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Children are an important source of COVID spread within households, a Canadian study finds. They are less likely than adults in the home to transmit the infection, however.
In a case-ascertained antibody surveillance study, the researchers found that the transmission rate within households was 50%.
“We found children did spread COVID, and to more than a third of household contacts,” lead author Maala Bhatt, MD, MSc, associate professor of pediatrics at the University of Ottawa, Ontario, Canada told Medscape Medical News. “Adults were more likely to transmit to other members of the household than children, but still, children were a significant source of spread within the household.”
The study was published online April 12 in CMAJ Open.
Isolation Uncommon
Researchers included 695 unvaccinated participants from 180 households in Ottawa who were evaluated between September 2020 and March 2021. “We started enrollment before vaccines were available at all,” said Bhatt, who is also director of emergency research at the Children’s Hospital of Eastern Ontario. In Canada, she said, healthcare workers got access to vaccines in January 2021, but others, including older adults, did not get access until summer.
Participants were identified if they tested positive for SARS-CoV-2 at an academic emergency department or at affiliate testing centers. People who heard about the research through the media could also self-identify to enroll. Each household had to have at least two participants, and at least one household member had to be under age 18 years.
Researchers focused on the secondary attack rate, which they defined as the proportion of household contacts who tested positive for SARS-CoV-2 antibodies once the index patient had been identified and enrolled.
Of the 180 index participants, 74 were children and 106 were adults. There were 515 household contacts (266 children, 249 adults). Of these contacts, 487 had antibody testing, and 239 (49.1%) tested positive. Of this latter group, 36.8% were asymptomatic, with rates similar for adults and children.
Adults were more likely than were children to transmit the virus (odds ratio [OR], 2.2; 95% CI, 1.3 – 3.6). Researchers could not determine the odds of transmission from asymptomatic cases compared with symptomatic cases.
The researchers found that the youngest children (age 3 years or younger) were least likely to spread the infection.
In 31.5% of the households, all household members were positive for the SARS-CoV-2 antibody. In 14.9% of households, no secondary transmission occurred.
The study pointed out not just the value of isolating and mask wearing, but also how difficult it can be within a household. The researchers found that just 8.7% of household contacts, or 45 people, were completely isolated from the index patients. Just 8.5%, or 44 people, reported they always wore a mask when indoors with the index patient.
Previous Findings Varied
Other research has produced various findings about household spread, the researchers said.
A meta-analysis that included 87 studies published between March 2020 and June 2021 from 30 countries found an overall secondary attack rate of 19%. These studies, however, had diverse methodologies related to factors such as the frequency of testing and type of tests used, said the current researchers.
Another study looked at household contacts of 92 child index cases in England and found that children 10 years or younger transmitted to 25% of contacts, while those between 11-18 years transmitted to 30% of contacts. While the results are similar to those of the new study, the spread may not all have occurred within the home, as community prevalence was high during the study, the current researchers said.
A third study examined a large provincial health administrative data set with swab results and found that transmission occurred in 27% of households. In that research, younger children were more likely to spread the infection than those aged 14-17 years.
The current study, even though it was conducted before widespread vaccine availability, has important messages for public policy, Bhatt said, including the importance of masking in indoor places. And with the variants now circulating, she believes the findings are still relevant. Addressing the role of children, who often are asymptomatic or have very mild symptoms, needs research attention, the researchers write.
Need for Lowering Transmission
“This provides another interesting piece of the puzzle of how household transmission works,” said Laura Sauvé, MD, MPH, chair of the Infectious Diseases and Immunization Committee for the Canadian Paediatric Society, who commented on the study findings for Medscape. Sauvé, who was not part of the research team, is also an investigator at the British Columbia Children’s Hospital and clinical assistant professor of infectious diseases at the University of British Columbia.
The study, she said, shows the importance of asymptomatic spread within households and reinforces the belief that household transmission is common, regardless of what actions are taken.
“One concern I had was despite the fact that their findings were that children and adults spread COVID-19 about the same amount and that hospitalized participants (which are nearly always adults) were more likely to spread COVID-19 to household members, the authors seemed to conclude that children were the most important culprits in household spread,” said Sauvé. She expressed doubt about this interpretation, which could be advanced to support increased restrictions that have significant developmental, educational, and mental health implications for children.
Bhatt welcomed the chance to clear up what she believes has been a misunderstanding of the study results. “Kids are not the most important culprits,” she said. “We do specifically say that adults spread more [than younger household members].” And, she emphasized, ”This is not to imply that kids should not be in school.” Rather, she said, ”we should be using the tools we have to lower transmission.”
This study was funded by the Children’s Hospital Academic Medical Organization, the PSI Foundation (COV-6), and the Ontario COVID-19 Rapid Research Fund. Bhatt and Sauvé have disclosed no relevant financial relationships.
CMAJ Open. Published online April 12, 2022. Full text
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