SINGAPORE - Transmission via contaminated surfaces at the Jurong Fishery Port was possible, though unlikely to be the dominant route of Covid-19 spread, said experts here.

The largest active cluster in Singapore, which has more than 1,000 cases linked to it, began after a number of fishmongers who had visited the fishery port tested positive.

Singapore's director of medical services, Associate Professor Kenneth Mak, said at a July 20 press conference of the multi-ministry task force tackling Covid-19 that the infection was likely introduced from Indonesian or other fishing boats.

However, the exact mechanism of transmission from the fishing boats to stall operators at the port "isn't entirely clear", he had noted then. Still, fomite transmission, which refers to objects or materials likely carrying infection, through contaminated surfaces and at the stalls, remains a strong possibility.

Professor Paul Tambyah, president of the Asia Pacific Society of Clinical Microbiology and Infection, said the outbreaks in the fishery port are a throwback to what happened at the Huanan seafood market in Wuhan, China, where the Sars-CoV-2 virus was first detected.

However, Prof Tambyah noted that while there was extensive environmental contamination detected in the market there, no data is currently available on the environmental samples from the Jurong fishery port.

"If the environmental samples (taken from the Jurong fishery port) were all negative, that would make person to person contact the most likely mode of transmission," he said.

"If, on the other hand, there was extensive environmental contamination like in Wuhan, then surfaces are likely to be more important (in facilitating transmission of the virus)."

The Straits Times has contacted the Ministry of Health on whether environmental samples were taken at the fishery port.

However, Associate Professor Raymond Lin, director of the National Public Health Laboratory at the National Centre for Infectious Diseases, said there is still no conclusive evidence on the relative importance of human contact, airborne spread and surface transmission as modes of transmission.

"In the same way, there has been no consensus among experts on the mode of transmission associated with Wuhan seafood market in 2020," he said.

Pointing to studies done on previous clusters, Prof Tambyah noted that fomite transmission had also been postulated as a possible route of transmission in Singapore's first church cluster.

An infected couple from Wuhan who had attended a service at the Life Church and Missions Singapore in January last year were thought to have sat in the same seats as those who were subsequently infected, thereby seeding the cluster.

Similarly, a study looking at a cluster in a Tampines Housing Board block in July last year did not rule out the possibility of fomite transmission, after two households which had no contact at all, were both infected, he noted.

Professor Dale Fisher, a senior infectious diseases consultant at the National University Hospital, said that while surface transmission is plausible, it is not the most likely in this instance.

Rather, markets are "very favourable sites for amplification" once the virus has been introduced there.

The reasons for this could include crowding, cold surfaces which favour virus survival in the environment, and the possibility of people removing their masks when lifting heavy crates, for instance.

Some may not have acted early if they started exhibiting mild symptoms, he noted.

Assoc Prof Mak had acknowledged during the press conference on July 20 that it is challenging to maintain safe distancing measures due to the port's hot and humid environment and the nature of activities like the carrying of heavy barrels of fish and ice.

Some mask-off activities and close interactions between people would likely have taken place too, he added.

Professor Teo Yik Ying, dean of the National University of Singapore's Saw Swee Hock School of Public Health, said the increased infectivity of the Delta variant could also help to facilitate fomite transmission.

"An individual infected with the Delta variant actually has a much higher viral load in the body, and he is effectively expelling much more virus particles (by)... talking, coughing, or sneezing, thus people nearby are exposed to a much higher viral load," he said.

"Because an infected person is effectively expelling higher amount of viruses, surfaces could now be contaminated with a higher amount of viruses, and thus that facilitates people being exposed through fomite transmission."

Prof Teo also noted that the incubation period of the Delta variant is much more variable and could sometimes be as short as one to two days, compared with the five to 10 days typically observed for the original Sars-CoV-2 virus. This led to a more aggressive spread in a shorter time frame, he added.

So, while the likelihood of Covid-19 transmission through frozen fish or its packaging cannot be ruled out, the experts concurred that the risk of such infection remains relatively low.

"Ultimately, the risk can be made negligible if the person frequenting wet markets or preparing food practises proper hand hygiene," said Prof Teo.