Obesity and coronavirus: how can a higher BMI increase your risk?

Sarah Boseley Health editor

Less than a year ago, Boris Johnson was taking a stand. Milkshakes, he said, should not be taxed.

“If we want people to lose weight and live healthier lifestyles, we should encourage people to walk, cycle and generally do more exercise. Rather than just taxing people more, we should look at how effective the so-called ‘sin taxes’ really are, and if they actually change behaviour.”

Since then, the world has been hit by a coronavirus pandemic, with the prime minister one of those infected. And Johnson, at least on the face of it, has changed his mind.

Related: Labour welcomes PM's 'conversion' on obesity after coronavirus scare

Obesity was a problem before Covid-19, but this is an infectious disease that hits obese people disproportionately hard, no matter their income.

Countries with high obesity rates from western Europe to the US are struggling to keep people alive in intensive care units. Britain, long described as “the fat man of Europe” also has the highest number of Covid-19 deaths in Europe. Almost 30% of adults in the UK are classed as obese. The figure is nearly 40% in the US, where Covid deaths have topped 100,000.

It is striking, experts say, that wealthy countries appear to have higher mortality rates than impoverished ones. Africa so far has not experienced the explosion of deaths that Europe has. There will be many factors, and absence of good data collection will be one. But low rates of obesity, type 2 diabetes and the chronic disease of the heart and other organs are likely to play a part.


Epidemics of infectious diseases behave in different ways but the 1918 influenza pandemic that killed more than 50 million people is regarded as a key example of a pandemic that occurred in multiple waves, with the latter more severe than the first. It has been replicated – albeit more mildly – in subsequent flu pandemics.

How and why multiple-wave outbreaks occur, and how subsequent waves of infection can be prevented, has become a staple of epidemiological modelling studies and pandemic preparation, which have looked at everything from social behaviour and health policy to vaccination and the buildup of community immunity, also known as herd immunity.


Is there evidence of coronavirus coming back in a second wave?


This is being watched very carefully. Without a vaccine, and with no widespread immunity to the new disease, one alarm is being sounded by the experience of Singapore, which has seen a sudden resurgence in infections despite being lauded for its early handling of the outbreak.

Although Singapore instituted a strong contact tracing system for its general population, the disease re-emerged in cramped dormitory accommodation used by thousands of foreign workers with inadequate hygiene facilities and shared canteens.

Singapore’s experience, although very specific, has demonstrated the ability of the disease to come back strongly in places where people are in close proximity and its ability to exploit any weakness in public health regimes set up to counter it.


What are experts worried about?


Conventional wisdom among scientists suggests second waves of resistant infections occur after the capacity for treatment and isolation becomes exhausted. In this case the concern is that the social and political consensus supporting lockdowns is being overtaken by public frustration and the urgent need to reopen economies.

The threat declines when susceptibility of the population to the disease falls below a certain threshold or when widespread vaccination becomes available.

In general terms the ratio of susceptible and immune individuals in a population at the end of one wave determines the potential magnitude of a subsequent wave. The worry right now is that with a vaccine still months away, and the real rate of infection only being guessed at, populations worldwide remain highly vulnerable to both resurgence and subsequent waves.

Peter Beaumont


Prof Barry Popkin, a professor of nutrition at the University of North Carolina at Chapel Hill School of Public Health in the US, says obesity “weakens greatly our immune system”.

The European Centre for Disease Prevention and Control says 73% of critically ill patients with Covid-19 in Italy, Spain, Sweden, Switzerland and the Netherlands are obese. 

In the UK, according to Prof John Wilding, the president of the World Obesity Federation, obesity doubles the chances of death.

The non-profit organisation is publishing a dossier of evidence and guidance about obesity in Covid-19 on its website on Wednesday, setting out what has been established so far.

According to a yet to be peer-reviewed study of the electronic health records of 17 million adult NHS patients, the risk of a coronavirus-related hospital death increases from between 1.5 to 2 times for people with a body mass index of 30, which is the lowest level of obesity, to more than 2 for those with a BMI of 40 or more. “It is a very significant increase,” says Wilding . “And the reasons for that are probably pretty complicated.”

Although not an intensive care doctor, Wilding says one of the reasons is the difficulty ventilating somebody who is obese.

“It’s much harder to ventilate you effectively if you have a higher bodyweight. The lung capacity for the body size is lower, so there’s less reserve in the system. So of course, if you do have a severe respiratory infection, that affects the ability of the lungs to help get oxygen into the blood. The system is going to have to work a lot harder for you than it is for somebody who is much lighter and much smaller.”

It could make sense to advise everyone who is morbidly obese to shield – but GPs do not always put people on the scales or keep a record. So there is no register of who they are.

When Johnson was admitted to St Thomas’ hospital in London with Covid-19, he was reported to have weighed 17-and-a-half stone (111kg), which would have given him a BMI of about 36. BMI is a ratio of weight to height: 18.5 to 25 is normal and 30 is obese.

Over 40 is morbid obesity – and the point at which people can be referred to specialist care and may be eligible for bariatric surgery to reduce their stomach size. Johnson was vulnerable when he was infected – and he appears to have recognised his weight was a factor. 

“I’ve changed my mind on this. We need to be much more interventionist,” he told senior ministers and advisers, according to the Spectator’s political editor, James Forsyth.

This is exactly what doctors at the British Obesity and Metabolic Surgery Society (BOMSS) have been waiting to hear; they have long feared that the UK is not taking the condition seriously.

Hospitals deal with the consequences of obesity – such as soaring rates of type 2 diabetes, blindness and amputations – and the cost to the NHS is high. But the UK offers only a tenth of the stomach-reducing surgery that France does: 6,000 operations a year compared with 60,000.

Prof David Kerrigan, the president of BOMSS, and colleagues wrote an open letter to the prime minister to draw his attention to last year’s call by the Royal College of Physicians for obesity to be recognised as a disease – not a lifestyle choice.

“There has been a curious reluctance on the part of the NHS to grasp this particular nettle,” they said, adding that bariatric surgery to shrink the stomach and reduce appetite was often regarded as a “quick fix”.

But “a quick fix is precisely what is needed if we are to avoid needless suffering, ICU admission and death in patients with obesity who subsequently become infected with Sars-CoV-2”, they wrote to Johnson.

Related: How weight loss surgery saved my life

Like so much of Covid-19, it is unclear why people who are obese are at higher risk of severe illness.

Shaw Somers, a consultant bariatric surgeon and a former BOMSS president, says people who are overweight have what is known as a pro-inflammatory state.

“Excess fatty tissue, when it reaches a certain point, starts to secrete certain hormones which make your body think that it’s inflamed. When it becomes extremely severe, at very, very heavy weight, it is the thing that drives the damage to all their organs. Their body thinks it’s chronically inflamed and this just basically trashes a lot of their essential systems like the kidneys, the lungs, the heart, etc,” Somers says.

“With severe obesity, your immune system is working overtime. What we know from Covid is, those who do badly have an exaggerated inflammatory response that comes on after seven to 10 days. Their immune system goes berserk and kills them. And we think that obesity just amplifies it and just makes it much worse.”

People who are obese also fare worse if they get Sars, flu or pneumonia. The highest risk is with the highest BMI, but it is a sliding scale. Even those who are overweight, with a BMI of 25, are running health risks.

“If you look at the prime minister, for instance, his BMI is 36. So it’s not far from 40, where he’d be eligible for NHS treatment,” says Somers. “If his BMI is 36 and he got another illness like blood pressure or diabetes, he would be eligible for NHS treatment for his obesity.

“Even someone who is maybe a couple of stone overweight will have a degree of impairment of the body function and the degree of impairment of the resilience of their body. Because of that obesity, they may not be aware of it until they get serious illness like Covid-19, such as bad influenza. Then it will hit them.”

Somers says the UK should be treating far more people who cannot reduce their weight by themselves. It does not have to involve surgery. He talks about the gastric balloon, for example, which is swallowed in a deflated state and then inflated to the size of a grapefruit in the stomach. Four months later, it effectively disintegrates and is passed in the normal fashion.

Wilding hopes that people will be thinking of living healthier lifestyles as we come out of lockdown.

“We would encourage people to try and eat a healthier range of foods, fresh foods, and try and make sure the constraints we’ve all had to go through are not actually putting us at even higher risk of developing severe infection or severe illness from coronavirus because we put on weight while sitting at home worrying about it,” he says.

In the aftermath of Covid-19, it is likely that more people will cycle and walk in order to avoid crowded trains, tubes and buses. More dedicated cycle lanes may be built.

The link between obesity and Covid-19 may encourage better nutrition, curbs on junk food and better access to obesity treatment for far more people when they need it.

“There are some positive things that can come out of this,” Wilding says.