Restaurant cluster shows how air conditioning helps coronavirus spread, study says

A study of 10 coronavirus patients from three families who dined at the same restaurant in southern China has suggested that air conditioning aided the spread of the virus.

Health authorities believe that Covid-19 is mainly transmitted through respiratory droplets produced when a coronavirus carrier coughs or sneezes.

But the study, conducted by scientists in the southern city of Guangzhou, said droplet transmission alone could not explain the cluster of infections.

"Strong airflow from the air conditioner could have propagated droplets" between three tables, according to the research.

Restaurants should increase the space between tables and improve ventilation to reduce the risk of infection, according to the team of researchers led by Jianyun Lu of the Guangzhou Center for Disease Control and Prevention.

An article about the research has been released ahead of its publication in the July edition of Emerging Infectious Diseases, the open-access and peer-reviewed journal published by the Centers for Disease Control and Prevention of the United States

Family clusters of infections have helped the spread of a pandemic that has now infected more than 2 million people and killed more than 130,000.

The researchers said Guangzhou cluster began with someone who had returned from Wuhan, the central Chinese city where the coronavirus was first reported in December, on January 23. The study identified this person as the first patient of the group.

Without knowing that he was sick, the person had lunch with three family members the next day at a windowless restaurant with an air conditioner on each floor of the 5-story building.

Two other families sat at adjacent tables, with about 3.3 feet in between each table. The diners shared the space for about an hour, the report said.

The first patient had a fever and cough later that day and went to the hospital. Within two weeks, four members of the first patient's family, three members of the second family and two of the third family had become ill with Covid-19.

After an investigation, the researchers said the only known source of exposure for the second and third families was the first patient in the restaurant.

"We conclude that in this outbreak, droplet transmission was prompted by air-conditioned ventilation. The key factor for infection was the direction of the airflow," the report said.

The researchers said their findings were "less consistent" with aerosol transmission " the spread of the virus through tiny droplets that can float in the air and travel longer distances than bigger respiratory droplets.

This mode of transmission could have potentially caused a greater number of infections in the restaurant.

But the other 73 customers who dined on the same floor tested negative for Covid-19 and no staff members of the restaurant were found to be infected, the study said.

And six smear samples from the air conditioner's outlet and inlet all tested nucleotide negative, it said.

The lack of infections in other parts of the restaurant could also be explained by "the lower concentrations of aerosols at greater distances," it added.

The Guangzhou team said that the research had limitations because they did not conduct an experimental study simulating the airborne transmission route.

They also didn't perform blood tests on other family members and diners whose swab tests turned out negative to rule out the possibilities of those people being asymptomatic carriers and the source of the infection.

Separate studies have suggested that the nasal swab tests are more likely to return false negative results on coronavirus carriers who have no symptoms because of the lower viral load in their respiratory specimens.

Copyright (c) 2020. South China Morning Post Publishers Ltd. All rights reserved.