What Do We Know About Children and Coronavirus Transmission?

Key Points:

• With just a few weeks remaining before schools in the U.S. are scheduled to reopen, and the federal government encouraging in-person schooling, there remain many questions about the risk COVID-19 poses to children and their role in transmission of the disease. Indeed, other countries have not reopened schools with the levels of community transmission found in the U.S., coupled with its insufficient testing and limited contact tracing.
• Our review of the latest available data indicates that, while children who are infected with COVID-19 are more likely to be asymptomatic and less likely to experience severe disease (though a small subset become quite sick), they are capable of transmitting to both children and adults.
• What remains unclear and where evidence is still needed is: whether children are less likely to be infected than adults and, when infected, the frequency and extent of their transmission to others. There is some evidence for an age gradient in infectiousness, with younger children less likely and older children more likely to transmit at levels similar to adults.
• While other countries that reopened schools have generally not experienced outbreaks in school settings, almost all had significantly lower levels of community transmission than the U.S. and greater testing and contact tracing capacity. Moreover, several disease clusters connected to schools and children have been reported.
• Taken together, the evidence indicates that where there is already widespread community transmission, as in many areas in the U.S., there is clearly a risk of further spread associated with reopening schools. The risks of reopening need to be considered carefully in light of the recognized benefits of in-person education.

Introduction

Policymakers in the United States are struggling to decide whether and how to reopen schools and daycares, at the same time that parents and caregivers are trying to weigh the risks and benefits of different approaches to schooling for their children. Indeed, our latest national poll found that most parents are worried about their child or a family member, as well as teachers and staff, getting sick from coronavirus if schools reopen and think it is better to wait.

With only a few weeks remaining until most U.S. schools are scheduled to begin, the White House for months has emphasized the importance of reopening schools and having students physically present in classrooms, although more recently has supported a more flexible approach for schools and parents. For its part, the Centers for Disease Control and Prevention (CDC) recently released updated guidance, including a review of the data, for school administrators, parents, and caregivers facing decisions around schooling, while also issuing a statement that having children physically present in reopened schools this fall was of utmost importance. CDC states, that, “No studies are conclusive, but the available evidence provides reason to believe that in-person schooling is in the best interest of students, particularly in the context of appropriate mitigation measures similar to those implemented at essential workplaces.”

Still, many large school districts, especially in locations where there is widespread community transmission, have opted for a virtual rather than an in-person start to the school year, citing the risks that the virus poses to students, teachers, staff, and households. Our recent analysis found, for example, that 1.5 million teachers are at high risk of severe disease if infected with coronavirus, due to underlying health conditions and age. In addition, we found that millions of seniors live in households with school-aged children.

Struggles surrounding decisions around schooling and childcare reflect some of the most perplexing aspects of the COVID-19 pandemic so far: understanding the risks the virus poses to children and their role in transmission of the disease. While researchers have been actively investigating these topics since the emergence of the pandemic, our understanding is, in many cases, still incomplete. Here, drawing on published literature and expert opinion, we summarize what is known about children and coronavirus, and what the information gaps remain.

Current Evidence on COVID-19 and Children

Disease severity is significantly less in children, though a small subset become quite sick: We know children of all ages can indeed be infected with coronavirus but the evidence at this point is quite clear that overall, children who become infected experience a milder disease course than adults. As one expert review panel stated, we are “essentially certain” the risk of death and of severe illness from COVID-19 in children is extremely low. Another expert panel organized by the National Academies of Medicine reported that “compared with adults, children who contract COVID-19 are more likely to experience asymptomatic infection or mild upper respiratory symptoms”, and that over 90 percent of children testing positive will have no or mild symptoms. While the infection has been known to cause an inflammatory condition (MIS-C) in some children, and these cases are serious (342 have been reported to date), so far such cases are very rare and most children who do experience the condition eventually recover.

One reflection of the generally milder disease course in children is that the reported number of COVID-19 deaths among children under the age 18 in the U.S. is less than 1% of reported COVID-19 deaths, even though children make up 22% of the U.S. population [see Box 1].

Evidence is mixed about whether children are less likely to become infected when exposed. Regarding risk of infection there is some evidence that, compared to adults, children are less likely to become infected when exposed to the virus. One prominent study estimated susceptibility to infection for those under 20 years of age to be about half that of those over 20 years, and several large-scale seroprevalence studies have found lower prevalence of infection in children, especially younger children, compared to adults. Still, the evidence is somewhat mixed on this point, with other studies showing children, especially older children (>10 years) having been infected at rates similar to adults in some places. A recent (pre-publication) review of the evidence concludes there is “significantly lower” susceptibility to infection for children under 10 compared to adults, but the same conclusion cannot be drawn for children 10 and older.

A number of biological explanations have been posited for this difference between children and adults in severity and infection risk, including less expression of a key receptor in the upper airways of children, and some level of pre-existing immune cross-reactivity to SARS-Cov-2 in many children due to recent exposure to related viruses (such as common cold viruses). Still, investigations continue and more evidence is needed to fully understand if lower infection rate in children is a real phenomenon and what explains it.

Children do transmit to others but more evidence is needed on the frequency and extent of that transmission. We do not yet know for sure how common transmission from children is compared to transmission from adults. We know that children are capable of transmission to others, but the frequency and extent of this transmission remains under investigation, and this is a question where only weak, and sometimes contradictory evidence, is available.

For example, multiple studies indicate that children have viral loads and shed virus in similar amounts to adults, which might indicate risk of transmission is similar across age groups. Without a doubt transmission from child-to-child and child-to-adult can occur, but a number of studies find children, particularly young children, are less likely to be source of infection in households and other settings, compared to adults. A number of these studies conclude that a majority of documented transmissions between children and adults have occurred from adults to children, rather than the other way around, but more detailed information is needed to fully understand this dynamic. There is new evidence, including from a recent comprehensive study from South Korea, suggesting there is an age gradient in terms of transmission risk from children, meaning younger children (<10 years old) are less likely to transmit compared to adults, while older children (10 and older) may transmit at levels similar to adults.

Evidence is so far lacking partly because doing studies and collecting the necessary information has been challenging for a number of reasons, including:

• Children have been less likely to be tested for coronavirus infection compared to adults. Testing (particularly in the US where testing capacity constraints have been common) has been focused on symptomatic patients (especially those with severe symptoms), and children exhibit fewer and milder symptoms compared to adults.
• Because children are less likely to be symptomatic, they may also be less likely to be identified as the “index” case during a contact tracing investigation and therefore their role in transmission could be underrecognized.
• Children may have different social mixing patterns compared to adults, coming into close contact with others at different rates. This may complicate comparisons of transmission between children and adults. On the one hand, children may have had fewer contacts than adults during times when schools and daycares were closed, but, on the other, may have a greater number of close contacts than adults when in a more typical school environment.

Transmission in School Settings

Most countries that have reopened schools have not experienced outbreaks but almost all have had significantly lower rates of community transmission than the U.S. and higher testing and contact tracing capacity. In many countries around the world, schools and daycares have reopened following a period of lockdown. So far, most have not seen cases surge after re-opening. For the most part, the lack of a surge in cases and the ability to control any outbreaks in most countries that have re-opened schools is in large part explained by their significantly lower levels of community transmission at the time schools re-opened compared to much of the U.S. now, as well as their greater testing and contact tracing capacity.

In Table 1, we compare U.S. COVID-19 data to 13 other countries that have reopened schools. We looked specifically at the 7-day average of cases, cases per million, and test positivity rate as of the date each country reopened schools compared to the U.S. as of the most recent date. As shown, all countries that have reopened had significant lower case counts and cases per million than the U.S. currently has and all but one (Japan) had lower positivity rates. For example, when South Korea opened schools on June 8, the country had a 7-day average of just 44 cases nationwide (or 0.9 per million population), after more than two months of limited transmission; it also already had widespread testing (a test positivity rate of 0.3%) and contact tracing in place, meaning it was well positioned to respond quickly to any school outbreaks. By contrast, the U.S. is currently averaging at more than 65,000 cases a day (or 195 per million population) and its test positivity rate is 8%. Many other countries, such as Finland, Iceland, New Zealand, and Vietnam, similarly had low case numbers, cases per million, and positivity rates when they opened schools and have avoided large school-based outbreaks so far.

At the same time, school associated outbreaks have occurred in some countries. Not all countries have avoided school-associated outbreaks. For example, schools in Canada, Chile, France, and Israel have reported outbreaks, sometimes significant ones, necessitating re-closure of schools in some cases. Israel, in particular, has had several outbreaks at schools after reopening, including an outbreak in a high school where 13.2% of students tested positive for coronavirus as did 16.6% of staff and school-based cases have been linked to increasing community transmission. The country reopened when it had significant fewer cases, cases per million, and positivity rate than the U.S. does today (see Table 1); it reopened its entire school system with few social distancing or other mitigation measures. In Sweden, a country which never closed its schools for children under 16, health authorities have reported that teachers are no more at risk in schools than adults in other professions and community transmission was not affected by schools being open. There have been school outbreaks and deaths among school teachers in Sweden, but insufficient testing and contact tracing at Swedish schools make it difficult to draw definitive lessons from the Swedish school experience.

Like schools, daycare-associated outbreaks also have been reported, including in the U.S., such as in California and Texas.

Conclusion

Taken together, the evidence indicates that while children are much less likely than adults to become severely ill from COVID-19, they do transmit virus. It is still unclear to what extent children, especially younger ones, are likely to get infected or transmit the infection to others compared to adults. However, where there is already widespread community transmission, as is the case in many areas in the U.S., there is clearly a risk of spread associated with reopening schools. This challenge may be more pronounced where testing and contact tracing capacity is limited. As such, the risk of re-opening schools needs to be considered carefully in light of the recognized benefits of in-person education.