Stress levels and genetics may partly explain why some people who test positive for coronavirus are able to beat the disease more effectively than others.
Scientists also speculate that the initial dosage of the infection that a person is exposed to might impact the severity of a patient’s condition – as would age.
It comes as Boris Johnson was taken into intensive care on Monday evening.
The prime minister, 55, had been self-isolating at home after testing positive for Covid-19, but was taken to St Thomas’ hospital after his condition deteriorated.
Other high-profile public figures such as Matt Hancock, 41, and the Duke of Wales, 71, who both also tested positive for the disease, have since recovered.
“Probably one of the most likely might be initial dose, the inoculum of the virus that you get in the first place,” says professor Robin May, professor of Infectious Diseases at the University of Birmingham.
“If you get a small dose of the virus, enough to cause the infection but your immune system gets on top of it quite early you get mild symptoms,” he said.
“Whereas if you get a really big dose of the virus on day one, the virus is replicating faster than your immune system can keep up and you might develop more severe systems.
“If you breathe in a few hundred viral particles or a few million – if someone sneezes in your face, you would get a huge amount of droplets.”
Professor May emphasised that there was currently no data to back up speculation over dosage, or other factors relating to contracting coronavirus, but added that it did apply to other types of infection.
He added that stress “significantly” impacted on immunity and that the prime minister’s job may have made him particularly susceptible.
“You would imagine it’s been a fairly stressful job for the past few weeks,” he said.
“Certainly we know from other infections that high levels of stress can make those infections worse, so that’s something that could be a player here.”
But others suggest that an individual’s genetics may have more of an impact on an individual’s ability to fight the disease.
“People like Boris Johnson probably have stress for breakfast, dinner and tea,” says professor Keith Neal, Emeritus Professor of the Epidemiology of Infectious Diseases at the University of Nottingham.
“There is little doubt that if you get run down you are probably more susceptible to infection and get them worse – to what degree that matters is different.
“These are the types of people that seem to thrive on being stressed, and stress is different to different people.
“He [Boris] has a stressful job and if he was trying to work and not rest when he was ill, I can’t believe that has helped but I wouldn’t put it down as the main factor.”
Prof Neal said that genetics were likely to be a “strong factor” in a person’s recuperation ability, especially in cases such as the Prince of Wales.
“Boris is a little bit fatter than the Prince of Wales and we know that people who are overweight are more at risk,” he added.
“Age clearly puts you at increased risk…[but] I suspect that the Prince of Wales is in pretty good shape for his age.
“He also comes from a long line of long-life people, from his grandmother and mother and father – nearly one hundred and still alive, so he’s probably got a good genetic stock in that sense.”