Evidence Summary: TK-6 Schools and COVID-19
Transmission
This is a summary
document of the evidence thus far that informs safe and successful in-person
instruction in TK-6 schools in the context of the COVID-19 pandemic. The
overall topics covered include: frequency of infection in elementary-aged
students; why they get it less often and with less severe disease than adults;
transmission patterns in elementary-school aged students; transmission patterns
in TK-12 schools; and the evidence for COVID-19 transmission mitigation
strategies particular to the school context.
This summary is not
comprehensive, but focuses on the best evidence we have to inform us regarding
the safety of in-person instruction for TK-6 students. The studies cited are
chosen for their rigor, rather than because they support a specific position
regarding whether or not it is safe to be open. We have learned a considerable
amount since March 2020 regarding schools, through scientific studies of
schools or camps that have been open in the U.S. or internationally. Because
change is the only constant in the COVID-19 pandemic, we will continue to gather
and monitor the evidence carefully, to inform safe and successful schooling.
Why
Children Get COVID-19 Less Frequently and Have Less Severe Disease
In epidemiological
studies globally and nationally, the evidence suggests that children seem to get COVID-19 less
frequently than adults. Originally it was thought that they might be less
frequently diagnosed due to less testing because children are more often
asymptomatic or have less severe symptoms. However, population-wide studies in Iceland and Spain using antibody tests that assess prior infection at any time find
that children have lower rates of infection compared to adults.
There are two
general explanations for why children get COVID-19 less frequently and have
less severe disease compared to adults. The first is that they produce fewer
ACE-2 receptors. Essentially, ACE-2 receptors are the doorway into human cells
for SARS-CoV-2, the virus that causes COVID-19. A study from May 2020 showed that elementary students produce fewer
ACE-2 receptors than middle and high school-aged students, who produce fewer receptors
than receptors adults. Consequently, children have fewer doorways into the body
for the virus, which leads to fewer infections and less severe infections for
those who catch the virus.
The other
explanation is that, because childrenās immune systems are used to fighting off
common colds, they are better primed to fight off COVID-19. Other viruses in
the same family (coronaviruses) as the SARS-CoV-2 virus cause the common cold.
Since they are in the same family of virus, some parts of the virus, including
something called the S2 spike, are very similar. There is a study of children from 2011-2018 (before SARS-CoV-2 appeared) that
shows that more children (ages 1-16) had antibodies against the S2 spike than
young adults (17-25), likely because they have coughs and colds from other
coronaviruses more often than adults. It is likely a combination of these two
phenomenaāACE-2 receptor production and pre-existing antibodies to other
coronavirusesāthat explain why children get disease less frequently and less
severely.
Children
with COVID-19 Most Often Get It from a Household Contact
When children do get
COVID-19, the predominant pattern of transmission is to get the infection from
an adult household contact (someone the child lives with at home who has COVID-19).
High rates of household infection from adults to children have been seen in studies
from Chicago, India, Greece, Australia, Switzerland, South Korea, and China.
This has been seen even in settings where schools were open. For instance, a study of 10 early childhood centers and 15 schools (>6000 people)
found low rates in the schools overall (1.2%) and >90% of cases were from
the community, not from in-school transmission.
Transmission
Among or from Students Is Uncommon
A recent study in the Morbidity and Mortality Weekly Report (MMWR) from the
Centers for Disease Control and Prevention (CDC) found that for students, going
to schools was not associated with having a positive COVID-19 test, but that social
gatherings wereāincluding weddings, parties, and playdates. This likely
reflects the more controlled school environment leading to a low risk of
transmission. It may also be that families who were going to these types of higher-risk
social gatherings may have had other higher risk behavior such as decreased
mask use.
The study from Australia mentioned above investigated the cases where
there was transmission in school. It found that, of children who tested
positiveāa low number relative to the total number of studentsāonly 0.3% had
had contact with another child who was positive (child-to-child transmission). Child-to-adult
transmission occurred only 1% of the time. In contrast, adult-to-child
transmission occurred 1.5% of the time, and adult-to-adult transmission was
4.4%, almost 15 times higher than child-to-child transmission. This was in the
context of masks not being encouraged at the time in Australia, though small
groups and physical distancing recommendations were in place. The higher risk
of adults transmitting to others compared to children transmitting to others is
likely due to adults getting COVID-19 more often than children and youth, and adults
having worse symptoms like cough, which makes it easier to transmit the virus.
These data suggest
that adult-to-adult transmission is the most likely scenario for in-school
transmission. This indicates that we have more control over in-school
transmission, since adults are more likely to be able to adhere to policies for
mitigation strategies such as masking and physical distancing. To achieve low
in-school transmission, school communities will need to remain focused on
ensuring places like teacher/staff break rooms are well-controlled and on
effectively implementing the core mitigation strategies for staff as well as
for students.
Low
Risk of Transmission in Elementary Schools
The data indicate
that the risk of transmission in elementary schools can be low. Two studies from
early in the pandemic in Oise, one of the most heavily affected areas of France,
focused on elementary schools and the local high school. Both studies examined the presence of antibodies (evidence of
prior infection) to the SARS-CoV-2 virus in students and staff who had been
attending the open schools without any precautions (e.g., masking, distancing) in
place. The high school study showed evidence of potential spread within the
school, with 43% of teachers, 59% of other school staff, and 38% of students
with antibodies, compared to community prevalence of 9%. The elementary school
study included six schools and >500 students, with only 9% of students, 7%
of teachers, and 4% of non-teaching adults with antibodies, very similar to
community prevalence. The lower transmission in the elementary schools likely
reflects the lower infection rates and lower severity of illness in elementary
students. However, it also likely
reflects the much higher rates of student mixing in a traditional high school
curriculum. This highlights why a modified high school curriculum that creates
stable groups can substantially mitigate the risk of widespread in-school transmission
in high schools.
Lessons
About What Not to Do
In addition to the
studies above, a study from a middle and high school in Israel after re-opening in May illustrates
the need for mitigation strategies to support safe schools. The school
re-opened in May, with no physical distancing measures in place. Due to a heat
wave, they stopped requiring masking for two days and had closed windows with
air conditioners. During the two days without masking or proper ventilation, two
symptomatic cases were in the school, leading to an outbreak across more than
100 students and staff. This study highlights the risk of spread without
mitigation strategiesāteaching us what not to do. Core strategies
include masks, physical distancing, enhanced ventilation with open windows and
without strong inward-directed air currents, and symptom screening.
Testing
Students and Staff with Symptoms Can Prevent Outbreaks
Though approximately
40% of children do not have symptoms of COVID-19, symptom screening will still
identify children with a higher likelihood of COVID-19 compared to students
without symptoms. Screening students and staff and excluding those with symptoms
creates a system for preventing possibly infectious people with COVID-19 from
coming to school, thereby avoiding or breaking the chain of in-school
transmission. One potential option for getting cleared to return to school
after having symptoms includes getting tested. So, in addition to helping to
prevent in-school transmission, the screening and testing of symptomatic
students and staff provides ongoing data about COVID-19 in school communities.
Core
Mitigation Strategies
The successful
approach to preventing transmission in schools leverages layers of safety strategies.
Core strategies include: masks; physical distancing; small, stable groups; hand
hygiene; ventilation; screening for symptoms or close contact; and asymptomatic
testing. Each layer provides additional protection and, when used together,
have been associated with low or zero transmission, even in communities with
high COVID-19 prevalence (paper in-press at Pediatrics). A modeling study examined the efficacy of different mitigation strategies to
prevent in-school COVID-19 transmission. The study compared the efficacy of
masking, monthly and weekly testing of teachers and students, and stable groups
of students and staff, examining each strategy alone and then examining
combinations of strategies. The authors looked at how much each strategy could
decrease the proportion of symptomatic infections for teachers in high schools,
middle schools and elementary schools, and for students, and for household
members of students or teachers. They found that masks alone and stable cohorts
alone were more effective than even weekly testing of students and teachers.
This illustrates again the importance of masks and stable cohorts.
In Summary:
Though the evidence
continues to evolve, we know more now than we did in July regarding how to
prevent transmission in schools. We have learned from examples of what works
and what does not work. Core mitigation
strategies are necessary for safe and successful schooling. If those mitigation
strategies are implemented as several layers of safety, elementary schools can
be safe workplaces for teachers and other staff and safe learning environments for
children.