Updates since December 20, 2020:
Updated on February 23, 2021 to incorporate two recent studies, one from North Carolina and the other from Wisconsin, that illustrate that key mitigation strategies can substantially prevent in-school 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 children's 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 phenomena—ACE-2 receptor production and pre-existing antibodies to other coronaviruses—that 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 (>6,000 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 were—including weddings, parties, and playdates. This likely reflects that the more controlled school environment leads 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 each of the cases where there was COVID-19 transmission in school. It found that, of children who tested positive—a low number relative to the total number of students—only 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, including a 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 both ensuring places like teacher/staff break rooms are well-controlled as well as 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, including 661 students, teachers, staff, siblings and parents, found evidence of potential spread within the school, with 43% of teachers, 59% of other school staff, and 38% of students with antibodies, compared to the 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 running. During the two days without masking or proper ventilation, two symptomatic individuals were in the school, leading to an outbreak across more than 100 students and staff. This study highlights the risk of spread without mitigation strategies—teaching us what not to do. Core strategies include masking, 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 when infected, 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, for students, and for household members of students or teachers. They found that masking 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.
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.
Getting it right: Core Mitigation Strategies Reduce In-school Transmission to Much Lower Rates than Community Transmission
Two recent studies, one from North Carolina and the other from Wisconsin, illustrate that key mitigation strategies can substantially prevent in-school transmission. The North Carolina (PDF) study included 90,000 students and 10,000 teachers with community daily case rates as high as 29/100,000 population per day. The study engaged 11 school districts to focus on the "3 Ws" (wear your mask, wait 6 feet, wash your hands), including elementary, middle, and high schools. There were 773 cases in students or staff attending schools, but only 32 in-school transmissions, with zero student-to-teacher transmissions. The study authors report that if in-school transmission was similar to the transmission rate in the community, there would have been an estimated 900 in-school transmissions. The three outbreaks (defined as 5 or more cases) in the ABC schools were associated with lack of masking in pre-Kindergarten and in special needs classes during meal times. Increased mitigation strategies were put into place to address these outbreaks and prevent additional transmission.
A study of schools in Wisconsin had similar findings to the North Carolina study. Seventeen K-12 rural schools participated from August 31-November 29, including 4,800 students and 650 staff. These schools emphasized adherence to masking, stable groups of 11-20 students, and staff maintaining 6 feet distance, if possible. During 13 weeks of in-person learning, there were seven in-school transmissions in students and zero in staff. Community rates were high, with test positivity ranging from 7-40%. Masking compliance within the schools was high, ranging from 92-97% and there were no differences in masking by age, suggesting that younger students can adhere to masking comparably to older students.
The implication from these two recent studies is that with mitigation layers in place (masking, physical distancing, stable groups, hand hygiene), in-school transmission can be minimized. Schools can achieve much lower transmission rates than the community through the use of these mitigation strategies, likely because teachers and staff are good at creating routines and structures for students, and students are able to participate in the mitigation layers in the school setting.