SINGAPORE – It has been about three years since Sars-CoV-2, the virus that causes Covid-19, made its presence felt in the world.

Bivalent vaccines, which have been reformulated to target new variants, are now readily available here. The Straits Times looks at some of the questions that you may have about booster shots.

Q: I am not keen on getting another booster shot but I do not want to risk becoming very sick if I become infected. Is it safe to skip it?

A: The benefits of getting a booster shot every year are clear for those who are 80 and above, as well as those with severely compromised immune systems. These persons are at high risk of developing severe Covid-19 disease. They should go for regular booster shots to protect themselves, said Associate Professor Hsu Li Yang, an expert on infectious diseases.

The benefits of repeated boosters are not that clear for those who are young and healthy, because they have already been vaccinated and are at low risk of developing severe disease in the first place, he noted. 

Earlier in the week, Professor Ooi Eng Eong, an expert in emerging infectious diseases at Duke-NUS Medical School, was quoted as saying that people here who have received their first booster shot are well-protected against severe Covid-19, even with new variants.

The benefit of getting a fourth shot is minuscule for most people, except the elderly, those with poorly controlled diabetes, transplant patients and other vulnerable groups, he added.

Prof Hsu, who is vice-dean of global health at National University of Singapore Saw Swee Hock School of Public Health, said the protection of three mRNA jabs (or four Sinovac ones) lasts for a few months, after which people can get infected again.

The protection against severe disease also wanes over time but much more slowly, he added.

He said the bivalent vaccines do not offer a more significant boost than the original vaccines, partly because the “new” variants that they target are no longer the main circulating strains in the world.

Q: What about those in their 40s or 50s who are living with elderly parents in their 70s and 80s? Should they take a bivalent booster?

A: Generally, those below 60 are at lower risk of severe disease after the initial vaccination doses and a booster because they are fairly healthy, said Prof Hsu.

“So at an individual level, they derive little benefit from boosters other than being protected against infection for a short few months,” he added.

“However, this protection also means that they are far less likely to transmit Covid-19 to their close contacts, including elderly parents or immunocompromised household members.”

So it will still be good to get regular vaccine boosters for that reason, he said.

Q: Is it possible to get a new side effect from a second booster shot?

A: Covid-19 vaccines are one of the best studied vaccines in the world, and no new side effects have been reported with additional doses. In fact, the risk of side effects, especially the serious ones, drops with subsequent vaccine doses, said Prof Hsu.

The side effects are mostly mild, with the common ones being pain at the injection site, fatigue or fever. These typically resolve within a day or two, he noted.

Q: What about the risk of getting a stroke after taking a booster shot?

A: While there was such a concern in the United States with the Pfizer bivalent vaccine initially, the signal disappeared after millions of people were vaccinated.

“It had most likely occurred by statistical chance, and data from a higher number of vaccinations helped dispel that early signal,” said Prof Hsu.

The Ministry of Health has also said that it has not seen an increased risk of strokes among those who have received the Pfizer bivalent vaccine.