Singapore's Covid-19 hospital cluster, reinfected migrant workers and 'viral shedding' - causes for concern?

This Week In Asia spoke to doctors and other health specialists in the city state to see whether the latest developments offered any cause for concern.

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What is the Covid-19 situation in Singapore?

This is in addition to the between 10 and 50 imported cases Singapore has recorded each day as work pass holders and foreign students return to the city state.

Health Minister Gan Kim Yong, citing the worrying increase in local cases coupled with the resurgence of contagion globally, said last week the country was on "heightened alert".

His ministry this week locked down four wards at Tan Tock Seng Hospital, one of Singapore's largest, after 13 infections there were linked to a Filipino nurse who had been fully vaccinated, yet still tested positive for the virus on Tuesday.

All staff at the hospital would be tested, the ministry said, with visitors now barred from entering wards.

This week, it emerged that 27 men living in the same mega-dormitory as a Bangladeshi worker who tested positive last month had also tested positive for the virus. The Health Ministry said five of the men had likely been reinfected. At least two of the 24, who had all previously recovered from Covid-19, later tested negative.

How will it affect the travel bubble with Hong Kong?

But the uptick in infection is unlikely to immediately affect the arrangement, which stipulates that the travel bubble will be suspended if the seven-day rolling average of unlinked cases in either city exceeds five. As of Thursday, Singapore's seven-day rolling average of unlinked cases was one, because 15 of Thursday's infections were traceable. The seven-day average for Hong Kong is currently 0.29 unlinked infections.

Singapore's Education Minister Lawrence Wong, who co-chairs the country's virus task force, when asked on Friday said that the bubble suspension mechanism would stay in place. However, he added that travellers must be "mentally prepared" for the "fluid" situation to change, and said the bubble would not start if Singapore reached the suspension threshold before May 26. 

What about the new hospital cluster?

Dr Ling Li Min, an infectious diseases doctor at Rophi Clinic, said Singapore had been "very fortunate" to avoid an outbreak at a hospital until now, given that a certain level of asymptomatic and mildly symptomatic cases in the community was to be expected.

"The key now is to ensure that there is no further transmission within the hospitals, as there are many patients who are at risk from severe Covid-19 infections," she said. "[This] serves as a reminder that this virus is challenging. While we try our best, inadvertently, small breaches may sometimes occur."

Fellow infectious diseases specialist Dr Leong Hoe Nam, however, said he was "very worried" about the hospital cluster, adding that there had been an "obvious break" in infection control.

He suggested that a 57-year-old Singaporean who had stayed at the hospital on April 18 and tested positive 10 days later could have been the index case for the cluster - an assertion that has not been confirmed by the health ministry.

"The effect was magnified by the patient being a superspreader and he was likely unmasked throughout his stay," Leong said, adding that the presence of elderly patients who for health reasons may not be able to wear a mask or get vaccinated made transmission of the virus in a hospital setting particularly worrisome.

Reinfections? Can you catch the virus more than once?

It is still unclear how long someone who has recovered from Covid-19 is immune to reinfection, as the virus that causes it has not been around for very long. This is unlike measles, for example, which scientists have discovered tends to confer lifetime immunity once a person catches it once.

But a recent study led by Public Health England showed that most people who have had Covid-19 are protected from catching it again for at least five months. It projected that recovered patients have a 83 per cent lower risk of contracting the virus that causes the disease compared with those who had never had it.

US health-oriented news site Stat also has reported that reinfections are rare, but the scientists it cited said more data and analysis was required to determine the likelihood of catching Covid-19 again.

In Singapore, the health ministry has warned that immunity in those who previously had Covid-19 "can wane and caution is required around the infection risks in recovered persons".

The picture is further complicated by new strains of the virus that have emerged in different parts of the world.

What about if you've been vaccinated?

In response to a handful of recent cases in which people have tested positive despite having received a vaccine, Singapore's government said last week that it is possible for vaccinated individuals to catch the virus.

Officials stressed, however, that available vaccines were effective in preventing symptomatic disease for the vast majority of people who received them, and vastly reduced the likelihood of death.

Wong, Singapore's education minister, on Friday urged Singapore residents to not overreact to the news of vaccinated people catching the virus and "make the biggest mistake" of thinking that vaccinations do not work. "I think that would be disastrous because the vaccinations do work," he said.

About 15 per cent of Singapore's 5.7 million population has been fully vaccinated with either the Pfizer-BioNTech or Moderna vaccines, which are currently offered to residents aged 45 or older. Authorities earlier said they believed that the vaccinations could offer protection for 15 to 18 months, following which booster shots may be given.

Why else are recovered patients testing positive again?

Of the 27 migrant workers who tested positive in Singapore recently, health authorities said it is likely that only five were reinfected with the virus, with a further 20 thought to have been shedding viral fragments from an old infection.

Leong, the infectious diseases expert, said this referred to the process of the body trying to expel "dead" bits of virus that were no longer infectious, but which could still yield false positives on polymerase chain reaction (PCR) tests.

Jeremy Lim, an associate professor at the National University of Singapore's Saw Swee Hock School of Public Health, said some recovered patients could shed virus particles "for ages", even after a full clinical recovery.

He noted that Singapore had previously only declared Covid-19 patients fit for discharge from hospital once they were no longer shedding virus fragments, but later dropped this stipulation. "Some may never have stopped shedding dead viral particles. Frankly we just don't know," he said.

PCR tests themselves are not infallible, either - explaining the two migrant workers at the mega-dormitory who recently tested positive for the virus before later returning a negative result.

What kind of threat do mutant strains pose?

So far, Singapore has detected 342 imported infections linked to coronavirus variants, Kenneth Mak, the health ministry's director of medical services, said during a press conference last week.

Mak stressed that there had only been seven locally transmitted cases of the B117 variant and one local infection involving the B. 1.351 strain, with all the other cases detected as part of the quarantine process for incoming travellers.

Health authorities are still encouraging all Singapore residents who can get vaccinated to do so, though Mak noted that the government was also preparing for the possibility that current vaccines may not be as effective against future Covid-19 variants.

He said manufacturers were already looking at producing vaccines that can better combat new virus strains and "when such a product is available, we may plan to make that available as booster doses [among those] that have previously been vaccinated."

Given the surge of cases in India that has been attributed to the more virulent B. 1.617 strain, Singapore - along with several other jurisdictions around the world - has temporarily banned non-resident travellers from India from entering the country.

This article originally appeared on the South China Morning Post (SCMP).

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