November 10, 2020

If someone in your house has the coronavirus, will you catch it? Here's what the evidence says.

Daily Briefing

    More than 50% of Americans who lived with a person with Covid-19 became infected with the novel coronavirus within a week—indicating household transmission is both "common" and "quic[k]," according to a recent CDC Morbidity and Mortality Weekly Report (MMWR).

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    For the report, researchers examined the new coronavirus's transmission in households in Nashville, Tennessee, and Marshfield, Wisconsin, from April to September. Each household had an index patient—the person whom CDC defines as the first member of a household with Covid-19 symptoms to test positive for the coronavirus using a reverse transcription–polymerase chain reaction (RT-PCR) test—and at least one other person with no Covid-19 symptoms at the time of the index patient's illness.

    In total, the research, which is part of an ongoing CDC study on household coronavirus transmission, involved 101 index patients in 101 households who lived with 191 household members. Researchers remotely trained each household member on how to complete symptom diaries and collect their own specimens—either nasal swabs only or nasal swabs and saliva samples—for 14 days.

    Household coronavirus transmission 'is high, occurs quickly,' CDC finds

    The researchers found household transmission of the new coronavirus " is common and occurs early after illness onset."

    Of the 191 household members who lived with a person who had Covid-19, 53% became infected with the novel coronavirus within a week of the index patient's illness onset—and 75% of that subgroup became infected with the pathogen within five days of index patient's illness onset, the researchers found.

    According to the researchers, this 53% infection rate is higher than what has been documented so far. To date, research on household transmission of the virus has shown an infection rate of 20% to 40%, CNN reports.

    According to the researchers, the age of the index patient did not influence the coronavirus's transmission within a household. "Substantial transmission occurred whether the index patient was an adult or a child," the researchers wrote.

    The researchers also found many of the household members who contracted the coronavirus did not show symptoms when their infections were detected.

    "An important finding of this study is that fewer than one half of household members with confirmed [new coronavirus] infections reported symptoms at the time infection was first detected," the researchers wrote. "Many reported no symptoms throughout seven days of follow-up, underscoring the potential for transmission from asymptomatic secondary contacts and the importance of quarantine."

    In addition, the researchers found many index patients did not self-isolate to help curb the coronavirus's spread. According to the report, 69% of index patients reported spending more than four hours in the same room with at least one household member the day before illness onset and 40% of them reported spending more than four hours with at least one household member the day after illness onset.

    Meanwhile, 40% of index patients reported sleeping in the same room with at least one household member before illness onset and 30% of them reported sleeping in the same room with at least one household member after illness onset, the researchers found.

    The researchers concluded the new data suggests "that transmission of [the new coronavirus] within households is high, occurs quickly, and can originate from both children and adults."

    How to prevent household coronavirus transmission

    To curb household coronavirus transmission, researchers recommended people self-isolate—including by using a separate bedroom and bathroom, if feasible—and wear face masks.

    "Isolation should begin before seeking testing and before test results become available because delaying isolation until confirmation of infection could miss an opportunity to reduce transmission to others," the researchers wrote. "Concurrent to isolation, all members of the household should wear a mask when in shared spaces in the household."

    During an interview with CNN last month, Jenny Radesky, a spokesperson for the American Academy of Pediatrics, said, "Unfortunately you need to keep the whole household under isolation. The patterns of Covid-19 spread suggest that clusters of people who live close together are at highest risk of getting it from one another. If children are part of that household, they may show minimal symptoms but still be contagious" (Carbajal, Becker's Hospital Review, 11/3; Miller, NBC News, 10/30; LaMotte/Langmaid, CNN, 10/30; CDC's Morbidity and Mortality Weekly Report, 10/30).

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