(CNN) -
"The dose makes the poison" is an adage attributed to Paracelsus, a Swiss physician-philosopher of the early Renaissance.
Basically, it means that any substance can become toxic if administered in a high enough concentration.
Even too much water can remove electrolytes and be potentially fatal.
Seeing the coronavirus through that lens, that the "dose" of the virus you receive can make the difference between being asymptomatic, getting mildly ill, or getting seriously ill, can be helpful when thinking about protection against covid-19 due to temperatures colder and increased cases.
SARS-CoV-2 is likely to behave like other viruses
The concept of needing a certain dose of a pathogen - an organism that causes disease - to trigger an infection has been shown to be the case for many viruses, such as influenza viruses, poxviruses and others, Erin Bromage explained. , associate professor of biology at the University of Massachusetts, Dartmouth.
'If you hit an animal with a low enough dose, it will be able to defend itself without developing any disease at all.
If you get a magic number from an infectious dose, an infection will set in and that animal will succumb to the disease of that particular pathogen.
But if you attack them with more than the infectious dose, in most situations, a high dose of pathogens - like a high dose of a virus, for example - leads to more severe results.
Then the dose becomes really important, "he said, emphasizing the" dose-dependent "relationship.
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It is not because you need a certain number of virus particles to infect a cell, it simply increases the chances that one of those virus particles will enter the cell and infect it, setting off the chain reaction.
Another way of looking at it is like conception: you don't need millions of sperm to fertilize an egg, you only need one, but men produce millions of sperm to improve the chances that one will reach the egg, overcome its defenses and fertilize it.
Receive a one-time viral dose or spread it over time
There is another dimension to the viral dose and it has to do with time.
It is not the dose of the virus that is simply obtained at a given moment that matters;
It can also be the sum of the viral doses you receive over a certain period of time.
“Some people speculate about it: is that why, for example, bus drivers or people who work in emergency rooms are more likely to have more adverse outcomes?
Because they are exposed to higher doses or because they are in an environment where they are exposed to it for a long period of time and get a larger swath of it in them? "
Bromage asked.
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In mid-October, the US Centers for Disease Control and Prevention (CDC) changed its definition of "close contact" to include brief and multiple exposures that add up to 15 minutes or more. with an infected person.
Previously, the CDC defined close contact as 15 minutes of continuous exposure to an infected individual.
It's not just about the virus, it's also about the host
The virus itself is not the only organism that plays a role, it also has to do with the individual.
"Each person has a different amount of virus that they require," Bromage said.
"Someone who is immunosuppressed, or someone who is stressed, for example, may need less challenge [from the virus] to get the same results than someone who is in a healthy condition."
Collectively, the probability of infection depends on the physiology of the potential host, as well as their personal behaviors and health habits, such as smoking, diet, physical activity, and sleep.
An elderly or ill host in the face of large and recurring exposures is clearly the worst case scenario.
But a medically fragile person could get sick even with a low dose of the virus;
conversely, a healthy person may be overwhelmed with a high enough dose.
A tragic example is the death of the young and apparently healthy doctor Li Wenliang in Wuhan, China.
On December 30, 2019, he raised the alarm privately after seeing seven cases of a SARS-like illness among patients at his hospital;
Four days later, the police charged him with "seriously disturbing social order" and "spreading rumors online".
Shortly after spending time caring for critically ill patients, Li developed symptoms of Covid-19;
he died less than a month later.
He was only 34 years old.
Even so, we are only talking about probabilities.
Determining the exact stage that leads to infection is much more difficult to do.
"We just can't study the exact viral dose that would make someone sick because it is completely unethical," said Dr. Monica Gandhi, Ph.D. in infectious diseases and professor of medicine at the University of California, San Francisco.
That's because it would mean consciously exposing people to progressively higher doses of the virus to determine when an infection occurs.
“I don't think we are going to reach that value.
We can do it in animals - there has been a ferret model and there has been a hamster model - now two animal models in which the more you give, the sicker the animals get is useful, and it can give us an idea.
But, unfortunately, we will not know with humans how much is needed, "he said.
Viral dose and viral load
"There's an interesting two-sided dance between viral [dose] - what goes in - and viral load - what goes out -" Gandhi said.
Viral load is the amount of virus that an infected person has in his body;
Some, but not all, studies have shown that the sicker a COVID-19 patient is, the higher the viral load.
“If you get a lower dose of virus, then you can take care of it calmly and isolate it.
You have this asymptomatic infection and may be less likely to produce a high viral load;
it produces a lower viral load and is therefore associated with less severe disease, "he said.
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He noted that in places and situations that embraced universal use of masks, such as on an Argentine cruise ship and several U.S. food processing plants, the asymptomatic infection rate, above 80%, was more than double the infection rate. asymptomatic estimated by the CDC with a rate of approximately 40%.
The mask appears to reduce the dose by filtering out some of the viral particles.
The connection between masks and vaccines
However, there is another fascinating and important way to think about virus dosing.
If the dose is small enough, it may not cause disease, but it could still elicit an immune response, similar to a vaccine.
This is a remarkable and critically important concept.
In fact, Gandhi and his co-author, Dr. George Rutherford of UCSF's department of epidemiology and biostatistics, wrote a perspective article in the New England Journal of Medicine in September making the argument that by wearing a mask, a person, if exposed, you would receive a smaller dose of the virus than you would otherwise, which would prevent disease but still activate the body's immune system.
In their article, she and Rutherford made the comparison to "variolation," which is what is called a similar process when people deliberately expose themselves to a little pox matter from a sick person to create immunity.
This was before the introduction of the smallpox vaccine.
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"We'll give them a little bit of the virus and they'll get a little sick and then they'll develop an immune response, and it totally worked," he said.
It could also help explain why some people never develop symptoms of Covid-19, but still have antibodies to the virus.
But more research will be needed to confirm this.
Because people cannot control whether, much less the viral dose they receive, Gandhi reiterated what most public health experts have said to keep us as safe as possible: keep a physical distance from those who are not in our home, choose outdoor spaces instead of indoors, practice good hand hygiene and - please - wear a mask!
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