North America is about to enter cold and flu season. Covid is on the uptick and may spread to millions of people this winter depending on how many people get a booster and how well the boosters work. Every year the U.S. Centers for Disease Control and Prevention (CDC) has to decide in the winter to spring which four flu strains are most likely to hit the United States next year. Strains circulating in Asia are often the ones that infect Americans, Canadians and Mexicans in the coming Winter. Thus, the flu vaccine is a cocktail of the four flu strains likely during following flu season.
The talk of infectious diseases begs the question: how many germs and viruses are sufficient to make you sick? Is it one germ? Is it 100, 1,000? What is the “infectious dose?”
For a pathogen to make us sick, it must overcome a lot. First it has to enter the body, bypassing natural barriers such as skin, mucus, cilia and stomach acid. Then it needs to reproduce; some bacteria and parasites can do this virtually anywhere in the body, while viruses and some other pathogens can only do so from within a cell. And all the while, it must parry attacks from the body’s immune system.
So while we are constantly inundated by microbes, the number of microbes that enter our bodies is usually too low to get past our defenses. (A tiny enough dose may even serve to remind our immune system of a pathogen’s existence, boosting our antibody response to keep us protected against it.)
When enough pathogens do manage to breach our defenses and start to replicate, we get sick. Often this is just a numbers game. The more invaders you’re fighting off, the more likely you are to feel ill.
The answer may surprise you. It depends on the attacker. For many infectious diseases it takes quite a few. Also, germs and viruses that cause infections are often fragile, not surviving long on surfaces we touch. Others are easy to spread. The norovirus that sometimes gets passed around on cruise ships requires only 18 viruses to infect someone. It’s easy to transfer from one to another. It can live on surfaces people touch and infect them days later.
You may remember hearing that some Covid strains are more contagious than others, as is common with many viruses.
Most animal models require a high dose of the virus — 10,000 to 1 million “plaque-forming units” (PFUs), where each unit is enough to infect a cell in tissue culture and kill it. Observational studies in humans, however, suggest that the infectious dose may be around 100 to 400 PFU on average, though again this method offers only a very rough guideline.
These studies suggest that one reason the virus is so easily transmissible is because it has a relatively low infectious dose, similar to other respiratory viruses such as RSV and “common cold” coronaviruses (and lower than the infectious dose of most strains of influenza virus).
How hard is it to come in contact with 100 to 400 PFU of Covid? Pretty easy if you’re around sick people:
A recent preprint shows that infected patients can exhale up to 800 viral RNA copies per minute for about eight days after their symptoms began. Even though we can’t directly translate RNA copies into the amount of live virus particles, if even half of those RNA copies are from a currently infectious virus, it’s theoretically possible to get a dose large enough to start an infection in just a minute of close contact.
Let’s say you’ve caught Covid before and maybe even have had a vaccine. You can still become infected with Covid but your host defenses kick into high gear.
Antibodies generated from vaccination or prior infection will bind to the invading microbe. These will interfere with its ability to attach to a host cell, or single out the microbe for ingestion by cells called neutrophils. And if a virus does manage to invade a host cell, it will be targeted for destruction by memory T cells.
The above paragraph suggests that if Covid does manage to evade your defenses it will require a much bigger dose to do so. In addition, your host defenses may fight off the invaders before you have time to feel ill. At the very least any infection you get will be mild and short lived. Vaccination doesn’t stop all infections, but it does reduce the viral load, viral shedding and reduce the symptoms.
When Covid took hold health authorities tried everything. Offices and many stores were closed to prevent people congregating. People were told to stand six feet apart to reduce the likelihood of breathing viruses shed by infected people. Those with Covid wore masks to reduce the viruses transmitted, while those around them wore masks to reduce the number inhaled. People hated masks but I suspect they helped reduce Covid (unless people like me used them as an excuse to continue with daily activities). For that matter I saw people driving alone in cars while wearing masks. That’s a little bit much, however.