OBESE young people should be prioritised for Covid vaccines as the programme is rolled out to under-40s, according to a new study.

Researchers found that even those with a BMI of just over 23 - which should put them in the healthy weight range - were at increased risk from the infection, with the danger of hospitalisation climbing by an average of 5% and ICU admission by 10% for every additional BMI unit.

Risk of death began increasing at BMIs of 28 and above, but was independent of other health conditions such as diabetes.

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However, the study - published in the Lancet journal Diabetes & Endocrinology - found that the effect of excess weight was greatest in people aged younger than 40, but had "little or no effect" on patients chances of developing severe disease if they were over 80.

Dr Carmen Piernas, lead author of the study and a nutrition scientist at Oxford University'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.”

The study is the largest to date to evaluate the impact of bodyweight across all BMI ranges on the risk of more serious illness or death from Covid.

As well as obesity, the researchers found that the risk of a hospital admission or death due to Covid increased steadily as BMI reduced from 20 - with anything below 18.5 considered underweight.

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The findings are based on health records from 6.9 million people living in England, including more than 20,000 Covid-19 patients who were hospitalised or died during the first wave from January 24 to April 30.

The researchers found that the effect of excess weight on the risk of severe Covid was greatest in young people aged 20 to 39.

In this age group, each additional BMI unit above a BMI of 23 was associated with a 9% increased risk of hospitalisation, falling to 4% in 60-79-year-olds and 1% in those aged 80 to 100.

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However, the overall incidence of severe Covid-19 among people aged 20 to 39 years of age was lower than all other age groups, with 922 hospital admissions compared to 4,678 in the over-80s.

Obesity also increased the risk of severe Covid more in Black people compared to white people and other ethnic minorities.

The authors said the exact mechanisms underpinning an association between obesity and severe Covid "remain elusive".

Some studies have suggested that it could be linked to the fact that obesity can create "a pro-inflammatory state" within the body which leads to other weight-related diseases, such as Type 2 diabetes.

However, the researchers noted that adjusting for these other conditions "did not abolish most of the excess risk".

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Nonetheless they added that "since most other obesity-related risks are improved with weight loss, weight-loss interventions might reduce Covid-19 disease severity."

Professor Paul Aveyard, who co-led the study, 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.”

In a linked comment article, Professor Krishnan Bhaskaran of the London School of Hygiene & Tropical Medicine, said: “People with a BMI of more than [40] were recognised early on in public health guidance as being at increased risk of severe Covid-19 outcomes, and BMI is now being used in risk prediction tools that are informing vaccine prioritisation, highlighting the importance of obtaining a detailed understanding of the association between BMI and Covid-19 outcomes to inform policy.

"Key future research priorities will be to establish whether BMI affects vaccine efficacy, and to understand whether people outside the healthy BMI range (18·5–24·9) are at increased risk of post- Covid-19 sequalae [after-effects of disease].

"Further careful epidemiological study of these and other emerging questions will inform the ongoing public health response to this new disease that is likely here to stay.”