SINGAPORE - Singapore kicked off its Covid-19 vaccinations for seniors aged 70 and above last week (Jan 27).

The goal is to provide enough vaccines for all citizens and long-term residents by the third quarter of this year (2021), if there are no unforeseen disruptions to vaccine shipments. Are there benefits to receiving the vaccine early, and how important is it for the elderly to get their shots? Professor Lam Kong Peng, executive director of the Singapore Immunology Network (SIgN) under the Agency for Science, Technology and Research; and Dr Zhang Biyan, SIgN immunology research fellow, answer these questions.

Q: What are the benefits of getting vaccinated early?

A: Vaccinating against a pathogen is akin to training an army to protect the country. Vaccines "teach" your immune cells to recognise the "enemy" pathogen, and arm the immune system with the correct "weapons" (antibodies or cytotoxicity) to defeat the virus.

As with many things, practice makes perfect. The memory cells in the immune system have a knack of inducing stronger and faster responses during secondary encounters with the pathogen. With prior immunity from the vaccine, any future encounters with the pathogen will result in a more effective clearance and can also further strengthen and refine existing immune responses.

This process of building an immunological memory takes time. In the case of Pfizer-BioNTech's vaccine, it takes about two weeks after the second dose, and two doses of the vaccine spaced 21 days apart, to achieve more than 90 per cent protection against the virus. The timeline is similar for Moderna's vaccine with two doses spaced 28 days apart.

The Sars-CoV-2 virus that causes Covid-19 has a long incubation period of up to 14 days and can remain undetected in asymptomatic individuals. In dealing with such a highly contagious and evasive virus, it is almost like fighting an invisible enemy and we must be ready when the enemy strikes.

Q: Why should the elderly take the vaccine?

A: As we age, our immune system wanes. Our immune cells may not be as efficient as when we were younger. Vaccination boosts our immune system and prepares our immune cells for the encounter with the virus.

The elderly are some of the most vulnerable to severe complications if they contract Covid-19. Reports have also indicated a higher mortality rate amongst older Covid-19 patients. For instance, the United States Centres for Disease Control and Prevention (CDC) found that eight out of 10 Covid-19 deaths in the US were adults 65 years and older. Therefore, it is important for seniors to be protected via Covid-19 vaccines.

Q: Is the vaccine effective and safe for the elderly?

A: About 25 per cent of subjects in the phase III clinical trial from both Pfizer-BioNTech and Moderna are above the age of 65. Reliable scientific data from both trials has shown a good safety profile of the vaccines in this age group. Two doses of both vaccines have been proven to be effective in offering more than 90 per cent protection.

In addition, a recent separate smaller study using the Moderna vaccine, and published in the New England Journal of Medicine, found that binding- and neutralising-antibody responses in adults above age 56 appeared to be similar to those previously reported among younger vaccine recipients aged 18 to 55.

As the risk of being severely ill or dying from being infected with Sars-CoV-2 appears to greatly outweigh the risk of an adverse reaction from the vaccination, especially in the high-risk elderly population, seniors should be encouraged to take the vaccine.

Q: Since there is a risk of adverse effects, why should I be vaccinated?

A: Although there have been several reports of anaphylaxis - serious allergic reactions post inoculation with the Covid-19 vaccine - the number of these cases are extremely low: 11.1 cases per million of Pfizer's doses, and 2.5 cases per million of Moderna's doses.

As of Jan 27, there were three cases of anaphylaxis in Singapore. As all who received vaccinations in Singapore are closely monitored, the symptoms in these three people - who are in their 20s and 30s - were promptly detected and treated. All have recovered from the episode and were discharged from the hospital after a day's observation or treatment.

In the event of rare anaphylactic responses to vaccines, safeguards are in place at vaccination sites to provide prompt medical treatment should any adverse events arise.

Due to the continued relative openness of Singapore's economy and dependence on international trade through the aviation and maritime industries, we remain vulnerable to waves of Covid-19 infection for as long as global cases remain high and our population does not have existing immunity to the disease.

With the recent re-emergence of Covid-19 cases in the community, albeit still in relatively low numbers, a successful vaccination programme is likely to significantly prevent the spread of Sars-CoV-2 in our population from reaching epidemic proportions.

The herd immunity threshold for Covid-19 is estimated to be about 60 to 70 per cent. However, the emergence of more contagious variants from the UK (B117) and South Africa (B1351) has prompted an increase in the recommended herd immunity threshold to about 80 to 90 per cent. To reach these high numbers for effective protection of everyone in the population, those suitable for vaccinations should be encouraged to take them.

As viruses cannot survive or replicate outside a living host, successful vaccination programmes globally are also likely to slow down the emergence of new Sars-CoV-2 variants. This is one key piece of ammunition to beat the virus at its game by breaking the transmission chain through effective global vaccination efforts that begin at the national level.

Q: Who should defer taking the vaccine?

A: The Pfizer-BioNTech Covid-19 vaccine is not recommended in Singapore for those who are aged 16 and under, pregnant or severely immunocompromised.

Those with a history of severe allergic reactions, involving anaphylaxis, should not take the vaccine yet. Individuals who are unsure of their suitability to be vaccinated should consult their doctors or health care providers.