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When a person is exposed to Covid, does the amount matter? Conventional wisdom has long been that French kissing a highly contagious person in a bar will lead to a much worse case than inhaling some virus fragments on a walk in the park. The higher the initial “viral load”, the sicker you will get.
Actually, probably not. Judging by new research, the math doesn’t work.
Before we dig any deeper, let’s consider the notion of a fatal dose and how it might lead to incorrect intuition about viral infection. With substances like lead or arsenic, the initial dose defines the effect: if it is greater than what the human body can tolerate, we will get sick or even die. Viruses, by contrast, use human cells to multiply at exponential rates, typically doubling every two hours if allowed to grow unhindered. The effect on the body, then, depends on how far they can spread.
To get an idea of what this looks like with Covid, a group of researchers built mathematical models to reflect how the virus infects human cells, how it reproduces itself, and how people end up feeling sick. The research has yet to be peer-reviewed, but from this mathematician’s perspective, the intuition is compelling.
Covid viruses bombard us regularly, just as we’re walking, but they don’t always make it into our airways. They can get caught in nose hair or infect a cell but not reproduce. Just as sex does not always result in pregnancy, exposure does not guarantee infection. There is an element of randomness.
However, once a virus begins to reproduce, its impact depends on its interaction with the immune system (specifically, the fast-acting part called the innate immune response). Infected cells produce a substance called interferon, which protects nearby cells from infection. The exponential growth of interferon-producing cells eventually catches up with the exponential growth of the virus(1), but as the body fills with interferon, people feel really sick.(2)[2]
Now consider how this works with different viral loads with realistic numbers. Suppose a person is infected with a small amount of virus at noon. Over the next five hours, that amount doubles three times, growing to eight times the original amount. The immune response at 5 pm, and the attendant discomfort, would be only slightly more developed than if the person was suddenly infected with eight times the original amount at 5 pm AND considering how fast the exponential growth is, from that time, the progression and peak of the disease would be essentially identical.
So how do other studies conclude that viral load matters? The researchers fire pipettes of active virus directly into the airways of mice, instantly infecting large portions of their lung cells. Such a specific infection in such a vulnerable part of the body never happens to humans in real life. It would be like sucking on a turkey syringe full of Covid. Nobody does that.
In short, don’t worry about viral load. If you want to avoid infection, masks help, and good masks help even more. But if you get infected, you’ll end up feeling just as bad as if you hadn’t worn a mask. And in that case, being vaccinated is the best way to avoid getting really sick.
(1) When unhindered, a given virus recreates itself about eight times. Different people produce different amounts of interferon in their innate immune response. But ultimately, interferon stops the virus from growing, which is what I mean by the immune system “catching up.”
(2) So, on the one hand, you want a healthy interferon response to fight the virus, but too much and you end up feeling very sick.
This column does not necessarily reflect the opinion of the editorial board or of Bloomberg LP and its owners.
Cathy O’Neil is an opinion columnist for Bloomberg. A mathematician and data scientist, she founded ORCAA, an algorithmic auditing company. She is the author, most recently, of “The Shame Machine: Who Profits in the New Age of Humiliation.”
More stories like this are available at bloomberg.com/opinion
Source: www.washingtonpost.com