Superspreader events are behind a large proportion of COVID-19 cases and the spread of the SARS-CoV-2 virus generally. Research suggests that between 10 and 20 percent of infected individuals are responsible for around 80 percent of cases. Superspreader events are where a large number of people are infected during a single gathering—the nomination of Amy Coney Barrett to the Supreme Court, held at the White House Rose Garden, being a recent example.
At the start of March, Vietnam had recorded 20 cases of coronavirus. By the end of the month, this figure had increased by almost 250.
In a research letter published in the CDC journal Emerging Infectious Diseases, scientists led by Nguyen Van Vinh Chau, director of the Hospital for Tropical Diseases in Ho Chi Minh City, have documented a superspreader event that took place in the country in the middle of March.
The event took place at a bar in Ho Chi Minh City for St. Patrick’s Day. The venue had two indoor areas, neither of which had any mechanical ventilation. Two of the three entrances tended to be kept closed during opening hours to help the air conditioning system to recycle the indoor air, the report said. While there was an outdoor space, the 43-year-old man had stayed inside during the party, arriving at around 10 p.m. local time and leaving around 2.30 a.m. the next morning.
In the two weeks before his symptoms appeared, he had traveled to Thailand and around Vietnam, between Hanoi and Ho Chi Minh City. He arrived at hospital on March 18 suffering from fever, cough, fatigue, a headache and muscle aches. He tested positive for SARS-CoV-2.
With contact tracing, the researchers were able to find 18 additional confirmed coronavirus cases that resulted from the man attending the St. Patrick’s Day celebration. Twelve of these were at the bar on the evening of the party, with six more being contacts of these people. Of the patients who attended the party, four were in close contact with the man having attended the event with him, and another was a waiter.
The researchers say that two or three of the confirmed cases were asymptomatic but had transmitted the virus to their contacts. None of the 19 confirmed cases from the cluster had symptoms on the night of the party. Five developed symptoms in the days and weeks that followed.
Analysis of 11 of the patients’ viral sequences showed they were either 100 percent identical or extremely similar, “but were different from sequences obtained from other cases in Ho Chi Minh City during the same period.”
The researchers say their results provide more evidence to show asymptomatic transmission of the virus, and how enclosed settings should be considered “high risk” for its spread. “As of September 15, 2020, only 30 cases of locally acquired infection had been reported in Ho Chi Minh City, but this cluster represents the only documented superspreading event,” they wrote.
“Our results emphasize that persons in crowded indoor settings with poor ventilation may be considered to be at high risk for SARS-CoV-2 transmission.”
