Smoking is one of the leading causes of death in the world, killing more than eight million people a year, according to the World Health Organization (WHO). Now, it is thought that smoking may also make people more vulnerable to developing serious complications if they catch coronavirus.

In the United Kingdom, the health secretary, Matt Hancock, has said: "It is abundantly clear from the research into previous coronaviruses that smoking makes the impact of a coronavirus worse."

We do know that smokers contract more respiratory illnesses, including the common cold which is also a coronavirus, than non-smokers. They also tend to have a higher rate of bacterial pneumonia and tuberculosis. During the Middle Eastern Respiratory Syndrome (MERS) outbreak in 2012, smokers were found to be more likely to die than non-smokers if they caught the virus.

Furthermore, smokers are twice as likely as non-smokers to contract influenza and have more severe symptoms.

However, the evidence has been less clear when it comes to the relationship between smoking and COVID-19, the disease caused by the novel coronavirus, specifically.

There have been some concerns that smokers are more likely to contract the virus because of the action of putting your hand to your mouth or using unwashed, contaminated hands to roll cigarettes which can increase the transmission of the virus from hand to mouth.

Some smoking products such as water pipes can involve sharing mouthpieces and hoses, which could also facilitate the transmission of COVID-19.

However, in a study of China's outbreak in the New England Journal of Medicine, it was found that fewer than 15 percent of the patients with COVID-19 were current or former smokers. Given the number of smokers in China - roughly one-fifth of the population - this might imply that smokers are not necessarily at a higher risk of catching it.

There is, however, evidence to suggest that smoking increases the severity of the virus if you catch it. The largest study looking at 1,099 patients in China, found that smokers were 1.4 times as likely to have severe symptoms and 2.4 times more likely to be admitted to an Intensive Care Unit (ICU), need ventilation or die compared with non-smokers who caught coronavirus.

A further small study of 78 people with COVID-19 also found a statistically significant higher proportion of smokers in the group whose condition adversely progressed compared with the group that showed improvement or stabilisation.

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However, there have been some contradictory studies that did not reveal a statistical significance in death rates between smokers and non-smokers, though it is important to recognise these were both small studies.

Smokers are considered at higher risk of developing complications such as respiratory difficulties and pneumonia if they catch coronavirus because their baseline respiratory health is likely to be poor.

They are also at an increased risk of having an underlying respiratory condition such as chronic obstructive pulmonary disease (COPD), a type of lung disease which could be exacerbated by the virus.

Additionally, smoking is a risk factor for developing other non-respiratory-related issues like heart disease and cancer which are also associated with complications for those who catch coronavirus.

All these conditions increase your need for oxygen or reduce your body's ability to utilise the oxygen properly, putting patients at risk of developing serious lung complications like pneumonia when infected with COVID-19.

Research shows that smoking compromises the immune system and increases susceptibility to infections.

Cigarette smoke suppresses the function of immune cells but also causes activation and recruitment of inflammatory cells into the lungs, which leads to the release of other chemicals, further changing the function of immune cells adversely. This may explain why smokers may be more susceptible to developing the serious consequences of COVID-19.

Another interesting theory that has been put forward as to why smokers are more at risk, relates to a protein called an angiotensin-converting enzyme-2 (ACE2) receptor.

This receptor is found on the surface of the cells of the respiratory tract, and the COVID-19 virus must sit within this receptor in order to be able to replicate and spread.

We are aware that smoking can increase the number of ACE2 receptors, so smoking may give the virus more receptors to use to invade cells and duplicate itself. It is therefore thought that this may be a cause of smokers' poor outcomes with COVID-19.

This remains a speculative theory and we have yet to understand what a raised or low ACE2 receptor expression may mean to the prognosis for people who contract COVID-19. Further studies will hopefully shed more light.

It seems most likely that the increase in severity of COVID-19 symptoms in smokers is a result of a combination of factors, including smoking-induced health conditions, poor baseline respiratory health, the ACE2 receptor hypothesis, comorbidities (concurrent conditions) and a suppressed immune system.

All this points to one conclusion, really. It is time to give up smoking.