Evaluation at Kaiser Permanente, who was not involved in the study.