IT is a question scientists are hoping a major international study will answer.

Most people will have heard of at least one case where a family member seems to dodge Covid-19 despite living in close proximity to others who have the virus.

"If we can understand why these people never become infected, we might be able to develop new treatments," says Professor Neil Mabbott, an expert in immunopathology at the University of Edinburgh.

He said it is likely that many factors contribute to an individual's risk of developing the virus, in common with other infections.

"It could be due to genetic differences in genes that affect our susceptibility to the disease," said Prof Mabbott.

"We know how mutations in some genes are linked to developing serious Covid-19 disease. It is equally likely that mutations in other genes may influence whether you develop symptoms or not.

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He said it is also possible that the person who didn't "develop disease" may not have been exposed to the infected person when they were at their most infectious. 

"I put develop disease in quotes for a reason. It is equally possible that the person did become infected, but didn't develop Covid-19 symptoms, or if they did they were very mild. 

"Perhaps they were only exposed to a small amount of virus. 

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"Without lots of testing it is difficult to be sure that they weren't infected, but responded differently."

He said the person's age may also play a role with younger people more likely to experience milder symptoms than an elderly person.

"Unknown to themselves, they may even have already had an infection and developed some natural immunity. They may have responded more strongly to the vaccines, or have had more boosters, a different type of vaccine etc. 

"None of this is coronavirus-specific. The same is true for other infections. Some people seem to dodge them or develop only mild symptoms, while others do not."

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Prof Mabbott said the same reasons were likely to apply for Omicron, but added: "This variant is much more transmissible. So further studies are necessary to determine whether these individuals have a similar level of protection against Omicron."

He pointed towards an "interesting and important" study published last month in Nature Magazine which suggests some people develop natural immunity to Covid-19 after having the common cold.

"They noted that some health care workers that did not become infected had pre-existing memory T cells that could recognise a core component of the coronavirus that causes Covid-19.

"These cross-protective T cells were probably generated in response to previous infections with the seasonal coronavirus that cause the common cold. 

"Since these T cells recognise features of the coronavirus other than the spike protein, it is possible that they may also be effective against Omicron."