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Cited by 124SARS-CoV-2 and COVID-19 ResearchCOVID-19 epidemiological studiesInfection Control and Ventilation

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Outbreak Associated with SARS-CoV-2 B.1.617.2 (Delta) Variant in an Elementary School — Marin County, California, May–June 2021

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Lam-Hine, Tracy, McCurdy, Stephen A., Santora, Lisa, Duncan, Lael, Corbett-Detig, Russell, Kapusinszky, Beatrix, Willis, Matthew·2021·MMWR. Morbidity and Mortality Weekly Report
DOI 10.15585/mmwr.mm7035e2PMC PMC8422870
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Abstract

On May 25, 2021, the Marin County Department of Public Health (MCPH) was notified by an elementary school that on May 23, an unvaccinated teacher had reported receiving a positive test result for SARS-CoV-2, the virus that causes COVID-19. The teacher reported becoming symptomatic on May 19, but continued to work for 2 days before receiving a test on May 21. On occasion during this time, the teacher read aloud unmasked to the class despite school requirements to mask while indoors. Beginning May 23, additional cases of COVID-19 were reported among other staff members, students, parents, and siblings connected to the school. To characterize the outbreak, on May 26, MCPH initiated case investigation and contact tracing that included whole genome sequencing (WGS) of available specimens. A total of 27 cases were identified, including that of the teacher. During May 23-26, among the teacher's 24 students, 22 students, all ineligible for vaccination because of age, received testing for SARS-CoV-2; 12 received positive test results. The attack rate in the two rows seated closest to the teacher's desk was 80% (eight of 10) and was 28% (four of 14) in the three back rows (Fisher's exact test; p = 0.036). During May 24-June 1, six of 18 students in a separate grade at the school, all also too young for vaccination, received positive SARS-CoV-2 test results. Eight additional cases were also identified, all in parents and siblings of students in these two grades. Among these additional cases, three were in persons fully vaccinated in accordance with CDC recommendations (1). Among the 27 total cases, 22 (81%) persons reported symptoms; the most frequently reported symptoms were fever (41%), cough (33%), headache (26%), and sore throat (26%). WGS of all 18 available specimens identified the B.1.617.2 (Delta) variant. Vaccines are effective against the Delta variant (2), but risk of transmission remains elevated among unvaccinated persons in schools without strict adherence to prevention strategies. In addition to vaccination for eligible persons, strict adherence to nonpharmaceutical prevention strategies, including masking, routine testing, facility ventilation, and staying home when symptomatic, are important to ensure safe in-person learning in schools (3).

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FIGURE 1 Classroom layout and seating chart for 24 students in index patient’s class, by SARS-CoV-2 testing date, result or status, and symptoms — Marin County, California, May–June 2021
FIGURE 1 Classroom layout and seating chart for 24 students in index patient’s class, by SARS-CoV-2 testing date, result or status, and symptoms — Marin County, California, May–June 2021CC BY · PubMed Central
FIGURE 2 Timeline of SARS-CoV-2 illness onset* after onset in the index patient (A) and presumed transmission† pathway (B) among students, siblings, and parents, relative to onset in the index patient — Marin County, California, May 2021 * Symptom onset date or specimen collection date, if asymptomatic. † Presumed transmission based on phylogenetic and epidemiologic analyses.
FIGURE 2 Timeline of SARS-CoV-2 illness onset* after onset in the index patient (A) and presumed transmission† pathway (B) among students, siblings, and parents, relative to onset in the index patient — Marin County, California, May 2021 * Symptom onset date or specimen collection date, if asymptomatic. † Presumed transmission based on phylogenetic and epidemiologic analyses.CC BY · PubMed Central
FIGURE 3 Phylogenetic tree*,† of SARS-CoV-2 whole genome sequences and specimen collection dates§ from a COVID-19 outbreak in an elementary school¶ — Marin County, California, May–June 2021 Abbreviations: SNV = single nucleotide variant; WGS: whole genome sequencing. * Phylogenetic tree was created with UShER, which uses the Fitch–Sankoff algorithm (a maximum parsimony-based phylogenetic placement approach). https://www.nature.com/articles/s41588-021-00862-7 † Specimen for the index patient was not available for WGS and is not included on the phylogenetic tree. § Dates in this diagram reflect the collection date for specimens that underwent WGS; thus dates might differ from those reported in the text for persons whose initial specimens were discarded. ¶ Branches are labeled with SNVs; cases (circles) are color-coded to indicate social relationship within the outbreak and labeled with the collection date for the specimen that was sequenced. Vertical lines represent genetically identical viruses; horizontal lines represent genetic descendants with additional SNVs. All sequenced specimens are classified as the SARS-CoV-2 B.1.617.2 (Delta) variant.
FIGURE 3 Phylogenetic tree*,† of SARS-CoV-2 whole genome sequences and specimen collection dates§ from a COVID-19 outbreak in an elementary school¶ — Marin County, California, May–June 2021 Abbreviations: SNV = single nucleotide variant; WGS: whole genome sequencing. * Phylogenetic tree was created with UShER, which uses the Fitch–Sankoff algorithm (a maximum parsimony-based phylogenetic placement approach). https://www.nature.com/articles/s41588-021-00862-7 † Specimen for the index patient was not available for WGS and is not included on the phylogenetic tree. § Dates in this diagram reflect the collection date for specimens that underwent WGS; thus dates might differ from those reported in the text for persons whose initial specimens were discarded. ¶ Branches are labeled with SNVs; cases (circles) are color-coded to indicate social relationship within the outbreak and labeled with the collection date for the specimen that was sequenced. Vertical lines represent genetically identical viruses; horizontal lines represent genetic descendants with additional SNVs. All sequenced specimens are classified as the SARS-CoV-2 B.1.617.2 (Delta) variant.CC BY · PubMed Central

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Metadata

Authors
Lam-Hine, T., McCurdy, S. A., Santora, L., Duncan, L., Corbett-Detig, R., Kapusinszky, B., & Willis, M.
Corresponding author
Lam-Hine, Tracy
ORCID
Venue
MMWR. Morbidity and Mortality Weekly Report
Publisher
Centers for Disease Control MMWR Office
Published
2021-09-03
Volume
70
Issue
35
Pages
1214-1219
ISSN
  • 0149-2195 (ISSN-L)
  • 1545-861X
DOI
10.15585/mmwr.mm7035e2
PMC
PMC8422870
OpenAlex
W3198028366
Cited by
124
References
4
Citations by year
2026: 4 · 2025: 7 · 2024: 8 · 2023: 20 · 2022: 65 · 2021: 20
FWCI
8.94
Type
journal article
Language
en
Open access
Locations
  • MMWR Morbidity and Mortality Weekly Report (publishedVersion)
  • Europe PMC (PubMed Central) (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • eScholarship (California Digital Library) (submittedVersion, cc-by)
Funders
  • Pfizer
  • California Department of Public Health
  • Moderna
  • Centers for Disease Control and Prevention
Topics
  • Health Sciences › Medicine › Infectious Diseases › SARS-CoV-2 and COVID-19 Research
  • Physical Sciences › Mathematics › Modeling and Simulation › COVID-19 epidemiological studies
  • Health Sciences › Medicine › Pulmonary and Respiratory Medicine › Infection Control and Ventilation
Keywords
COVID-19 outbreak, Delta variant, elementary school, attack rate, whole genome sequencing, contact tracing, non-pharmaceutical interventions, Marin County, children and adolescents, vaccine effectiveness, asymptomatic transmission, household transmission
MeSH terms
School Teachers, COVID-19, SARS-CoV-2, COVID-19 Vaccines, Adult, California, Child, Disease Outbreaks, Humans, Masks, Schools, Contact Tracing

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