The case study, published Wednesday by the Centers for Disease Control and Prevention, is one of the most detailed looks at how covid-19 moves through communities and shows how a single person can set off a chain reaction of infections. The transmissions — traced to a funeral and birthday party held three days apart — took place before major social distancing policies were implemented and may have facilitated transmission of covid-19 to the broader Chicago community. (The CDC also previously investigated how those aboard the Diamond Princess cruise ship that had docked in Japan became infected.)

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The CDC report begins the night before the funeral. The traveling friend — dubbed “Patient A1.1” as the first patient in the first transmission “generation” in the first cluster discovered — shared a takeout meal eaten from common serving dishes with two family members of the deceased at their home. The meal lasted about three hours.

At the funeral, Patient A1.1 hugged the friends who had been at the dinner and other family members to “express condolences.”

Two days later, one of the dinner hosts showed symptoms of the coronavirus. Two days after that, the other host got sick, too. A third family member who had hugged Patient A1.1 at the funeral also got sick.

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Around the same time, Patient A1.1, who was then still experiencing mild respiratory symptoms, attended a birthday party with nine other people. They hugged and shared food at the three-hour party. Seven of the attendees soon became ill.

Within about a week of the onset of symptoms, the condition of the first dinner host deteriorated. The person was hospitalized, put on a ventilator and subsequently died.

Another family member visited the dinner host at the hospital and — without any personal protective equipment, according to the CDC — provided “personal care” and gave hugs. Three days later, that person developed a fever and cough, too.

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Meanwhile, two of the birthday party attendees became critically ill and were put on ventilators. Both died. The five others experienced mild symptoms of cough and low-grade fever.

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While one of the critically ill patients was hospitalized, a family member and a home-care professional who visited that person developed probable covid-19, the CDC said. The visiting family member, in turn, probably transmitted it to a household contact.

Three of the symptomatic birthday party attendees went to church six days after developing their first symptoms. Another church attendee who sat within one row for 90 minutes, talked to them and passed the offering plate with them also developed symptoms.

The CDC hypothesizes that these clusters may have facilitated transmission of covid-19 more broadly in Chicago and that they show why social distancing measures — and in particular, avoiding gatherings with multiple people — have been critical as the virus moved out of retirement communities, cruise ships and other more contained places. In New Orleans, local officials blamed another type of “super-spreading” event, Mardi Gras, for accelerating the transmission there. Outbreaks also have been traced to a French ski resort and an Italian soccer game.

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“[T]hese findings highlight the importance of adhering to current social distancing recommendations, including guidance to avoid any gatherings with persons from multiple households and following state or local stay-at-home orders,” the CDC said.