At least 80% must get Covid-19 shots to achieve herd immunity

At least 80 per cent of Singapore's population would have to be vaccinated to achieve herd immunity against Covid-19, said the Health Ministry's chief health scientist, Professor Tan Chorh Chuan.

Assuming the efficacy of a Covid-19 vaccine is 90 per cent, vaccinating 80 per cent of the population would mean that about 72 per cent of them would become protected, he told The Straits Times on Wednesday.

Prof Tan strongly encourages everyone who is suitable to get the vaccination because this would protect them and their loved ones with whom they are in close contact, and reduce the likelihood of spread in Singapore.

"Where most of a population has immunity to the virus, this herd immunity indirectly reduces the risk of infection for those who are not immune to it, thereby limiting infection clusters," he said.

"The estimates for herd immunity vary generally around 60 to 70 per cent of the population, but a vaccine coverage of at least 80 per cent would be prudent."

On Monday, Prime Minister Lee Hsien Loong disclosed Singapore's Covid-19 vaccination plan in a televised address, and said the first shipment of Covid-19 vaccines by United States pharmaceutical giant Pfizer and its German partner BioNTech would arrive before the close of the year.

Priority for the first shipments will be given to healthcare workers, those on the front lines, as well as elderly and vulnerable patients, but there will be enough vaccines for everyone by the third quarter of next year, said the Prime Minister.

Prof Tan said elderly people would include those who are 60 years and older.

"As this group is very large (close to a million people), planning is ongoing for the vaccination roll-out, but it is likely that the elderly in institutional care settings such as nursing homes will be prioritised," he said on Wednesday.

If the elderly and people with multiple health problems are vaccinated, symptomatic infection rates would fall sharply among these groups, who are at much higher risk of severe disease and death, and much more likely to need hospital care if they become infected, he said.

Priority is also given to healthcare workers and front-line personnel, as vaccination would greatly reduce the risk of healthcare services suffering if many healthcare workers get sick and are not able to work.

Front-line personnel would include non-medical workers at healthcare facilities, as well as people who are involved in quarantine operation, or working in border control, Prof Tan said.

"Beyond that, when a high proportion of the total population has been vaccinated, the risk of large outbreaks and super-spreading events would be markedly reduced," he added. Achieving herd immunity against Covid-19 helps protect those who cannot be vaccinated because of their age or their medical conditions.

These include people with a history of anaphylaxis, or severe allergic reactions, who should not receive the Covid-19 vaccine at this time, said Prof Tan, as well as pregnant women.

It may not be advisable for patients who have medical conditions or are on drugs that suppress the immune system to get the vaccine, but more detailed guidance would be provided for this group.

In clinical trials for the vaccine, elderly people had similar side effects as younger adults, such as pain at the injection site, headache, tiredness and fever, but these were less common and milder than for younger people, said Prof Tan.

Singapore also has advance purchase agreements for other vaccines, including those by Moderna in the US and Sinovac in China.