At this point in the omicron-driven COVID-19 surge, you’ve probably heard some version of this story at least once: One person in a friend or family member’s household tested positive for the virus, but no one else did. Or maybe it was in your house, when your roommates or spouse or kids all tested positive, but you kept getting negative tests.
If the virus is so transmissible, how can people sharing bathrooms or beds have such different outcomes?
“The research has indicated from the beginning that when COVID is introduced into a household, high rates of transmission occur, but I didn’t say 100%,” said Dr. William Schaffner, a professor in the division of infectious diseases at Vanderbilt University. “Some people manage to escape even though they’re in close quarters with others. For some reason or another, the infectious agent doesn’t always infect everyone in close proximity.”
The same thing used to happen before COVID, too — a flu, cold or stomach bug would tear through a family, but some people escape.
In most cases, uninfected people in the household are still getting exposed to some extent. But everything from your age to the date of your last shot will affect whether the infection actually sticks. The level of antibodies circulating in an individual’s blood is a key factor, said Dr. Peter Chin Hong, an infectious disease expert at UCSF.
Antibodies are proteins made by your immune system that stop a virus from binding to your cells, or tag virus particles and infected cells for other parts of your immune system to destroy. They develop in both people who have been infected with COVID and in response to the vaccine. Your body begins to make lots of antibodies immediately after a shot or an infection, but ramps down production over time.
If you’re unvaccinated and have never been exposed to the virus that causes COVID-19, your body doesn’t know how to produce the right antibodies yet.
What if you were unvaccinated and have never had COVID, but still didn’t catch the virus going through your home? You may “have recently had COVID asymptomatically, developed antibodies and not know it,” Chin-Hong noted.
Immunocompromised people may produce fewer antibodies after being infected or vaccinated, making them more likely to catch COVID.
Another factor at play is how long, and how closely, you’ve been exposed to infected people in your home. If you can completely isolate from people who are sick, including using a separate bathroom, it reduces the chances of spreading the virus. But keep in mind that most people are already highly contagious before they develop overt symptoms.
The biggest wild card, though, may be human biology, including your gender and genes. “There’s so much we don’t know about transmission of disease,” Schaffner said. “This is an area where genetic diversity may be playing into the likelihood in getting infected … Some people, just given their constitution, are more likely to get it than others, but we don’t know what those differences are.”