The role of schools in transmission, and the value of school closure, has been one of the most contentious issues over the course of the COVID-19 pandemic. There is ongoing debate about exactly how much SARS-CoV-2 risk is posed to individuals and communities by in-person schooling. While there is general consensus that it should be possible to open schools safely with adequate mitigation measures, there is little data and even less agreement as to what level of mitigation is needed.

Many ecological studies have shown an association between in-person schooling and the speed and extent of community SARS-CoV-2 transmission (1–3), though these results have been far from uniform (4). While there have been numerous outbreaks of SARS-CoV-2 in schools and school-like settings (5–7), studies conducted outside of outbreak settings have suggested that, when mitigation measures are in place, transmission within schools is limited and infection rates mirror that of the surrounding community (8, 9).

However, the ways in which in-person schooling influences community SARS-CoV-2 incidence are complex. Schools play a unique role in the social fabric of the United States and other countries, and often create potential transmission connections between otherwise disparate communities. Even if transmission in classrooms is rare, activities surrounding in-person schooling, such as student pick-up and drop-off, teacher interactions, and broader changes to behavior when school is in session could lead to increases in community transmission.

There is also a growing body of evidence that younger children (e.g., those less than 10 years of age) are less susceptible to infection when exposed (10), though it is unclear if they are less likely to pass on the virus once infected (11, 12), or if this reduced susceptibility is offset by the increased number of contacts students make when in school (13). Even when students are infected, the risk of severe disease and death among teenagers and young children is low (14). This means that one of the main reasons for a focus on schools is not the risk to students, but the risk that in-person schooling poses to teachers and family members, as well as its impact on the trajectory of the overall epidemic. Yet, few studies have focused on the risk in-person school poses to household members (15).

Different interpretations of the evidence and local politics have led to massive heterogeneity in approaches to schooling across the United States during the 2020-21 school year (16), running the gambit from complete cessation of in-person learning to opening completely with no mitigation measures in place. Most schools that have opened have made some efforts to mitigate transmission, but there is much diversity in the approaches adopted.

This hodgepodge of approaches to schooling creates a massive natural experiment from which we can learn about what does, and does not, work for controlling school associated SARS-CoV-2 spread. However, there is no central repository for the measures taken across the over 130,000 schools in the United States, nor is there a central repository for health outcomes in these schools, making studies difficult. Where data are available they are often restricted to traditional public school systems, though 28% of Pre-K through 12th grade students are in private or charter schools, and rarely can it be linked with individual- or household-level outcomes.

The COVID-19 Symptom Survey provides a unique opportunity to collect and analyze data on schooling behaviors and SARS-CoV-2 related outcomes from households throughout the United States. This survey is administered through the Facebook platform in partnership with Carnegie Mellon University and yields approximately 500,000 survey responses in the United States each week (17). It includes questions on symptoms related to COVID-19, testing and, since late November 2020, the schooling experience of any children in the household. Analysis weights adjust for non-response and coverage bias (see supplement for details).

We analyzed data collected over two time periods during the 2020-2021 school year (Nov. 24, 2020-Dec. 23, 2020 and Jan. 11 2021-Feb. 10, 2021). Of 2,142,887 total respondents in the 50 US states and Washington DC during this period, 576,051 (26.9%) reported at least one child in Pre-K through high school living in their household (Table S1-S2, Fig. 1A). Forty-nine percent (284,789/576,051) of these respondents reported a child living in the household engaged in either full- (68.8%) or part-time (46.0%) in-person schooling, with substantial variation both within and between states (Fig. 1, Table S3). Overall, in-person schooling increased between the two periods from 48% to 52%, though decreases were observed in some states (e.g., Arizona) (Fig. S1, Table S3).

After adjusting for county-level incidence and other individual- and county-level factors (but not school-based mitigation measures; Tables S1-S2, Fig. S2), living in a household with a child engaged in full-time in-person schooling is associated with a substantial increase in the odds (adjusted odds ratio [aOR] 1.38, 95% CI 1.30-1.47) of reporting COVID-19 like illness (CLI, fever of at least 100 °F, along with cough, shortness of breath, or difficulty breathing), loss of taste or smell (aOR 1.21, 95% CI 1.16-1.27), and report of a positive SARS-CoV-2 test result within the previous 14 days (aOR 1.30, 95% CI 1.24-1.35) (Fig. 2A, Table S4).

When stratifying by grade level (restricted to households reporting a child/children in a single grade strata), we find that the strength of the associations increase with grade level, from no association in households with only Pre-K and Kindergarten students (Fig. 2A, Table S4) to a substantial positive association in households with only high school aged students.

The association between COVID-19 outcomes and reporting a child in the household engaged in part-time in-person schooling is attenuated but still statistically significant for CLI (aOR 1.21, 95% CI 1.13-1.29), loss of taste and smell (aOR 1.18, 95% CI 1.13-1.24) and reporting a positive test (aOR 1.09, 95% CI, 1.03-1.14). Among those reporting part-time schooling the association between grade and COVID-19-related outcomes is less clear (Fig. 2A, Table S4).

For those students engaged in any form of in-person learning, the most common mitigation measure reported was student mask mandates (88%), followed by teacher mask mandates (80%), restricted entry (66%) and extra space between desks (63%) (Table S5). The distribution of mitigation measures reported was similar between those reporting full- and part-time in- person schooling, though most measures were slightly more likely to be reported in the part-time setting (Fig. 2B). Overall, respondents reporting a household child engaged in in-person school reported a mean of 6.7 (IQR 4-9) mitigation measures in place at any school attended by a household child. Those reporting only children in part-time schooling reported more mitigation measures (mean 7.3, IQR 5-10) than those reporting only children in full-time schooling (mean 6.4, IQR 4-9). There is substantial geographic heterogeneity in the number of mitigation measures reported (Fig. 1D, Fig. S3, Tables S5-S6), with households in South Dakota reporting the least (mean 4.6, IQR 2-7), and households in Vermont reporting the most (mean 8.9, IQR 8-11) mitigation measures.

We find a clear association with the number of mitigation measures implemented and the risk of COVID-19 outcomes among adult household members responding to the survey after adjustment for individual and county level factors. Each measure implemented is associated with a 9% decrease in the odds of CLI (aOR 0.91, 95% CI 0.89-0.92), a 8% decrease in the odds of loss of taste or smell (aOR 0.92, 95% CI 0.91-0.93) and an 7% decrease in the odds of a recent positive SARS-CoV-2 test (aOR 0.93, 95% CI 0.92-0.94) (Table S7). Regression treating each individual mitigation measure as having an independent effect show that report of daily symptom screening is clearly associated with greater risk reductions than the average measures (Fig. 3, Table S8), with some evidence that teacher mask mandates and cancelling extra-curricular activities are also associated with larger reductions than average. In contrast, closing cafeterias, playgrounds and use of desk shields are associated with lower risk reductions (or even risk increases); however this may reflect saturation effects as these are typically reported along with a high number of other measures. Notably, part-time in-person schooling is not associated with a decrease in the risk of COVID-19-related outcomes compared to full-time in-person schooling once we account for other mitigation measures.

To explore what, if any, levels of mitigation are associated with elimination of the excess risk posed by in-person schooling, we conducted analyses where we limited the in-person exposure group to schools with 0, 1-3, 4-6, 7-9 and 10 or more mitigation measures in turn (Fig. 4, Fig. S4, Tables S9-S10). We found that when we limited the in-person group to cases where 7 or more mitigation measures were in place the risks associated with in-person schooling had largely disappeared, with complete absence of increased risk with 10 or more mitigation measures. Among those reporting 7 or more mitigation measures, over 80% reported student and teacher mask mandates, restricted entry, extra space between desks and no supply sharing, and over 50% reported student cohorting, reduced class size and daily symptom screening.

The results presented here show a clear association between in-person schooling and the risk of COVID-19-related outcomes in adult household members, as well as evidence that implementation of a moderate number of school-based mitigation measures is adequate to eliminate this risk. However, in-person schooling and mitigation measures are not distributed at random in the population (Fig. 1, Tables S1-S3, S5-S6, S9-S10). For instance, households with a student attending in-person schooling tend to be in counties that are a higher percentage white (Fig. S2), and contain survey respondents who are more likely to have recently eaten out or gone to a bar (Table S2). Despite our best efforts to adjust for local incidence, individual behavior and other potential confounders, it is possible that unmeasured factors are responsible for the observed associations.

To address the possibility that the association with in-person schooling could be the result of differences between urban, suburban and rural counties, local patterns of incidence, or other differences between those more and less likely to send children to school in-person we performed several stratified analyses (Fig. 5). When stratifying by counties classified by size and metro status, incidence and propensity to avoid in-person schooling, we found few systematic or statistically significant deviations from the overall estimate of the relative risk associated with full- and part-time in-person schooling. The notable exception is an apparent increase in the risk associated with in-person schooling in households with the highest propensity to have children attending in-person classes (Fig. 5C).

While we were not able to specifically look at the relationship between in-person schooling, mitigation measures and risk to teachers, we were able to look at the risk associated with reporting paid work outside the home among those reporting teaching pre-K through high school as their current occupation. Teachers working outside the home work were more likely to report COVID-19-related outcomes (e.g., Test positive aOR 1.8, 95% CI 1.5-2.2; Fig. S5, Table S11), but increased risk was commensurate with the risk of working outside the home in other groups, such as those working in healthcare (aOR 1.7, 95% CI 1.5-1.9) and office work (aOR 1.6, 95% CI 1.5-1.7).

The results presented here provide strong evidence that in-person schooling poses a real risk to those living in the households of students, but that risk can be managed through commonly implemented school-based mitigation measures. However, much still remains unknown. We were unable to measure the risk posed by in-person schooling to the students themselves, nor were we able to specifically assess how different policies impact teachers and other school staff. This study also provides limited insight into the mechanisms by which in-person schooling increases risk, and it remains possible that classroom transmission plays a minor role, and other school related activities drive risk.

Furthermore, despite its size, this internet-based survey is cross-sectional, necessarily short and subject to response biases; though its size and robustness of the results allay some of these concerns. We were unable to evaluate compliance with or investment in reported mitigation measures. Though we adjust for several county-level measures of socioeconomic status and these data were not available at the individual level, such factors are known to be associated with COVID-19 risk and attitudes about in-person schooling. Additional, more formal, studies that span schools with multiple policies and approaches are critical to finding definitive answers to these questions. However, our results were robust when we looked with strata of urbanization, background COVID-19 risk, and propensity for in-person schooling (Table 5), and when we examined alternative measures of individual and household COVID-19 occurrence (Fig. S6-S8).

The debate around in-person schooling in the United States has been intense, and has exacerbated differences in approach between independent school systems and individual families nationally. The lack of coordination has provided an opportunity to learn about the risks of in-person schooling, and the degree to which mitigation measures may reduce risk. While online surveys have their unique limitations, the wide reach of the Facebook COVID-19 Symptom Survey has allowed us to gather data from households engaged in heterogeneous schooling activities throughout the country in a way few other study designs could. In analyzing these data, we find further support for the idea that in-person schooling carries with it increased COVID-19 risk to household members; but also suggests common, low cost, mitigation measures can greatly reduce this risk.