On April 8, 2020, this report was posted online as an MMWR Early Release.

Investigation of COVID-19 cases in Chicago identified a cluster of 16 confirmed or probable cases, including three deaths, likely resulting from one introduction. Extended family gatherings including a funeral and a birthday party likely facilitated transmission of SARS-CoV-2 in this cluster.

SARS-CoV-2, the virus that causes coronavirus disease 2019 (COVID-19), has spread rapidly around the world since it was first recognized in late 2019. Most early reports of person-to-person SARS-CoV-2 transmission have been among household contacts, where the secondary attack rate has been estimated to exceed 10% (1), in health care facilities (2), and in congregate settings (3). However, widespread community transmission, as is currently being observed in the United States, requires more expansive transmission events between nonhousehold contacts. In February and March 2020, the Chicago Department of Public Health (CDPH) investigated a large, multifamily cluster of COVID-19. Patients with confirmed COVID-19 and their close contacts were interviewed to better understand nonhousehold, community transmission of SARS-CoV-2. This report describes the cluster of 16 cases of confirmed or probable COVID-19, including three deaths, likely resulting from transmission of SARS-CoV-2 at two family gatherings (a funeral and a birthday party). These data support current CDC social distancing recommendations intended to reduce SARS-CoV-2 transmission. U.S residents should follow stay-at-home orders when required by state or local authorities.

During January 1–March 20, 2020, specimens that tested positive for SARS-CoV-2 at hospital, commercial, or public health laboratories were reported to CDPH; each triggered an epidemiologic investigation. Contact tracing interviews were conducted with patients with confirmed COVID-19 using a structured questionnaire designed to identify the date of symptom onset and any person with whom the patient had close contact since that date. The type of contact and setting in which the contact occurred were recorded. Close contacts of patients with confirmed or probable COVID-19 were interviewed and enrolled in active symptom monitoring using Research Electronic Data Capture software (REDCap, version 8.8.0, Vanderbilt University, 2020). Patients were classified as having confirmed COVID-19 if SARS-CoV-2 was detected by real-time reverse transcription–polymerase chain reaction testing of a nasopharyngeal or oropharyngeal specimen. Patients were classified as having probable COVID-19 if they developed new symptoms of fever, cough, or shortness of breath within 14 days of contact with a patient with confirmed or probable COVID-19 but did not undergo laboratory testing (consistent with CDC recommendations,* the Illinois Department of Public Health prioritizes testing for hospitalized patients and other high-risk groups).

In February 2020, a funeral was held for a decedent with a non-COVID-19, nonrespiratory cause of death. A close friend of the bereaved family (patient A1.1) attended the funeral; patients in this investigation were referred to by their family cluster letter (A or B), then by the assumed transmission generation (1–4), and finally, in sequence order within each generation (1–7)† (Figure 1). Patient A1.1 had recently traveled out of state and was experiencing mild respiratory symptoms; he was only tested later as part of the epidemiologic investigation and received a diagnosis of confirmed COVID-19. The evening before the funeral (investigation day 1), patient A1.1 shared a takeout meal, eaten from common serving dishes, with two family members of the decedent (patients B2.1 and B2.2) at their home. At the meal, which lasted approximately 3 hours, and the funeral, which lasted about 2 hours and involved a shared “potluck-style” meal, patient A1.1 also reported embracing family members of the decedent, including patients B2.1, B2.2, B2.3, and B3.1, to express condolences.

Patients B2.1 and B2.2 subsequently developed confirmed COVID-19 with onset of symptoms 2 and 4 days, respectively, after the funeral; patient B2.3 developed probable COVID-19 with symptom onset 6 days after the funeral (investigation day 8). Patient B2.1 was hospitalized on investigation day 11, required endotracheal intubation and mechanical ventilation for acute repiratory failure, and died on investigation day 28. Patients B2.2 and B2.3 were managed as outpatients, and both recovered.

During investigation days 11–14, another family member who had close physical contact with patient A1.1 at the funeral (patient B3.1) visited patient B2.1 on the acute medical inpatient ward, embraced patient B2.1, and provided limited personal care, while wearing no personal protective equipment (PPE). Patient B3.1 developed signs and symptoms consistent with COVID-19, including a fever and cough on investigation day 17, 3 days after last visiting B2.1. Patient B3.1 had also attended the funeral 15 days before symptom onset but described more extensive exposure while visiting patient B2.1 in the hospital.

Three days after the funeral, on investigation day 5, patient A1.1, who was still experiencing mild respiratory symptoms, attended a birthday party attended by nine other family members, hosted in the home of patient A2.1. Close contact between patient A1.1 and all other attendees occurred; patient A1.1 embraced others and shared food at the 3-hour party. Seven party attendees subsequently developed COVID-19 3–7 days after the event (Figure 2), including three with confirmed cases (patients A2.1, A2.2, and A2.3) and four with probable cases (patients A2.4, A2.5, A2.6, and A2.7). Two patients with confirmed COVID-19 (A2.1 and A2.2) were hospitalized; both required endotracheal intubation and mechanical ventilation, and both died. One patient with a confirmed case (A2.3) experienced mild symptoms of cough and subjective low-grade fever, as did the four others who received diagnoses of probable COVID-19. Two attendees did not develop symptoms within 14 days of the birthday party.

Two persons who provided personal care for patient A2.1 without using PPE, including one family member (patient A3.1) and a home care professional (patient C3.1), both developed probable COVID-19. It is likely that patient A3.1 subsequently transmitted SARS-CoV-2 to a household contact (patient A4.1), who did not attend the birthday party, but developed a new onset cough 3 days following unprotected, close contact with patient A3.1 while patient A3.1 was symptomatic.

Three symptomatic birthday party attendees with probable COVID-19 (patients A2.5, A2.6, and A2.7) attended church 6 days after developing their first symptoms (investigation day 17). Another church attendee (patient D3.1, a health care professional) developed confirmed COVID-19 following close contact with patients A2.5, A2.6, and A2.7, including direct conversations, sitting within one row for 90 minutes, and passing the offering plate.

The patients described in this report ranged in age from 5 to 86 years. The three patients who died (patients A2.1, B2.1 and A2.2) were aged >60 years, and all had at least one underlying cardiovascular or respiratory medical condition.