Bengaluru: In an update to an earlier study on a Covid-19 outbreak in Guangzhou associated with a restaurant, researchers from the Guangzhou Center for Disease Control and Prevention in China analysed video data from the eatery and ruled out close contact and surface transmission of the virus.
The original Guangzhou restaurant study, carried out by the same institute, had analysed the incidence of an outbreak in an air-conditioned restaurant in Guangzhou during the early days of the pandemic. The study found that multiple individuals across different tables at the restaurant were infected because they were in the path of the air conditioning draft from an infected patient.
The update to the study, published by a different group of researchers, analysed CCTV footage in detail, including 40,000 surface touches and 13,000 episodes of close contact during the duration of the entire meal. The data ruled out both surface transmission and close contact, further supporting the explanation that the disease spread via the air-conditioning draft.
The study was published last week in the Journal of Infection.
How the transmission occurred
On 23 January 2020, a family traveled from Wuhan and arrived in Guangzhou. The next day, one individual (who was infected), had lunch with three other family members at the restaurant. Two other families sat on either side of the index patient’s table.
Later that day, the index patient experienced symptoms. Within 10 days or so, a total of nine others, including three from the same index patient’s table and five from two neighbouring tables, had fallen ill.
Last year’s study had concluded that while infections on the same table can be explained by close contact outside the restaurant, the infected individuals on the other tables — which were placed on either side of the index patient’s table — contracted the virus because they were sitting in the same pathway of the draft from the air conditioning unit as the index patient. Thus, it was a case of long-range airborne transmission.
The study played a key role in highlighting the role of aerosol transmission, as opposed to close contact spread through heavier droplets that travel shorter distances.
It has since been established that the virus does spread as aerosols in air currents over longer ranges, but the primary mode of transmission continues to remain close contact, super spreader events at short range.
What CCTV footage showed
To confirm the nature of long-range transmission by an airborne virus, researchers obtained the full length, high resolution CCTV footage from the restaurant of the day of the meal.
All diners and staff were monitored by three high resolution video cameras. The researchers analysed the footage spanning two and a half hours, from the time the index patient entered the restaurant, to the time the last diner left.
The objective was to establish, or rule out, evidence of close contact or fomite/surface transmission.
The team consisted of six trained analysts, processing the data second by second. They analysed more than 40,000 instances of surface touches and 13,000 episodes of close contacts in the 2.5 hours.
Detailed analysis showed that diners and staff spent 20 per cent of their time in close contact with others, and 90 per cent of the meal time touching surfaces. However, the data showed that no close contact occurred between diners from different tables.
The team concluded that they could safely rule out surface and close contact transmission. They state that the evidence fully supports long-range airborne transmission in the restaurant, as was concluded by the original paper.
(Edited by Manasa Mohan)