Acute gastroenteritis (AGE) outbreaks commonly occur in congregate settings, including schools and childcare facilities. These outbreaks disrupt institutions, causing absences and temporary facility closures. This study analyzed the epidemiology of school and childcare AGE outbreaks in the United States.
We analyzed AGE outbreaks occurring in kindergarten to grade 12 schools and childcare facilities reported via the National Outbreak Reporting System in the United States from 2009 to 2019 and compared this information to 2020 data. Outbreak and case characteristics were compared using the Kruskal-Wallis rank sum test, χ2 goodness-of-fit test, and Fisher exact test.
From 2009 to 2019, there were 2623 school, 1972 childcare, and 38 school and childcare outbreaks. School outbreaks were larger (median, 29 cases) than childcare outbreaks (median, 10 cases). Childcare outbreaks were longer (median, 15 days) than school outbreaks (median, 9 days). Norovirus (2383 outbreaks; 110 190 illnesses) and Shigella spp. (756 outbreaks; 9123 illnesses) were the most reported etiologies. Norovirus was the leading etiology in schools; norovirus and Shigella spp. were dominant etiologies in childcare centers. Most (85.7%) outbreaks were spread via person-to-person contact. In 2020, 123 outbreaks were reported, 85% in the first quarter.
Schools and childcare centers are common AGE outbreak settings in the United States. Most outbreaks were caused by norovirus and Shigella spp. and spread via person-to-person transmission. Fewer outbreaks were reported in 2020 from the COVID-19 pandemic. Prevention and control efforts should focus on interrupting transmission, including environmental disinfection, proper handwashing, safe diapering, and exclusion of ill persons.
Acute gastroenteritis illnesses spread rapidly in schools and childcare facilities, causing outbreaks and disrupting the learning process. Better understanding of etiologies and epidemiology of these outbreaks is needed to inform effective prevention and control measures.
This study analyzes 4756 school and childcare acute gastroenteritis outbreaks in the United States reported through the National Outbreak Reporting System from 2009 to 2020. Most outbreaks were caused by norovirus or Shigella spp. and were spread via person-to-person transmission.
Approximately 179 million cases of acute gastroenteritis (AGE) occur in the United States every year1 ; outbreaks of AGE commonly occur in congregate settings, where many people gather in close proximity, including schools and childcare centers.2,3 In the United States, there are 130 930 primary and secondary schools serving 56.4 million students primarily 5 to 18 years old.4 Additionally, approximately 230 000 home and center-based childcare programs serve ∼10 million children from birth to 5 years old.5
With many children in 1 location, the presence of diapered children in childcare facilities, and many shared surfaces, the spread of enteric pathogens can happen quickly in schools and childcare facilities.6 To prevent the spread of AGE pathogens, guidelines from the American Academy of Pediatrics recommend excluding symptomatic children and staff from school or daycare.7 Illness control regulations in schools and childcare facilities vary between jurisdictions8 and, despite these regulations, outbreaks in schools and childcares still occur. AGE outbreaks disrupt childcare and school operations, causing temporary closures of a facility or school district9 and costing millions of dollars in direct and indirect economic costs.10 Beginning in March 2020, the unprecedented COVID-19 pandemic led to closures of 95% of schools for in-person learning and two-thirds of childcare centers across the United States, affecting more than 55 million children.11,12
To collect information on foodborne, waterborne, and enteric outbreaks in the United States, the Centers for Disease Control and Prevention (CDC) launched the National Outbreak Reporting System (NORS) in 2009, through which state, local, and territorial health departments can report outbreaks.13 Other studies have published NORS AGE outbreak data in other congregate settings, including camps,14 colleges and universities,15 and long-term care facilties.16 This paper summarizes AGE outbreak characteristics in kindergarten to grade 12 (K–12) schools and childcares reported through NORS from 2009 to 2019 and compares these with outbreaks reported in 2020. In doing so, this paper documents the AGE outbreak burden in these settings and provides data to inform prevention and infection control measures.
NORS is a dynamic, voluntary reporting system through which state, local, and territorial health departments in the United States report foodborne and waterborne disease outbreaks and enteric disease outbreaks spread via contact with environmental sources, infected persons or animals, or unknown modes of transmission to the CDC. In NORS, an outbreak is defined as 2 or more cases of similar illness with a common exposure or epidemiologic link.13
NORS collects aggregate demographic, epidemiologic, clinical, and laboratory data for each outbreak. All NORS outbreak reports contain the number of primary cases, mode of transmission, exposure state, and date the first case became ill. Other data are collected when available, including setting, suspected and laboratory-confirmed etiologies, attack rates, case demographics, and symptoms. In NORS, outbreak etiologies are reported based on available laboratory, clinical, or epidemiologic information. Confirmed etiologies have 2 or more cases with laboratory test confirmation; suspected etiologies include those with no testing or with a single positive specimen. If no laboratory testing is available, suspected etiologies can be reported based on clinical and epidemiologic information. When available, the number of cases with an outbreak-associated outpatient medical visit, emergency department visit, hospitalization, or death is collected along with the number of cases for whom this information is available. Mode of transmission is the primary means of pathogen spread; in NORS, person-to-person transmission is defined as direct contact with infected persons, their bodily fluids, or the local environment where both the exposed and infected persons were simultaneously present.13
We included AGE outbreaks reported through NORS that occurred at a K–12 school or childcare from January 1, 2009, through December 31, 2020. Study data were extracted from NORS on February 9, 2022. Our analyses focused on outbreaks occurring from 2009 to 2019; 2020 outbreak data are reported separately. Outbreak setting was classified by reviewing reported settings, with additional review of case demographics and comment fields, as necessary. Schools were defined as public or private elementary, middle, or high schools primarily serving grades K–12; childcare centers were defined as center or home-based childcare centers primarily serving children ≤5 years old, including preschools. Outbreaks that occurred at a facility serving preschool and school-age children in a single location (eg, pre-K to grade 6) were classified as “school” outbreaks, whereas outbreaks that affected both a childcare center and separate school location were classified as “both school and childcare” outbreaks. For foodborne outbreaks, the outbreak setting was where the food was prepared or eaten.
Because NORS collects data on nonenteric foodborne and waterborne outbreaks, analysis was restricted to AGE outbreaks. AGE symptoms included bloody stools, diarrhea, gastroenteritis, nausea, vomiting, or abdominal cramps. Outbreaks caused by a non-AGE etiology (eg, Legionella spp.) were excluded. Suspected and laboratory-confirmed outbreak etiologies were grouped by genus. Outbreaks with >1 suspected or laboratory-confirmed etiology were classified as multiple etiology outbreaks.
Outbreaks identified as occurring at a university, technical school, or college; in a primary or secondary boarding school; outside of normal school operations (eg, sports tournament, camp, community meeting); or only among staff or faculty (eg, teacher appreciation breakfast) were excluded. Outbreaks where exposure occurred in multiple states were excluded (Supplemental Fig 4).
Analyses were conducted using the R Environment for Statistical Computing (version 4.0.4).17 Outbreak size, duration, attack rates, and reported symptoms were compared using the Kruskal-Wallis rank-sum test. Outbreak seasonality was assessed using a χ2 goodness-of-fit test, and outbreak etiologies were compared using the Fisher exact test. Rates of health care utilization for outpatient, emergency department, and hospital visits were calculated using the number of cases with reported information as a denominator and were compared using Fisher exact test.
Between 2009 and 2020, 42 619 outbreaks were reported through NORS; 5145 (12.1%) reported an outbreak setting of “school/college/university” or “child daycare,” and 4756 (92.4%) met our inclusion criteria (Supplemental Fig 4).
Of these, 4633 outbreaks occurred from 2009 to 2019; 2623 (56.6%) in schools, 1972 (42.6%) in childcare facilities, and 38 (0.8%) in both a school and childcare facility (Table 1). During this period, outbreaks were reported by 49 states, Puerto Rico, and Washington DC. A total of 166 787 cases were reported; 132 557 cases (79.5%) in schools, 29 318 (17.6%) in childcare facilities, and 4912 (2.9%) in a school and childcare outbreak (Table 1).
From 2009 to 2019, most outbreaks (n = 2947, 63.6%) occurred during October through March compared with April through September (P < .001). This fall/winter predominance was most marked among school outbreaks (n = 1904, 72.6%, P < .001) and outbreaks reporting a viral etiology (n=1775, 72.8%, P < .001) (Fig 1). Childcare outbreaks occurred consistently throughout the year (51.6% from October through March), but seasonal variation was observed by etiology: most childcare outbreaks reporting a viral etiology occurred during October through March as compared with April through September (n = 479, 66.5%, P < .001) and a majority reporting a bacterial etiology occurred during April through September compared with October through March (n = 499, 61.8%, P < .001) (Fig 1).
From 2009 through 2019, outbreaks lasted a median of 12 days (interquartile range [IQR]: 6–22). Childcare outbreaks (median, 15 days; IQR: 8–26) lasted significantly longer than school outbreaks (median, 9 days; IQR: 4–18; Table 1) (P < .001). Bacterial outbreaks (median, 19 days; IQR: 10–32) lasted significantly longer than viral outbreaks (median, 10 days, IQR: 5–18) (P < .001). The median number of cases was significantly higher in school outbreaks (29 cases; IQR: 13–63) than childcare outbreaks (10 cases; IQR: 5–20;Table 1) (P < .001).
Person-to-person transmission (n = 3971, 85.7%, Table 2) accounted for most school and childcare outbreaks during 2009 through 2019. Foodborne (n = 123, 2.7%), environmental contamination (n = 14, 0.3%), animal contact (n = 11, 0.2%), and waterborne (n = 10, 0.2%) were uncommon modes of transmission (Table 2). For the remaining outbreaks (n = 504, 10.9%), the mode of transmission could not be determined.
Most school and childcare outbreak reports (n = 3741, 80.7%) during 2009 through 2019 included a suspected or laboratory-confirmed etiology. Nearly one-fifth of outbreaks did not report an etiology (n = 892, 19.3%). Most outbreaks reported a single suspected or laboratory-confirmed viral (n = 2438, 52.6%) or bacterial (n = 1086, 23.4%) etiology; outbreaks caused by a parasite (n = 112, 2.4%), chemical/toxin (n = 6, 0.1%), or that had multiple etiologies (n = 99, 2.1%) accounted for less than 5% of reported outbreaks. Viral etiologies accounted for 65.2% school outbreaks and 36.5% of childcare outbreaks. Bacterial etiologies were more commonly reported in childcare outbreaks (41.0%) than school outbreaks (9.9%) (P < .001) (Table 1).
From 2009 through 2019, norovirus (n = 2383, 51.4%) and Shigella spp. (n = 756, 16.3%) were the most commonly reported pathogens, representing 67.7% of outbreaks. Norovirus was the leading pathogen in school outbreaks (n = 1689, 64.4%), whereas norovirus (n = 686, 34.8%) and Shigella spp. (n = 537, 27.2%) were reported at a similar frequency in childcare outbreaks (Table 1). The next most common pathogens included Escherichia coli (n = 138, 3.0%), Salmonella spp. (n = 138, 3.0%), and Cryptosporidium spp. (n = 90, 1.9%), all 3 of which were more commonly reported in childcares outbreaks than in schools (Table 1).
From 2009 to 2019, 1012 (38.6%) school outbreaks and 1051 (53.3%) childcare outbreaks reported attack rate data for both students/attendees and staff. Among outbreaks with complete data, the median attack rate was 15.8% (IQR: 7%–31%) for students/attendees and 7.7% (IQR: 0%–23%) for staff. The median attack rates for childcare attendees (18.9%, IQR: 10%–36%) and childcare staff (13.3%, IQR: 0%–33%) were significantly higher than attack rates for students (12%, IQR: 6%–25%, P < .001) and staff (5.2%, IQR: 1%–14%, P < .001) in schools.
Most outbreak cases in 2009 through 2019 occurred among students or childcare attendees; a median of 97.1% of cases in school outbreaks were 5 to 19 years old and a median of 80.0% of cases in childcare outbreaks were <5 years old. Most cases reported diarrhea (median, 82.3%; IQR: 40%–100%) or vomiting (median, 75.0%; IQR: 42%–100%). Compared with school outbreaks, a higher percentage of cases childcare outbreaks reported diarrhea (medians, 100% vs. 54.5%, P < .001) and a lower percentage reported vomiting (medians, 52.2% vs. 87.0%, P < .001).
From 2009 to 2019, 2594 (98.9%) school, 1969 (99.8%) childcare, and 36 (94.7%) school and childcare outbreaks reported 6409 outpatient visits, 1604 emergency department visits, 694 hospitalizations, and 5 deaths. Childcare outbreaks reported higher rates of outpatient visits (24.2 vs. 5.5 per 100 case-patients, P < .001), emergency department visits (4.8 vs 1.5 per 100 case-patients, P < .001), and hospitalizations (1.8 vs 0.3 per 100 case-patients, P < .001) than school outbreaks (Fig 2). Health care utilization varied by pathogen, with bacterial (48.4 outpatient visits per 100 case-patients; 4.6 hospitalizations per 100 case-patients) and parasitic (61.2 outpatient visits per 100 case-patients; 2.4 hospitalizations per 100 case-patients) pathogens having the highest rates of health care utilization across both settings (Fig 2). In our dataset, 5 deaths were reported: 3 in childcare outbreaks (3 different E coli O157:H7 outbreaks) and 2 in school outbreaks (a norovirus outbreak and an outbreak with no reported etiology).
The number of outbreaks reported per year ranged from 139 in 2009 to 716 in 2016; 544 outbreaks were reported in 2019 (Fig 3). From 2012 to 2019, an annual median of 532 outbreaks were reported in these settings; by setting, an annual median of 280 school, 230 childcare, and 4 school and childcare outbreaks were reported. In contrast, in 2020, 123 outbreaks (76 school and 47 childcare outbreaks) were reported (Fig 3); 105 (85%) from January to March 2020. No outbreaks were reported in April, May, August, or December 2020 (Supplemental Fig 5). In 2020, most outbreaks reported norovirus (n = 80, 65.0%), followed by Shigella spp. (n = 10, 8.1%), E coli (n = 4, 3.2%), Salmonella spp. (n = 3, 2.4%), and Staphylococcus spp. (n = 1, 0.8%); 25 outbreaks (20.3%) reported no etiology. Most outbreaks (n = 96, 78%) were person-to-person transmission, followed by foodborne (n = 2, 1.6%) and unknown (n = 25, 20.3%).
K–12 schools and childcares are common AGE outbreak settings. During 2009 through 2020, 4756 outbreaks in these settings were reported to CDC through NORS. From 2009 through 2019, 4633 outbreaks were reported (median, 457 outbreaks per year) causing 166 787 cases, 6409 outpatient visits, 1604 emergency department visits, 694 hospitalizations, and 5 deaths. Norovirus and Shigella spp. were the most commonly reported etiologies; norovirus caused two-thirds of K–12 school outbreaks, and norovirus and Shigella spp. together caused two-thirds of childcare outbreaks. Across settings and etiologies, person-to-person transmission dominated.
In March 2020, almost all K–12 schools and two-thirds of childcare facilities in the United States shut down because of the COVID-19 pandemic.11,12 Outbreaks reported in these settings through NORS dropped from 544 in 2019 to 123 in 2020; only 18 outbreaks were reported between April and December 2020. Other studies have also shown a sharp decrease in AGE pathogen activity in 2020, coinciding with nonpharmaceutical interventions (eg, social distancing) implemented to prevent the transmission of COVID-19.18–21 Similar to the 2009 to 2019 outbreaks, most 2020 outbreaks were caused by norovirus and spread via person-to-person transmission. Comparatively, Shigella spp. caused fewer 2020 outbreaks, likely because of Shigella spp.’s summer seasonality.22
Consistent with this study, norovirus and Shigella spp. are significant AGE pathogens in young children and adolescents22,23 and common causes of outbreaks in schools and childcares.3,6,24–29 Our study period (2009–2020) occurred after US rotavirus vaccine introduction in 2006, after which norovirus became the dominant pediatric viral gastroenteritis etiology in the United States.30–32 Additionally, our study is larger and covers a longer period and larger geographic area than other studies and may more accurately capture the relative national burden of different school and childcare outbreak etiologies. In comparison, pathogens, such as E coli, Salmonella spp., rotavirus, and sapovirus have played a larger role in school and childcare outbreaks reported previously.33–35 An analysis of 29 childcare AGE outbreaks in North Carolina attributed 45% of outbreaks to a viral etiology, mostly rotavirus and norovirus, but only 1 outbreak to Shigella.33 In a 2008 systematic review of childcare outbreak reports, E coli, Salmonella spp., norovirus, and rotavirus were the leading pathogens, with Shigella spp. only representing 5% of outbreaks.34 In our analysis, Shigella spp. was the second leading cause of childcare outbreaks; only 7% of outbreaks were caused by other bacterial pathogens and 1% by viral pathogens other than norovirus.
In this study, person-to-person transmission dominated in school and childcare outbreaks, which is consistent with other outbreak reports in these settings34,36–39 and evidence that norovirus and Shigella spp. are predominantly spread via person-to-person transmission.3,22 A systematic review of childcare outbreaks found person-to-person transmission to be the prominent mode but also found 16% of reported outbreaks were spread via foodborne transmission34 ; many school and childcare outbreak reports in the literature also mainly describe outbreaks spread via foodborne transmission.2,40–42 However, in our study, <3% of outbreaks were spread via foodborne transmission. Although both norovirus and Shigella spp. can be spread via food43 and cause foodborne disease,43 our study emphasizes the importance of person-to-person contact as the dominant mode of transmission in both schools and daycares and across both bacterial and viral etiologies.
School outbreaks were larger, shorter, more seasonal, and had fewer health care encounters than childcare outbreaks and were predominately caused by norovirus. Norovirus has a distinct winter seasonality, is characterized by short, mild illness,23 and can present as vomiting only.44–46 In the United States, K–12 schools are typically closed for parts of June to August, whereas childcare facilities operate year-round. Conversely, bacterial outbreaks, particularly Shigella spp., were more commonly reported in childcare facilities; these pathogens can cause more severe and longer diarrheal disease22 with less pronounced seasonality.47,48 Bacterial outbreaks may spread more in childcares because of diapered children, poor hand hygiene, and younger age of the children.22,49 Additionally, childcare outbreaks may have been more severe because of the younger age and presumably less developed immune systems of childcare attendees.
Although NORS is a national surveillance system gathering timely data on many outbreaks, it has limitations. NORS reporting has increased over time,3 likely because of increased reporting capacity by health departments. Although the number of reported school and childcare outbreaks also increased over our study period, NORS’ limited data collection does not allow us to discern if this is a true increase or a reporting artifact. NORS reporting is also voluntary; not all jurisdictions report all outbreaks through NORS, and some deprioritize reporting of outbreaks spread via person-to-person transmission. For these reasons, this analysis summarized the data on a national level and did not explore any state or regional trends. NORS data likely underrepresent the true number of AGE and person-to-person outbreaks in schools and childcare facilities in the United States, especially from 2009 to 2011 after NORS was initially launched. Additionally, outbreak etiologies can be reported based solely on clinical or epidemiologic data; outbreaks without confirmatory laboratory testing could be misclassified. Given that sapovirus, rotavirus, and astrovirus outbreaks can present similarly to norovirus outbreaks, some outbreaks reporting no etiology or norovirus may have been caused by other pathogens.50 This analysis did not include secondary cases or transmission outside of the school or childcare center. However, children infected in school and childcare outbreaks can be a source of transmission to household members and contribute to a pathogen’s spread in the wider community.51–53
Shigella spp. and norovirus vaccines are in development,22,54 and Shigella spp. can be treated with antibiotics.22 However, nonpharmaceutical interventions, including proper handwashing with soap and water,55–57 and exclusion of ill children and staff,58 can prevent both sporadic and outbreak associated AGE illnesses in these settings.7,59 Both norovirus and Shigella are also known to persist on contaminated surfaces60 ; common surfaces and diapering areas should be routinely cleaned and disinfected with products effective against these pathogens.61–65 Schools and childcare centers should enforce proper infection control strategies to prevent and control AGE outbreaks and reduce AGE-related absenteeism.
Schools and childcare centers are common settings for AGE outbreaks, accounting for a median of 457 outbreaks and 15 779 cases per year in the United States from 2009 to 2019. In 2020, reported AGE outbreaks decreased dramatically to 123 reported outbreaks for the year, correlating with the closure of nearly all K-12 schools and two-thirds of childcare centers because of the COVID-19 pandemic. Schools and childcare centers provide a prime setting for the spread of AGE pathogens, particularly norovirus and Shigella spp., through person-to-person contact. Outbreak prevention and control in these settings should focus on handwashing, cleaning and disinfection with effective products, and the exclusion of ill children and staff.
Kayla Vanden Esschert, Division of Foodborne, Waterborne, and Environmental Diseases, National Center for Emerging and Zoonotic Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA. Mary Ann K. Hall, Eagle Global Scientific, contracting agency for the National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, GA
Ms Mattison conceptualized and designed the study, analyzed and interpreted the data, drafted the initial text, tables and figures, and reviewed and revised the manuscript. Ms Calderwood designed data collection instruments, conceptualized and designed the study, and reviewed and revised the manuscript for important intellectual content. Mr Marsh conceptualized and designed the study, drafted initial data analyses, and reviewed and revised the manuscript. Ms Balachandran, Ms Kambhampati, Ms Gleason, Ms Wikswo, and Ms Lawinger designed data collection instruments and reviewed and revised the manuscript. Dr Mirza helped conceptualize and design the study, coordinated, and supervised data analysis and reviewed the manuscript for important intellectual content. All authors approved the final manuscript as submitted and agree to be accountable for all aspects of the work.
FUNDING: No external funding.
CONFLICT OF INTEREST DISCLOSURES: The authors have indicated they have no potential conflicts of interest to disclose. The findings and conclusions in this report are those of the author(s) and do not necessarily represent the official position of the Centers for Disease Control and Prevention.