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NCID executive director explains how Delta variant is fuelling COVID-19 surge

NCID executive director explains how Delta variant is fuelling COVID-19 surge

Singapore's National Centre for Infectious Diseases.

SINGAPORE: Health Minister Ong Ye Kung said on Friday (Sep 24) that despite Singapore's many safe management measures, the Delta variant "did not follow our script".

"It transmitted through the community and is driving up daily cases much more quickly than we expected, before our ramp-up plans were fully implemented and before our support systems are fully in place," Mr Ong said at a COVID-19 multi-ministry task force virtual press conference where more restrictions were announced.

But Mr Ong said that this is the nature of the virus, and that to avoid putting the healthcare system under strain, Singapore is "tapping on the brakes" to slow the rise in cases even while stepping up plans to increase the number of COVID-19 hospital beds and other measures.

"It is what every country has to contend with and the inevitable path forward, if we want to live with COVID-19 and regain our normal lives. So we need to ride out this wave of transmission as best and as safely as we can."

On Monday, Singapore reverted to groups of two for social gatherings and dining in, and default work-from-home arrangements.

What is so different about the Delta variant and how has it fuelled the surge in cases? Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases (NCID), explains:


In a study comparing the Alpha (B.1.1.7), Beta (B.1.315) and Delta (B.1.617.2) variants, NCID found that Delta was the most transmissible.

Compared to the wild-type virus, all Variants of Concern showed lower Ct values indicating a higher quantity of viral materials using polymerase chain reaction (PCR) tests. 

Ct stands for cycle threshold. PCR involves an amplification of the genetic material from the virus through a series of cycles so that there is sufficient material to be tested.

A lower Ct value means that fewer cycles were needed to create enough viral material for the test, implying that there was more viral material. Of the three Variants of Concern, Delta appeared to have the lowest Ct value.

"There was noticeable prolonged shedding among Delta cases. The study also demonstrated that the Delta cases tended to have pneumonia and more of the Delta variant cases needed oxygen/ICU and (resulted in) death," said Prof Leo.


NCID has examined 1,109 unvaccinated COVID-19 positive patients who were admitted to the centre from February to December 2020 and compared them to 1,113 COVID-19 positive patients who were admitted to NCID between Apr 28 and Aug 26 this year.

Of the 1,113 patients admitted to NCID in 2021, 750 (67.4 per cent) had data on variants and the vast majority (99.6 per cent) were infected with the Delta variant.

A higher proportion of unvaccinated and partially vaccinated COVID-19 patients above the age of 50 years in the 2021 group required supplemental oxygen (31 per cent) compared with the unvaccinated patients of the same age group in 2020 (14.1 per cent).

This suggests that the Delta variant is associated with an increased need for supplemental oxygen in unvaccinated patients as compared to the wild type.

Among the 633 COVID-19 patients 50 years and older who were admitted to NCID from Apr 28 to Aug 26 this year, the proportion of patients who required supplemental oxygen was highest in the unvaccinated group (38.9 per cent) compared with 19.3 per cent of the partially vaccinated group and 3.7 per cent of the fully vaccinated group.

Additionally, 1.7 per cent of the fully vaccinated patients aged 50 years or older required high dependency or intensive care compared with 12.1 per cent among the unvaccinated or partially vaccinated COVID-19 patients of the same age group in 2021.


"The findings show the beneficial effects of vaccine. In addition, there may be partial protection with one dose of vaccine before completing the two-dose regimen," said Prof Leo.

"However, it should be stressed that one should complete full vaccination of two doses for maximum protective effectiveness."

She added: "Generally, fully vaccinated patients had milder illness and most recovered uneventfully, unless they had underlying medical conditions that made them susceptible."

In addition, NCID studies showed that while the Delta variant can escape host immunity and cause infection in some vaccinated individuals, the immune system retains sufficient power to limit viral replication.

This prevents the cascade of hyperinflammation which causes severe infections in the second week of illness, said Prof Leo.

Viral loads decline faster in vaccinated than in unvaccinated individuals. This means a vaccinated person becomes less infectious earlier.


As of Sep 25, 82 per cent of Singapore's population have completed the full regimen of COVID-19 vaccination.

While vaccination is taking place, NCID is working with the Ministry of Health, Ministry of Home Affairs and Ministry of Manpower to monitor long-term immune response in key risk groups.

These include vulnerable elderly from assisted living facilities, frontline personnel and migrant workers. NCID has already enrolled about 1,800 people and the study is expected to cover two years.

This will allow NCID to monitor how a booster vaccine might affect immunity levels to COVID-19. The longer two-year study will also help identify risk factors for infection, and how well the vaccine can protect selected high-risk groups against COVID-19 once it becomes endemic.

Findings so far indicate that antibodies in older individuals are dropping but this does not mean that the vaccine is not useful. The evidence has shown that those above 50 who are vaccinated are substantially less likely to suffer severe illness than those who are not vaccinated.

Singapore has already announced booster shots for seniors aged 50 and above.


There are many factors at play which are compounded by the unpredictable nature of SARS-CoV-2, said Prof Leo.

Like any RNA virus, SARS-CoV-2 has the ability to mutate and to gain advantages to ensure survival. The Delta variant is a typical example that has high transmission capability, allowing the virus to spread with ease.

It can evade Singapore's immunity protection barriers, rendering the vaccine less effective in preventing infection, and lessening the effectiveness in protection against severe illness – although this is less affected compared to protection against infection.

"Given all these challenges, despite more than 80 per cent population vaccine coverage, we are seeing vaccine breakthrough cases and increasingly seeing more cases needing supplementary oxygen," said Prof Leo.

"The current surge of COVID-19 cases reminds us not to be complacent, to not underestimate the virus, to continue to gather more data to learn more about the virus, and to move cautiously and be nimble in our response."


Prof Leo said that specimens from local and imported cases are selected every week for gene sequencing of the entire virus genome.

The sequence information is then analysed by experts using bioinformatics software. The sequences are uploaded to the global SARS-CoV-2 sequence database GISAID, which has more than 3.6 million sequences.

GISAID categorises the groupings of sequences into clades and lineages, and includes identification of critical mutations which may affect the properties of the virus.

"We monitor the variants which are of potential significance, and these are classified by WHO in order of potential for increased transmissibility, disease severity or effect on vaccine efficacy," she said.

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Source: CNA/hm(cy)


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