Being obese doubles the risk of hospital treatment from Covid-19 and increases the risk of dying by nearly 50%, a global analysis suggests.
Obesity makes other diseases such as diabetes and high blood pressure more likely, the US researchers say.
Along with a weakened immune system, this can make these individuals more vulnerable to severe Covid-19.
They also warn a vaccine against coronavirus could be less effective in obese people.
This is based on the fact that flu vaccines don't work as well in those with a body mass index (BMI) of over 30.
The team, from the University of North Carolina, looked at data from 75 studies from around the world for their research, including nearly 400,000 patients.
They found that people with obesity and Covid-19 were twice as likely to end up in hospital and 74% more likely to be admitted to intensive care. They were also more at risk of dying from the disease caused by coronavirus.
Studies from the UK have shown similar risks for people carrying excess weight, prompting the government to introduce new measures to curb obesity.
Professor Barry Popkin, who led the study from the department of nutrition at the University of North Carolina, said the increased risks of being obese and having Covid-19 were "much higher than expected".
He said healthier eating had to be a priority in many countries, with fewer sugary drinks and much less junk and processed food in people's diets.
Obesity is linked to a number of diseases which also put people at higher risk of being ill with Covid-19.
It can also lead to more inflammation in the body, reduce the body's ability to fight off infections and put more strain on other organs, as well as the breathing.
"Vaccine researchers should look at how it affects obese individuals," Prof Popkin says of a coronavirus vaccine to protect against Covid-19.
He is concerned that a vaccine, when it is ready to be used, may be less effective in a population with a high percentage of people with obesity.
With obesity rising around the world and 20% of people overweight or obese in nearly all countries - in the UK and US it's close to 66% - understanding how treatments and vaccines work in this group is "critical", the research says.