It found the risk of worse outcomes from the virus starts rising in people with a BMI above 23.
Researchers have found a potential link between bodyweight and risk of severe coronavirus, especially for younger adults.
The study was based on more than 6.9 million people living in England and including data from more than 20,000 Covid-19 patients who were admitted to hospital or died during the first wave.
It found that the risk of worse outcomes from the virus starts rising in people with a body mass index (BMI) above 23kg/m squared, which is considered to be in the healthy range.
The healthy range of BMI is 18.5-24.9kg/m squared.
People with a BMI greater than 25kg/m squared are classified as overweight and those with a BMI greater than 30 kg/m squared are classed as obese.
According to the study, the risks of hospital admission were 5% higher for each one unit increase in BMI and the risk of ICU admission was 10% higher for each unit increase.
People who were underweight, with a BMI less than 18.5, also experienced worse outcomes from Covid-19, the research suggests.
The study published in The Lancet Diabetes And Endocrinology found that the effect of excess weight on the risk of severe disease was greatest in people aged 20 to 39, and decreased after age 60.
Increasing BMI had very little impact on the risk of severe coronavirus in people aged over 80 years.
However, the researchers report that the overall incidence of severe Covid-19 among people aged 20 to 39 was lower than all other age groups.
Dr Carmen Piernas, lead author of the study, from the University of Oxford’s Nuffield department of primary care health sciences, said: “Our study shows that even very modest excess weight is associated with greater risks of severe Covid-19 complications and the risks rise sharply as BMI increases.
“We also show that the risks associated with excess weight are greatest in people aged under 40 years, while weight has little to no effect on your chances of developing severe Covid-19 after age 80.
“These findings suggest that vaccination policies should prioritise people with obesity, especially now the vaccine is being rolled out to younger age-groups.”
Previous research has reported that obesity is associated with more severe outcomes after infection, but experts say this is the first to examine the consequences of excess weight on Covid-19 outcomes across the full range of BMI.
Researchers analysed records between January 24 and April 30 last year for outcomes linked to severe Covid-19.
During the study period 13,503 patients were admitted to hospital with the disease, 1,602 patients required treatment in an intensive care unit (ICU), and 5,479 patients died.
Most people with severe disease were aged over 60 years (72.1% of people admitted to hospital, 9,736/13,503, 55.7% of ICU admissions, 892/1,602, 92.5% of deaths 5,069/5,479).
Researchers found the risk of severe outcomes increased progressively above a BMI of 23kg/m squared and this was independent of other pre-existing health conditions, including type 2 diabetes.
The risks associated with higher BMI were greater for black people compared with white people, and there was no evidence that the risks for other ethnic groups differed from those of the white population.
Professor Paul Aveyard, who co-led the study, from the University of Oxford’s Nuffield department of primary care health sciences, said: “We don’t yet know that weight loss specifically reduces the risk of severe Covid-19 outcomes, but it is highly plausible, and will certainly bring other health benefits.
“Losing weight is hard and the recent NHS investment to improve access to weight management programmes could help to reduce the severity of Covid-19 at a population level and reduce the pressure on health care systems, while also lowering the risks for type 2 diabetes and some cancers.”
The authors say their study has several limitations, including that the analysis of the impact of BMI may be limited by the smaller sample of people with recent BMI measurements.
However, the findings did not change when the researchers excluded BMI measurements that were more than a year old at the start of the study period.