Living with someone who has COVID-19? Your odds of catching it are only 12%, Utah study finds.

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After his oldest daughter tested positive for the coronavirus, Cameron Robinson figured it probably wouldn’t take long before the disease had pinballed around his Utah County home, hitting him and his wife and his four other kids, too.

His 16-year-old had caught it at work and tried to quarantine in her room. But, sure enough, two days later, Robinson’s 7-year-old son complained of a fever. The boy vomited in the car on the way to the testing center where his results also came back positive.

What’s odd, though, is that’s where it stopped.

Despite hugging his son and wrestling with him at bedtime each of the nights before, Robinson never got sick. And no one else in the house did either.

“We just assumed everyone at home would get it,” Robinson said. “That’s what the county health department had told us would happen. So we were surprised when it didn’t, when it was just two of our kids.”

It turns out, according to new research from the University of Utah, that’s as strange as scientists previously thought. Even with COVID-19 being highly contagious, sharing a household doesn’t automatically mean sharing the disease, those at the U. found.

Damon Toth, an epidemiology professor there, worked on a study with colleagues this summer where they randomly tested the members of households across the state for coronavirus antibodies. Those tests can tell researchers if someone has previously had the disease by checking if their body created the cells needed to fight it.

The study was originally designed to determine how prevalent the virus has been in the state, Toth said. Depending on whether people get tested or not, the Utah Department of Health’s data might not be able to capture the real rate of disease. Random sampling can fill in the blanks.

And with it, the U. scientists concluded that about 1% of the population here has had COVID-19. (That’s about two times what has been detected by the state, which the researchers expected to find.)

But their data ended up providing more insight into the spread than they had planned.

Because the tests were done on an entire household instead of randomly by a single person, Toth said, the researchers found they had a way of looking at whether people living together were also getting sick together. When one person in a house had been sick, they discovered, there was only a 12% likelihood that another person they were living with would also have gotten infected.

Put another way, that means 88% of the time, a housemate did not get the virus after someone they were living with did.

“Everyone kind of does a double take with that number,” Toth said. “You would think if someone in your house, who you live with and eat with and talk with, had the virus, then it’s more likely than that you’d get it. … But it doesn’t necessarily mean you’re a goner.”

Overall, researchers tested 8,000 people across four counties in the state — Summit, Salt Lake, Davis and Utah — who were 12 years and older.

With only Robinson’s son falling ill after his daughter tested positive, that means roughly 16% of the others in the household contracted the virus from the original case. It tracks with the U.’s data.

While that’s good, Toth warns the study should be viewed with caution. Certainly, the 12% spread is surprisingly lower than the researchers anticipated for those in close quarters with a virus that transmits through the air. It’s still higher, though, than the risk to the general population: the 1% they found that had caught it overall.

That means a person is at higher risk for getting the disease if someone in their house has it than they are regularly, day to day. “It’s still pretty risky,” Toth said. “That’s nothing to sneeze at.”

Additionally, how contagious the virus is depends on the person carrying it. Researchers call people who carry a higher viral load “superspreaders.” It’s not clear what makes those individuals — about 1 in 5 who get COVID-19 — more infectious, but they spread the disease much further than the average sick person.

If a household had one of those individuals in it, Toth noted, the other members will surely catch the virus. The researchers found that happened in one Utah home, for instance, with six people living in it. One person had COVID-19 and spread it to all five others, Toth said.

The superspreaders are also more likely to get people sick at large venues, such as a workplace or social event. It’s part of why medical experts have urged such tight restrictions on gatherings and face masks. Community transmission is a serious concern with just one viral individual.

“It’s like a series of dice rolls with COVID-19,” Toth said. “Some people who get it can be very highly contagious. And then other people not really at all. That variability, it seems like for this virus, is very extreme.”

There’s still little known about when a superspreader is most contagious and for how long — but, in the case of families, it’s likely to occur at least sometime while they’re in the household because that’s usually the place where people spend most of their time.

Robinson’s family, for instance, got fairly lucky. Both kids had mild symptoms and have recovered well. For the Budworths in Sandy, though, it didn’t go so well.

The virus quickly spread through Mac and Shannon Budworth’s home in May. Both parents got it, and then it passed on to their four kids, ages 5, 7, 11 and 14.

“It spread through our family like wildfire,” said Mac Budworth.

They had tried to be careful but got sick after going to a family picnic before Memorial Day. The first few weeks were awful. Shannon felt like she was standing in front of an oven trying to catch her breath. Then, she had shooting pain in her arms and legs. Mac felt like his eyes were going to pop out of his head. He also alternated between chills and hot flashes.

It was lighter for their kids, fortunately, who mostly experienced headaches. But it’s continued now, even weeks later, with symptoms for the parents. They’ve both had shortness of breath, fatigue and heart palpitations. Mac’s liver has been inflamed. And Shannon has had seizures.

“All of the long-haul stuff that you hear about, we’ve experienced a lot of that,” Mac said.

So he was surprised by the finding that just 12% of household members in the U.’s study were infected when his family saw 100%. “It’s just not what we experienced at all,” he added.

The Budworths are a likely case, according to Toth’s work, where a superspreader could have been involved.

The research from the University of Utah mirrors other studies on the subject, including one out of China where the virus originated. That found a 19% transmission rate in households. Some have found as low as 4% in a home and others as high as 45%. None, though, go above 50%, Toth said.

But the U. study also has some limitations. For instance, researchers didn’t actually do antibody tests for children younger than 12. That was partially to avoid doing a blood draw on young children, Toth said. It was also because kids are generally less likely to get seriously sick from the virus (though they can still spread it).

It didn’t take into account, either, the type of household and if that could contribute to spread. For instance, what if it’s a multigenerational family living in one home, like many who have been affected in Salt Lake City’s Rose Park and Glendale neighborhoods? Or what if there are particularly tight quarters or bad ventilation?

Toth said the researchers have the data available to compare the square footage of the houses and that might begin to answer some questions about density.

The group also plans to break the spread down by age as much as possible. Maybe, Toth hypothesized, there’s a correlation between age and how contagious someone is or how superspreaders are concentrated in a certain area of Utah.

“Hopefully,” Toth said, “we can find some more useful or interesting things about this disease.”

Some of that, he hopes, will help families know more of what to expect when someone in their house tests positive.