Viral load. I first heard about it, as you may have done, via social media. A letter, supposedly from an ICU (intensive care unit) doctor, popped up on Instagram on Tuesday morning, March 24, warning that this was one reason it why, when it comes to the coronavirus, it is so important to avoid groups of people, even if you are young and healthy.
"Remember this: VIRAL LOAD. There will be a lot about this, why is it important?" the letter stated. "With this virus, the amount of virus in your blood at first infection directly relates to the severity of the illness you will suffer." It went on to explain how being in a group, "say in a pub or religious building or entertainment venue with 200 people and a large number don't have symptoms, but are shedding, you are breathing in lots of droplets per minute and absorbing a high dose of the virus."
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Young people who were flouting the social distancing warnings were, therefore, putting themselves at greater risk, the letter warned: "being under 60 and fit and well doesn't seem to be as protective as we first thought. Why? Viral load."
Many are also suggesting that viral load may be the reason why so many healthy doctors in China and now in Europe have died from the condition. Last week saw the death of the first British doctor from suspected coronavirus, Essex GP Dr Habib Zaidi, 76, while 41 Italian medics have died since the outbreak began there. The Chinese whistleblower doctor, Li Wenliang, 34, died after contracting the virus while working at Wuhan Central Hospital.
But what is the truth about viral load and what does this mean for families isolating at home? If one of you is sick and you're sharing the same space will that increase the viral load for the rest of you, making you more severely ill? This morning both of my teenagers complained of feeling ill. One was coughing and the other felt "achey".
They were both sitting on the bed together looking at their phones, so I suggested that they move apart. Professor Wendy Barclay, Head of Department of Infectious Disease, at Imperial College London and Action Medical Research Chair Virology, an expert in the flu virus, who is now researching the coronavirus, says when it comes to coronavirus viral load is indeed important in how the infection will play out.
"Viral load is simply a term for how much virus a person has got," says Professor Barclay. "In general with respiratory viruses, whether you get severely ill or only get a mild cold can sometimes be determined by how much virus actually got into your body and started the infection off. Think about the analogy of two armies at battle. The outcome of the infection is your own immune system, whose army is of a fixed size and doesn't change, versus the virus, whose army gets bigger and bigger as it replicates, but also starts off at different sizes.
"With flu, I quite often describe it like this: if you sit next to somebody and they cough right in your face, you'll probably get a huge amount of virus, but if there is someone on the other side of the bus who breathes, by the time the virus is diluted in all the other air you don't get very much. And the battle between the virus and your own immune system is weighed comparatively depending on the size of the army at the start."
This is why doctors are very badly affected. "The procedures doctors are doing mean they are up close to the face taking a swab, or doing an intubation (a procedure performed when you can't breathe on your own), and despite the fact that we hope that they are wearing protective equipment, they may be exposed to more virus in the air than the average person would normally be."
A study published in The Lancet on March 19 found the mean viral load of severe cases was around 60 times higher than that of mild cases. So what should families do at home? The NHS recommends that if one of you develops a continuous new cough, or a fever, that you should self-isolate, for seven days, but what if everyone develops symptoms at the same time? Will your children make each other "sicker" if they sit in the same room together coughing, as some are suggesting?
"No, no, I don't think there's any basis in that at all," says Professor Barclay. "Once they've got it, they've got it and their immune systems are going to fight it off as best they can. "The point is once you are infected, there isn't much evidence of super-infection so that if you believe all of you are infected, it's probably not going to make much difference. What happens, once the virus is in you, it starts replicating in your own body and the virus accumulates, it's making hundreds and thousands of copies of itself every few hours, so any further exposure at that point is probably irrelevant," explains Professor Barclay.
Even so, we probably shouldn't be sneezing in each other's faces, says Willem van Schaik, Professor in Microbiology and Infection at the University of Birmingham who emphasises the importance of hand-washing and sneezing into tissues. "If only because there might be other people entering your house - you never know - and it will protect them." Adults who fall ill could be well-advised to wear masks, says Prof van Schaik. "The guidance right now is to give the mask to the patient, as the patient will be coughing and sneezing and so that will minimise the amount of virus flying around."
If one of you develops symptoms and the others at home aren't infected - or at least aren't showing symptoms - Professor Barclay recommends "keeping as much distance as possible and avoiding face-to-face contact as much as possible". Which of course is easier said than done, especially when it comes to children and living together in a small home. She doesn't, however, recommend that parents wear masks when a child falls sick, as the child will likely find it too scary. It's also sensible to use separate towels, cutlery and plates, though washing with detergent will inactivate the virus, says Professor Barclay.
Separate your lives as much as possible and use separate bathrooms, if you have them. If a number of people in the household are infected but one person is not, they should keep their distance. "If one of you is not yet infected, then that's quite bad news for that person because they are getting coughed at by five people instead of one," explains Professor Barclay.
"This goes back to the concept of chickenpox parties. Years ago people said, 'If your child hasn't had chickenpox, take them to a party and get them infected, but that's a really bad idea because you are putting them in the way of a lot of virus, whereas with natural infections in the community, there will be much less virus."
If you want to reduce your chance of not only catching the virus but also reducing the severity of the infection you should observe the two-metre rule. "The point of social distancing," says Professor Barclay, "is that by standing further away from someone when they breathe or cough out the virus, it likely means fewer virus particles reach you and then you get infected with a lower dose and get less ill. Your own viral load, that starts off the battle, is going to be lower and you've got more chance of winning it."
And should we be trying to do this at home, especially given that one of the problems with the virus is that so many of us are asymptomatic and that it can take five days, or potentially more, after the initial infection before symptoms show? "It is possible that keeping a distance within the home is a sensible idea but practically speaking, particularly if children are involved that isn't completely possible," says Professor Barclay. "Just generally (it's about) not being as close to people as you generally are - that means fewer hugs and kisses I'm afraid."