People who get infected by coronavirus infect around half of their household members, with adults only slightly more likely than children to spread the virus, a US government study said on Friday.
A paper by the Centers of Disease Control and Prevention (CDC) is the latest to attempt to quantify the household transmission rate of the disease, with previous research varying widely but generally suggesting that adults are bigger drivers than children.
The new research by the CDC involved finding cases of “index” or initial patients with lab-confirmed coronavirus infection in Nashville, Tennessee, and Marshfield, Wisconsin, starting in April 2020.
Both the index patients and their household members were trained remotely to complete symptom diaries and obtain self-collected specimens, which were either nasal swabs only or nasal swabs and saliva samples, for 14 days.
The secondary infection rate when index patients were over 18 was 57 percent, which fell to 43 percent when the index patient was under 18. Overall there were far fewer children index patients than there were adults.
The person who first developed symptoms is the index patient.
Interpreting the findings, the authors of the paper wrote: “In this ongoing prospective study that includes systematic and daily follow-up, transmission of SARS-CoV-2 among household members was common, and secondary infection rates were higher than have been previously reported.”
“Substantial transmission occurred whether the index patient was an adult or a child,” they added.
Another important finding of the study was that fewer than half of household members with confirmed infections reported symptoms at the time infection was first detected, and many reported no symptoms throughout seven days of follow-up.
This underscores the potential for transmission for asymptomatic secondary contacts.
Other studies carried out abroad have at times found lower household infection rates.
The CDC said this might be because those studies didn’t have enough follow-up, or because those patients isolated in facilities outside their houses or applied more stringent mask use.