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As a physician and infectious disease epidemiologist, I've seen a lot of COVID-19 patients during the course of the pandemic, and there's a question I hear over and over:

How is it possible that my partner – or child or sibling or roommate – tested positive for COVID, and even though I slept in the same room or lived in the same house, I didn't come down with the virus?

Weren't they breathing out infectious particles for days on end? And I assume I was breathing them in.

There is an answer to this question. But it's a bit complicated.

First: Let's review how SARS-CoV-2, the virus that causes COVID-19, spreads. While some viruses are primarily passed through contact with the bodily fluids (Ebola) or skin (mpox) of someone who's infected, SARS-CoV-2 is easier to catch. It's spread mainly through the air in invisible aerosols (and to a lesser extent in large droplets) that the infected person emits while breathing, talking, sneezing, coughing, laughing or snoring. The aerosols can hang around in the air for hours, and others can inhale them.

So yes, if someone in your house is exhaling SARS-CoV-2-viral particles, you could breathe them in and become infected. But ... here's why that does not always happen.

There are two points to ponder. One: The person with COVID is not contagious at all times. Two: Different factors can reduce the risk of getting infected from a housemate.

The incubation period

For starters, someone who is sick with COVID-19 is not infectious from the moment they catch the virus to the moment they test negative (or their symptoms go away). Viruses like SARS CoV-2 have different stages.

It all starts when you're first exposed.

If you breathe in enough viral particles and your immune system doesn't vanquish the pathogens you've inhaled, the timer starts for your case of COVID. The virus will incubate in your body until symptoms begin to appear.

And when do you become contagious – marking the transition from the "latent" stage to the "infectious" stage? There's no way to know the exact moment this happens. It's a complex biological process, and for everyone it will be a little different.

But we can make certain inferences.

For example, many people wonder: Can you be contagious before symptoms of COVID appear?

Early on in the pandemic, the answer was definitely yes – and that's one reason why it was so hard to control COVID. We knew this from studies that showed transmission happening before the sick person had any symptoms. You would not necessarily know when you were exposed to someone who was contagious because they may have been "pre-symptomatic."

But things have changed since 2020. Now, most of our immune systems are able to recognize the virus – a result of previous exposures and/or being vaccinated. And the immune system's reaction to even a small amount of virus could be symptoms like coughing, sore throat, running nose, a fever. With our immune systems primed, the body's response comes much more quickly than it would have back in 2020 when SARS-CoV-2 was a novel pathogen. So those symptoms could appear early on after exposure — even before we're infectious. This was shown in a study published in Clinical Infectious Disease that examined people's symptoms compared to how much virus they were carrying across the days of their infection.

The infectious window

The infectious window – the period during which you're contagious — varies from person to person.

For some, it may only be a couple of days, whereas for others it can be a week or even longer (especially in people who are immunocompromised and can't easily clear the virus).

If you're lucky, your housemate will have a short window.

And then there's the matter of quantity.

Many if not most people will transmit only small amounts of SARS-CoV-2 after getting infected.

In fact, most of the spread of SARS-CoV-2 actually happens from a relatively small number of highly infectious people — called superspreaders. Research on transmission has shown a wide variation in how many people get infected by one person.

Some superspreaders are just biologically capable of shedding a lot of virus. One study published in November 2023 found an association between higher BMI in men and higher viral loads. Other superspreaders could have a big network of people they come into contact with – for example, infecting on a bus or in a choir setting.

You can reduce the amount of pathogen you're exposed to

Just breathing in a pathogen doesn't mean you'll get sick. This is the difference between exposure versus actual infection.

For a pathogen to cause disease, you must be exposed to enough of it – the minimum infectious dose — so it can overcome your immune defenses. Some pathogens can do their work with a tiny infectious dose, meaning even just a few microbes or viral particles are sufficient to infect you, while others require a much heftier exposure. That's why you're unlikely to catch COVID from, say, dashing into a grocery store for a quick shop and perhaps breathing in a very small amount of virus. The duration of exposure and the concentration of the pathogen in its preferred route (air, for this virus) affect your chance of getting sick.

We are exposed to small amounts of pathogens all the time, but usually it is not enough to cause disease. Sometimes, however, it is. A more sinister example of these principles is Coxiella burnetti, the bacteria that causes "Q Fever." It can spread to people from animals, including farm animals, and is on the list of "select agents" for bioterrorism concern. The bacteria has a very low infectious dose, is stable in the environment and can spread so effectively that even living miles downwind of a farm is a risk factor.

Reducing the dose you are exposed to is one way to avoid infection. For SARS-CoV-2, opening windows can dilute the cloud of aerosols by bringing in fresh air.

If your home has a fan with an air filter or you're using a HEPA air purifier, the infectious aerosol particles can become trapped in the filter rather than in your lungs.

To reiterate: the cumulative dose you inhale depends on the time you spend in a place and the concentration of viral particles there.

Masks that are worn correctly and consistently will also reduce that dose. If your housemate is infected and you wear an effective mask (a well-fitting N95, for example), the mask will trap most of the viral particles, reducing the amount you inhale. If the person with COVID masks up, that will decrease the quantity of pathogens they're emitting into the air as well.

Your immune system (or medications) could save you

If you've been vaccinated or had a prior infection, your immune system could be able to knock out the pathogen. The older you get, the less effective your immune system will be. And if you have a medical condition that makes your immune system weaker – like cancer or chronic diseases like diabetes – that could also play a role in whether exposure to viral particles will lead to infection.

For healthy people exposed to a sick person, antiviral drugs (or antibiotics for bacteria) can often avert infection after exposure — this is called post-exposure prophylaxis and is used for many infections already, including HIV, gonorrhea, chlamydia and syphilis.

In one study, researchers looked at whether those living at home with someone infected with COVID-19 were less likely to get sick if they used the antiviral paxlovid. The study found a signal — a 32% reduced risk of getting sick compared to placebo — but did not meet statistical significance.

So in conclusion: Yes, you can live with someone with COVID and not catch it. Because infectious disease transmission is complicated.

And remember, we do see weird things happening with infectious diseases every single day. So yes, in theory you could catch, say, mpox from dust particles on a patient's blanket. This actually happened in a famous case from the U.K., but it is exceedingly rare.

But don't fall for the trap of thinking that your single example represents the general trend.

When it comes to COVID, let me assure you: It is contagious – but remember, that doesn't mean everyone who is in contact with someone who's infected will catch it.

I was infected a few weeks ago. My mother and sister were both in the car with me hours before I felt sick and tested positive. My sister got COVID days later. But my mother never got it.

Abraar Karan is an infectious disease physician and researcher at Stanford University. He worked on the COVID-19 pandemic for the Massachusetts Department of Public Health and the mpox outbreak for the Los Angeles County Department of Public Health.

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