There is growing evidence that catching a cold may later reduce the severity of Covid-19.
Some scientists have even suggested deliberate exposure to some types of cold virus may ease the global impact of Covid-19 until a proper vaccine emerges.
Research is at an early stage, however, with many questions still to be answered, and people are being urged not to try to purposefully catch a cold.
But does the evidence stack up?
What’s the basis of the idea?
While the virus that causes Covid-19 – SARS-CoV-2 – is new to science, it belongs to a well-known family known as beta coronaviruses – and these cause around 15 to 20 per cent of common colds.
Most people recover from a cold in a week or so, as their immune systems attacks the virus. But in the process, the immune system “remembers” details of the virus, so it can quickly fight back if attacked again.
New studies suggest that similarities between the coronaviruses causing colds and Covid-19 explains why the immune system of some people never exposed to Covid-19 reacts as though it has seen it before.
What is the evidence ?
Around half a dozen studies in the US, Europe and Asia of stored blood from people never exposed to Covid-19 have found around 20 to 50 per cent react to the new virus. Tests found that T-cells – white blood cells which target and destroy cells infected with specific viruses – behave as if they have fought Covid-19 before, even though that's highly unlikely. In August, researchers in the US announced that these T-cells showed signs of having been made originally to fight coronaviruses that cause colds.
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Now researchers at the University of Rochester Medical Center, New York, have found that another key component of the immune system shows similar behaviour. In a study published earlier this week, the team found that people never exposed to Covid-19 have Memory B-cells (MBC) which also react as if they have seen the new virus before.
As with T-cells, the suspicion is that these antibody “factories” have been primed by previous exposure to other coronaviruses – including those which cause the common cold.
How good is the evidence ?
So far, the evidence comes from studies of fewer than 100 samples – so it’s hardly conclusive. However, it is well-known that infection with one virus can lead to immunity against another. The very idea of vaccination is based on the 18th century discovery that infection by a virus carried by cows triggers immunity against the related, but far more deadly, smallpox virus.
In the case of Covid-19, the Rochester team points out that the common cold coronaviruses and SARS-CoV-2 both have so-called Spike proteins on their surfaces.
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While not identical, they both share a set of molecules which MBCs recognise and attack. That suggests patients exposed to the right type of cold virus might suffer less from Covid-19.
So could deliberate exposure to cold viruses combat Covid-19 ?
Possibly – but most researchers would first want to see hard evidence that it cuts the risk of serious illness. Doing that would need large numbers of people, testing them for signs of previous coronavirus infections, and then watching how they fare if they catch Covid-19.
It’s possible that having had a cold has no impact – or even makes things worse. Arguably the best evidence for benefit comes from the Swine Flu pandemic of 2009.
As with Covid-19, many scientists expected this new form of virus to cause severe illness, simply because people had never been exposed to it before.
In fact, Swine Flu proved no more deadly than standard winter flu. Researchers later concluded the reason was many people had already developed some protection via previous infections by related viruses.
What happens next ?
Some researchers are already calling for urgent action on the cold virus idea. They point out even if an effective vaccine becomes available soon, wealthy nations have already cornered the market, and most countries will not have access to it until at least 2022.
This makes the cold virus idea a potentially valuable stop-gap measure that could significantly cut the global impact of Covid-19. However, research by the British Medical Journal suggests opportunities to put the idea to the test are already being missed.
In the meantime, experts warn that no-one should try deliberately contracting a cold to “protect” against Covid-19 – not least because most aren’t caused by coronaviruses.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK
