The iconic article “Cuddlers, Touchers, and Sitters” (formerly entitled “Modes of Transmission of Respiratory Syncytial Virus”) by Hall and Douglas1 stands to this day as one of the simplest recitations of the behavior of respiratory viruses and children. With an elegant and simple study design, Hall and Douglas vividly demonstrated that it’s the interactions between susceptible and infected persons that drive much of viral transmission. For those who do not recall this study, Hall and Douglas examined the likelihood of transmission from an infant infected with respiratory syncytial virus (RSV) to an adult caretaker who either sat with the infant on their lap, touched the infant while they laid in their crib, or sat next to the crib. We can now predict the outcomes—cuddlers were the most likely to get infected. While the classic diagram of the “chains of transmission” helps us break down some the factors that determine onward transmission, many respiratory viruses rely on time, proximity, and contact to spread.
With this lens, we read the Paul et al impressive study2 of the transmission of SARS-CoV-2 virus in households. Using public health data from the province of Ontario, Canada, investigators identified clusters in which the apparent primary case was a child. Some of the findings are not surprising. The age distribution of most secondary cases indicated that they were likely either siblings (0-20 years of age) or parents (30-50 years of age). Additionally, older children and teens were more likely than toddlers and young children to be the primary household case, consistent with a higher degree of social mobility. Applying thoughtful stratified and sensitivity analyses, these investigators also developed a more nuanced understanding of how, and potentially why, SARS-CoV-2 spreads more in some households than others.
First, we learned that while young children (those younger than 4 years) were less likely to be a child primary case, they were more likely to be a source case for other members of their households. This finding is at odds with early reports suggesting that young children rarely transmit to others.3 To understand this apparent contradictory observation, we must remember what happened to infants and toddlers during the early days of the pandemic: they were sequestered, staying at home alongside parents who were working and siblings who were learning remotely. For these households, there were few opportunities for virus to enter the family circle. Limited testing capacity, with a focus on testing elderly individuals, health care workers, and those ill enough to require hospitalization, also contributed to the impression that this new virus spared young children. Under these circumstances, we were understandably surprised when we diagnosed a young child with COVID.
Times changed and the Paul et al study was conducted as Ontario was getting back to business and Ontario’s children were getting back to school and daycare. Under these more realistic conditions, we see that not only can infants and toddlers get infected but they can also spread SARS-CoV-2. In fact, the assumption that infected children were at risk of spreading SARS-CoV-2 was at the core of safety plans in schools and daycares that emphasized the importance of masking and distancing of students throughout the school day.4 But these practices were inconsistent with the available observations that children were both less likely to have severe disease (true) and possibly less likely to spread it to others (not true). These safety plans were focused on reducing transmission between infected school staff and students, as well as protecting everyone’s family members. The Paul et al study now reveals that these measures have served an important purpose as the nation awaited the safety afforded by vaccines.
Still, there are some surprises in these data. The investigators’ finding that the youngest children are the most likely to spread the virus in household demands some consideration. To date, we do not have any evidence that the viral titer shed by young children is greater than that shed by teens and adults; in fact, most studies suggest that, in childhood, viral shedding may increase with increasing age.5 Additionally, prior work that has described that young children are more likely to have asymptomatic infections than older individuals6 and that asymptomatically infected individuals are less likely to transmit than individuals who have symptomatic infection.7 While ascertainment bias might be at play in this study (ie, symptomatic individuals more likely to be tested), this finding persisted when the analysis was limited to households where the primary case was symptomatic.
To understand why the youngest children may be more likely to transmit COVID to others once infected, we believe we need to consider the simple but elegant findings of Hall and Douglas—behavior matters! Infants and young children demand attention when sick. The youngest toddlers are unreliable maskers and do not always understand the messaging of a 6-ft distancing rule—nor should they. Cuddling and touching are part and parcel of taking care of a sick young child and that will obviously come with an increased risk of transmission to parents as well as to older siblings who may be helping to care for their sick brother or sister. Whether it is through large respiratory droplets being expelled when a young child does not “cover their cough” or the direct inoculation of mucus membranes that can happen when copious, infectious secretions end up on the hands of family members who overlook the need to clean their hands, these transmission pathways may provide frequent and easy opportunities for viral transmission.
The real challenge of these data may be what to advise families with young children with COVID. It’s hard to imagine a household wearing masks and continually cleaning their hands when there is a sick young child at home. Parents will always hold their young, sick children to provide comfort. Siblings will continue to use antics and toys to distract a crying younger brother or sister. The obvious solution to protect a household with a sick young infant or toddler is to make sure that all eligible members of the household are vaccinated. Cloaked with the protection of vaccine, household members need not fear the youngest family members with a runny nose come fall; the solution—vaccination—is now within their grasp.
Corresponding Author: Susan E. Coffin, MD, MPH, Children’s Hospital of Philadelphia, 2716 South St, Roberts Pediatric Research Building, Room 10-363, Philadelphia, PA 19146 (email@example.com).
Published Online: August 16, 2021. doi:10.1001/jamapediatrics.2021.2767
Conflict of Interest Disclosures: Dr Coffin reported grants as a Centers for Diease Control and Prevention coinvestigator at a Vaccine and Treatment Evaluation Unit site conducting COVID vaccine trials in children outside this work. No other disclosures were reported.
E; UNCOVER. The role of children in transmission of SARS-CoV-2: a rapid review. J Glob Health
. 2020;10(1):011101. doi:10.7189/jogh.10.011101
et al; CDC COVID-19 Response Team. Summary of guidance for public health strategies to address high levels of community transmission of SARS-CoV-2 and related deaths, December 2020. MMWR Morb Mortal Wkly Rep
. 2020;69(49):1860-1867. doi:10.15585/mmwr.mm6949e2