SINGAPORE - Two people have tested positive for Covid-19 during a mandatory testing exercise for residents and visitors of Block 506 Hougang Avenue 8.
Nine cases were earlier identified by the Ministry of Health (MOH), bringing the total number of cases to 11 people across at least four different households so far.
Although four of the units with confirmed cases are in the same stack, meaning they share the same unit number but are on different floors, MOH said its initial assessment is that airborne transmission along the stack is highly unlikely.
Polymerase chain reaction (PCR) testing for residents and visitors was conducted at the block last Friday and Saturday, said MOH and the National Development Ministry in a joint statement on Sunday (May 23). As of noon on Sunday, 403 of 405 people had tested negative.
MOH said the cases detected are largely among people under quarantine who later tested positive for the virus.
"This means that they have been isolated early and ring-fenced, and were not likely to be moving around while being infectious.
"Epidemiological investigations are ongoing to determine linkages and the source of transmission," MOH added.
The Hougang block is the only Housing Board block with infected cases seen in more than two households so far, the ministry noted.
PCR tests are also being conducted for all 243 residents of Block 559 Pasir Ris Street 51, as four Covid-19 cases have been detected in two different households in the block.
In its late update on Sunday, MOH said initial investigations have found that the units with confirmed cases do not share the same stack, MOH added, unlike in the Hougang block.
Health Minister addresses common questions on virus
In a Facebook post on Sunday, Health Minister Ong Ye Kung addressed questions that people had on Covid-19, including whether the testing operations at housing blocks mean the outbreak must be bad.
"No," the minister said. "Testing is a very effective way to prevent further spread. You should feel safer when you hear about all these testing ops.
"They help us detect and isolate people early, including persons who are asymptomatic or mildly symptomatic and would otherwise be missed."
The authorities will be doing even more testing and surveillance moving forward, he added.
The minister made the point that Singapore's three-day moving average of total community cases has been hovering around the same number - close to 30 - since May 16.
The same average for unlinked community cases was 7.7 on May 16, 12 on May 18 and six on May 21.
"We are monitoring the situation closely, to see if we are trending up, down or flat," he said.
He also debunked the notion that there is no point getting vaccinated as many vaccinated people are infected.
Mr Ong said there are 78 vaccinated individuals infected in the current outbreak - many of whom are front-line workers - compared with about 300 unvaccinated infections.
"International studies continue to show that vaccinations are very effective in preventing infections and severe illnesses," he said.
On the B1617 variant of the virus being airborne and very contagious, Mr Ong noted that Covid-19 has always been spread when a person coughs, sneezes, talks or sings, through droplets and aerosols in the air.
There are multiple factors that impact the risk of transmission, he said.
"Limited airflow and poor ventilation worsens spread. Large majority of infections are in enclosed places with close interactions without masks," he added.
"Hence we closed restaurants and big gatherings. Which is also why a big proportion of infections now actually come from home. To protect your family members, see a doctor immediately if unwell."
How the virus could have spread in Hougang block
Asked about the units with Covid-19 cases in the same stack at the Hougang block, Associate Professor Hsu Liyang, an infectious diseases expert at the NUS Saw Swee Hock School of Public Health, noted that the ministries have not disclosed whether these are directly above or below one another, or if there are unaffected units in between.
"If the floors are one above the other, then perhaps some quirk of ventilation or air conduction allows aerosol spread," he said.
"If they are in the same stack but on floors five and 10, for example, then it is unlikely."
Infectious diseases expert Leong Hoe Nam from the Rophi Clinic at Mount Elizabeth Novena said lifts are one possible source of transmission within the same HDB block.
"We know, consistently, that enclosed places are at high risk, whether it is buses, planes or cars. Lifts are like vertically moving cars," he said.
Dr Leong pointed to a study published in the International Journal of Infectious Diseases last month that showed how the virus lingered in the air inside a car after it was driven for 15 minutes, with the air conditioning on and the windows closed, by a Covid-19 patient who was not wearing a mask.
The study found that samples of the virus could be detected at least two hours after the drive, suggesting the risk of transmission by aerosolised virus.
"We tend to be more lax in a private enclosed space. We may clear our nasal mucus, blow or dig our noses, or adjust our masks for a breather," Dr Leong said.
These lapses in people's usual behaviour could put them at risk, he added.
On the theory that the virus could be spreading through sewer pipes, Dr Leong said that mode of transmission has been considered many times but has not gained mainstream traction.
"Our toilets have that layer of water that prevents the noxious air from coming out. That is an effective barrier," he said.
In a Facebook post on Sunday, Aljunied GRC MP Gerald Giam said the Aljunied-Hougang Town Council disinfected common corridors and lifts at the Hougang block on Saturday evening as an added precaution, after the MOH swabbing operation at the void deck was completed.
He also said some residents at the block had been unable to receive deliveries such as cooking gas cylinders over the last two days.
"We have contacted the relevant company to encourage them to resume deliveries as soon as possible so that you can cook your meals," he said.