Early reports of COVID-19 spreading between people focused on households, health care facilities and group living settings. But for the virus to spread widely through a community, it has to jump between people who don't live together. In February and March 2020, the Chicago Department of Public Health traced exactly that: a cluster of 16 confirmed or probable cases, including three deaths, likely all from a single introduction at two family gatherings.
Image from CDC's report on COVID-19 transmission at family gatherings in Chicago.
How it spread
Investigators interviewed patients and their close contacts, labeling each person by family cluster (A or B) and likely "generation" of infection.
| Event | What happened |
|---|---|
| A funeral (February 2020) | The index patient, a close friend of the bereaved family (A1.1), attended while mildly ill. Three family members fell ill 2, 4 and 6 days later; one (B2.1) was hospitalized, needed a ventilator, and died on day 28 of the investigation |
| A hospital visit (days 11–14) | Another family member (B3.1), who had also been close to A1.1 at the funeral, visited B2.1 in the hospital without protective equipment, embracing the patient and giving some personal care, and developed fever and cough on day 17 |
| A birthday party (day 5, three days after the funeral) | A1.1, still mildly ill, spent 3 hours at a party with nine family members, hugging and sharing food. Seven attendees fell ill 3–7 days later |
| Home caregiving | Two people who cared for an infected partygoer (A2.1) without protective equipment — a relative and a home care professional — fell ill; the relative likely infected a household member who hadn't been at the party |
| Church (day 17) | Three sick partygoers attended services six days after their symptoms began. Another congregant, a health care professional, caught the virus after talking with them, sitting within a row of them for 90 minutes and passing the offering plate |
Timeline of the cluster, by day of investigation. Credit: Centers for Disease Control and Prevention.
The cluster in numbers
- 16 cases: seven confirmed by laboratory test and nine probable — people who developed fever, cough or shortness of breath within 14 days of contact with a case but weren't tested, since Illinois was prioritizing tests for hospitalized and high-risk patients.
- Ages ranged from 5 to 86.
- Three people died — all older than 60, all with at least one underlying heart or lung condition.
- The median time from last contact with a case to first symptoms was 4 days.
Within three weeks of the index patient's mild symptoms, 15 other people were likely infected. The index patient alone apparently infected 10 people — despite having no household contacts, having only mild symptoms, and never seeking care; they were tested only as part of the investigation. So-called super-spreading events had played a big role in SARS and MERS, though their importance for COVID-19 was still debated.
Likely incubation periods for the 15 secondary cases. Credit: Centers for Disease Control and Prevention.
Limitations
- Probable cases weren't tested, so some may have had other illnesses — though influenza-like illness was declining in Chicago at the time.
- No genetic sequencing was available to confirm the links; some people, such as the health care professional, might have been infected elsewhere.
- People without symptoms weren't tested, so the cluster may have been larger.
The lesson
The funeral, the birthday party and church all took place before major social distancing measures. The findings supported CDC's advice to avoid gatherings — even with family — and Illinois's executive order banning public and private gatherings of any size outside a single household, and underscored following stay-at-home orders where required.
Sources
Based on Ghinai I, Woods S, Ritger KA, et al., "Community Transmission of SARS-CoV-2 at Two Family Gatherings — Chicago, Illinois, February–March 2020," Morbidity and Mortality Weekly Report 69(15), Centers for Disease Control and Prevention; a work of the United States government in the public domain. Original report.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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