A team of researchers has suggested that 70.4% of nearly 850 000 US household Covid-19 transmissions originated with a child, with younger kids most likely to spread the virus, they said.
The researchers, led by Boston Children’s Hospital, gave smartphone-connected thermometers to 848 591 households with 1 391 095 members, who took 23 153 925 temperature readings from October 2019 to October 2022. Fevers were a proxy for infection.
Of all readings, 57.7% were from adults. Most households (62.3%) reported temperatures from only one person, while 37.7% included multiple participants taking 51.6% of all readings, reports CIDRAP. Most children were eight or younger (58.0%), and more females than males participated in each age group.
Younger kids more likely to spread virus
A total of 15.8% of readings met the criteria for fever, making up 779 092 fever episodes, they reported in their study, published in JAMA Network Open.
The number of fever episodes predicted new Covid-19 cases, which the researchers said lends validity to using fever as a proxy for infection. Of these cases, 15.4% were considered household transmissions, the percentage of which rose from 10.1% in March to July 2021 to 17.5% in the Omicron BA.1/BA.2 variant wave.
Among 166 170 households with both adult and child participants (51.9% of households with multiple participants), there were 516 159 participants, 51.4% of whom were children. In these households, 38 787 transmissions occurred, 40.8% of which were child to child, 29.6% child to adult, 20.3% adult to child, and 9.3% adult to adult. The median serial interval between the index and secondary cases was two days.
Of all household transmissions, 70.4% began with a child, with the proportion fluctuating weekly between 36.9% and 87.5%. Paediatric transmissions reached a high of 68.4% the week of 27 September 2020, and fell to a low of 41.7% the week of 27 December 2020 (0.61 times less frequent). The next high was 82.0% the week of 23 May 2021, which stayed stable until 27 June (81.4%) and then declined to 62.5% by 8 August (0.77 times less frequent).
The percentage of household transmissions beginning with a child then rose to 78.4% by 19 September, hovering there until 14 November (80.3%) and then dropping to 54.5% the week of 2 January 2022 (0.68 times less frequent).
By 6 March, the proportion rose to 83.8%, fell to 62.8% the week ending 24 July (0.75 times less frequent), and then climbed to 84.6% the week of 9 October.
Children aged eight and younger were more likely to be the source of transmission than those aged nine to 17 (7.6% vs 5.8%). During most of the pandemic, the proportion of transmission from children was negatively correlated with new community Covid-19 cases.
In-person school contributed to transmission
“More than 70% of transmissions in households with adults and children were from a paediatric index case, but this percentage fluctuated weekly,” the study authors wrote.
“Once US schools reopened in autumn 2020, children contributed more to inferred within-household transmission when they were in school, and less during summer and winter breaks, a pattern consistent for two consecutive school years.”
The researchers said the finding that paediatric Covid-19 transmission was negatively correlated with new community cases during most of the pandemic is consistent with that of a previous study.
“When the incidence of Covid-19 increases, adults in the community are at higher risk of infection; this may increase the likelihood that adults become the index case in a household transmission and explain the negative correlation we observed,” they wrote.
“Also, when the Covid-19 incidence is low, overall use of non-pharmaceutical interventions might decrease, leading to increased incidence of non–SARS-CoV-2 pathogens, which may be more common in children.”
The authors concluded that children had an important role in the spread of SARS-CoV-2 and that in-person school also resulted in substantial spread.
Study details
Smart thermometer–based participatory surveillance to discern the role of children in household viral transmission during the covid-19 pandemic
Yi-Ju Tseng, Karen L. Olson, Danielle Bloch, et al.
Published in JAMA Network Open on 1 June 2023
Key Points
Question What role did children play in household viral transmission during the Covid-19 pandemic, when enveloped virus rates were low and relative proportions of Covid-19 were at a high?
Findings In a cohort study of 166 170 households with adults and children using smart thermometers, among 38 787 inferred household transmissions over three years, 70.4% had a paediatric index case. Rates dropped during school breaks.
Meaning These results suggest that children were important viral vectors in households during the pandemic, particularly when school was in session.
Abstract
Importance
Children’s role in spreading virus during the pandemic is yet to be elucidated, and measuring household transmission traditionally requires contact tracing.
Objective
To discern children’s role in household viral transmission during the pandemic when enveloped viruses were at historic lows and the predominance of viral illnesses were attributed to Covid-19.
Design, Setting, and Participants
This cohort study of a voluntary US cohort tracked data from participatory surveillance using commercially available thermometers with a companion smartphone app from October 2019 to October 2022. Eligible participants were individuals with temperature measurements in households with multiple members between October 2019 and October 2022 who opted into data sharing.
Main Outcomes and Measures
Proportion of household transmissions with a paediatric index case and changes in transmissions during school breaks were assessed using app and thermometer data.
Results
A total of 862 577 individuals from 320 073 households with multiple participants (462 000 female [53.6%] and 463 368 adults [53.7%]) were included. The number of febrile episodes forecast new COVID-19 cases. Within-household transmission was inferred in 54 506 (15.4%) febrile episodes and increased from the fourth pandemic period, March to July 2021 (3263 of 32 294 [10.1%]) to the Omicron BA.1/BA.2 wave (16 516 of 94 316 [17.5%]; P < .001). Among 38 787 transmissions in 166 170 households with adults and children, a median (IQR) 70.4% (61.4%-77.6%) had a paediatric index case; proportions fluctuated weekly from 36.9% to 84.6%. A paediatric index case was 0.6 to 0.8 times less frequent during typical school breaks. The winter break decrease was from 68.4% (95% CI, 57.1%-77.8%) to 41.7% (95% CI, 34.3%-49.5%) at the end of 2020 (P < .001). At the beginning of 2022, it dropped from 80.3% (95% CI, 75.1%-84.6%) to 54.5% (95% CI, 51.3%-57.7%) (P < .001). During summer breaks, rates dropped from 81.4% (95% CI, 74.0%-87.1%) to 62.5% (95% CI, 56.3%-68.3%) by August 2021 (P = .02) and from 83.8% (95% CI, 79.2%-87.5) to 62.8% (95% CI, 57.1%-68.1%) by July 2022 (P < .001). These patterns persisted over two school years.
Conclusions and Relevance In this cohort study using participatory surveillance to measure within-household transmission at a national scale, we discerned an important role for children in the spread of viral infection within households during the Covid-19 pandemic, heightened when schools were in session, supporting a role for school attendance in Covid-19 spread.
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