Smoking cigarettes increases the risk of severe coronavirus infection by dampening the immune response of the body, a study suggests.
Lab studies on airway models made from human stem cells reveals smoking stops key immune system molecules, called interferons, from working properly.
Interferons are messengers that tell infected cells to make proteins to attack the invading pathogen, and are essential for fighting off initial infection.
They also summon support from the wider immune system and warn uninfected cells to prepare for the virus.
The study found smoking stops this pathway from working properly and this causes up to a threefold increase in the number of human cells infected by the virus.
This finding goes against a growing body of evidence showing smokers are in fact less likely to catch the coronavirus.
For example, a study from Mexico published in the summer analysed data from almost 90,000 patients and found smokers were 23 per cent less likely than non-smokers to get diagnosed with Covid-19.
And the team also found smokers who did get infected were no more likely to need intensive care, be hooked up to a ventilator or die.
University College London academics also looked at 28 papers and found the proportions of smokers among hospital patients were ‘lower than expected’.
One of the studies showed that in the UK the proportion of smokers among COVID-19 patients was just five per cent, a third of the national rate of 14.4 per cent.
A May study discovered fewer than five per cent of 441 COVID-19 patients who needed to be admitted to an Italian hospital were smokers.
The scientists described it as a ‘very low’ number, given that a quarter of the general population are known to be hooked on cigarettes.
Pictured, microscopic images of human stem cell-derived airway tissue models with cell nuclei (blue) and SARS-CoV-2 virus infected cells (green); tissue exposed to cigarette smoke (right) had two to three times more infected cells than non-exposed tissue (left)
Smoking increases the risk of severe coronavirus infection by dampening the immune response. Lab studies on airway models made from human stem cells reveals smoking stops key immune system molecules, called interferons, from working properly (stock)
At the start of the pandemic, when little was known about SARS-CoV-2, the virus which causes Covid-19, researchers instinctively warned smokers would be at higher-risk due to the fact the coronavirus targets the respiratory system and the fact smoking has long been linked with lung cancer, strokes, diabetes and other chronic conditions.
However, studies soon emerged indicating smokers are at reduced risk. Experts floundered to explain how this could be, calling it weird and bizarre.
Since then, some other studies have been published stating smokers may be at increased risk, but scientists struggled to present real world data or a mechanism to support this.
In June, German researchers conducted a comprehensive review of the impact of smoking and vaping on coronavirus infection.
Both harden the arteries and raise the risk of developing lung and heart diseases — two risk factors for coronavirus — by up to seven-fold, they found.
This graphic sums up how the presence of coronavirus and cigarette smoke impacts human airways. It shows more coronavirus infects cells if the cells have been exposed to cigarette smoke. It also shows that when interferons are artificially introduced (bottom), there is no infection. This proves that smoke inhibits the interferon pathway and that is why smokers are at more risk of severe Covid than non-smokers
There have been conflicting reports on the impact of smoking on a Covid patient’s prognosis, with some studies finding it reduces risk, and others finding the opposite. Now, academics from the University of California Los Angeles have determined how smoking likely results in a more severe SARS-CoV-2 infection
They admitted smoking is more toxic on the body than vaping but warned research suggested vaping was ‘not a healthy alternative’.
The review — published in the European Heart Journal — did not actually analyse the hospital records of Covid-19 patients, however.
Others say nicotine may control the immune system, stopping it from dangerously over-reacting to infection – a phenomenon found to kill many Covid-19 patients.
In June, the World Health Organization declared smoking — which impairs lung function — may make people more susceptible to COVID-19.
‘If you think of the airways like the high walls that protect a castle, smoking cigarettes is like creating holes in these walls,’ says author of the latest study Dr Brigitte Gomperts, from UCLA.
‘Smoking reduces the natural defenses and that allows the virus to set in.’
The scientists used human stem cells from donors to create an airways analogue, called an air-liquid interface culture.
The researchers focused on this part of the respiratory system before the lungs because it is where mucus is formed and the majority of cilia live, little hairs designed to help move cilia, and any trapped infections, out of the body.
Some were left unabused, while some were exposed to cigarette smoke for three minutes every day over four days.
They were then both infected with SARS-CoV-2 to see how the virus behaved in both systems.
In the models exposed to smoke, the researchers state in their paper, published in Cell Stem Cell, there was between two and three times more infected cells.
