A: About 50% (with all the usual caveats about averages and estimates).
Transmission of SARS-CoV-2 among household members is common, from both children and adults. Prompt isolation of cases is crucial, and all household members should wear masks in shared household spaces.
While we all worry a lot about exposure to COVID-19 at large events, at school or work, while traveling, or even at the grocery store, within-household exposure has gotten less attention. A recent study published in MMWR (linked below) sheds some new light on how common this route of transmission might be.
The study followed 191 household contacts of 101 “index patients” (the first member of a household to be diagnosed with COVID-19) for 14 days after the onset of illness of the index case. None of the household contacts reported symptoms at the time of the index patient’s diagnosis.
The household contacts were trained remotely to complete daily symptom diaries and to collect their own specimens (nasal and/or saliva) each day. The study took place in Nashville, Tennessee and Marshfield, Wisconsin.
And what happened to those 191 household contacts? 102 of them — 53% — tested positive for SARS-CoV-2 in at least one specimen during the follow-up period.
Was that true regardless of whether the index case was a child or an adult? Pretty much. Here are the “secondary infection rates” (the proportion of household members who were infected by the index case) by the age of the index case:
< 12 years old: 53%
12-17 years old: 38%
18-49 years old: 55%
50 and over: 62%
And what about by the age of the household contact? In other words, were children and adolescents as likely as adults to get infected by the index case? Again, basically yes:
< 12 years old: 57%
12-17 years old: 47%
18-49 years old: 59%
50 and over: 43%
Two other interesting findings from this study:
#1. Twice as many laboratory-confirmed secondary cases were symptomatic (68) vs. asymptomatic (34).
#2. Among secondary cases who reported symptoms, those symptoms began 4 days (on average) after the index patient became ill. 75% of symptomatic cases developed symptoms between 3 and 5 days after the index case’s symptom onset.
What does this mean for you and your family? If someone in your household has a positive test result, isolate that person within the household as soon as possible. Everyone (including the sick person) should #maskup inside, particularly in shared spaces. Other household members should quarantine, monitor themselves closely for symptoms, and seek testing as recommended by your health care provider.
Bottom line: risk of within-household transmission from children and to children is not noticeably lower than from or to adults.
We hope your family stays safe and healthy.
