All 156 close contacts of a port worker who tested positive for Covid-19 after he was vaccinated have tested negative while in quarantine.

They will be tested again before being discharged.

More than 1,500 workers staying in the infected worker's dormitory in Brani Terminal Avenue have also been cleared of the virus, Transport Minister Ong Ye Kung said yesterday when he provided an update on the case three days after it was reported.

The 23-year-old Indian work permit holder received his first dose of the Covid-19 vaccine on Jan 25 and the second one on Feb 17.

He tested positive on April 7, was immediately isolated and taken to the National Centre for Infectious Diseases.

The Ministry of Health has said that the vaccine doses "likely account" for his lack of symptoms prior to the test on April 7.

Mr Ong yesterday described the case as a reminder that it is possible for vaccinated individuals to be infected. However, he emphasised that vaccines are still very effective in protecting people from the more severe forms of the disease and can "significantly reduce" the likelihood of the infected person passing on the virus to others.

"The case is just one data point, but it reinforced our understanding of the virus and vaccinations," he said. "We continue to strongly encourage everyone to take the vaccine when it is your turn, to protect yourself and others."

The patient is a lashing specialist employed by Seafront Support Company, which provides supply and lashing and unlashing services at Brani Terminal.

His job is to go on board ships to secure and unsecure containers before port cranes move them. Mr Ong said that he and other lashing specialists like him are not allowed to interact with the ship crew.

His case continues to be studied by the health authorities.

The Straits Times spoke to doctors who said the primary goal of vaccination is to prevent Covid-19, not infection, and so the port worker's case was not an instance of the vaccine failing.

This means that those who are vaccinated would have developed immune memory against the Sars-CoV-2 virus such that upon natural infection, the rapid immune response would prevent these individuals from becoming ill.

The instances of an asymptomatically infected patient passing on the disease are also relatively uncommon as they will not sneeze or cough to expel the virus.