SINGAPORE - There are some groups of people who should wait to receive a Covid-19 vaccine, including those with a history of severe allergic reactions, said a panel of experts on Thursday (Dec 17).

Serious allergies usually refer to people who, in response to a specific stimulus such as a bee sting or medication, experience swelling around the mouth, eyes or face, have difficulty breathing or experience a serious drop in blood pressure, said Associate Professor Lim Poh Lian, the director of the high-level isolation unit at the National Centre for Infectious Diseases.

Others who belong to certain groups - such as pregnant women, immunocompromised persons and those under the age of 16 - should also hold off on receiving the shots, as large-scale clinical trials have not involved such volunteers. This means there is not yet enough data to evaluate the safety of a Covid-19 vaccine on these groups of people.

But the experts speaking at a webinar hosted by The Straits Times on Thursday (Dec 17) afternoon noted that barring these groups of people, those here who are offered a Covid-19 vaccine should take it, especially as Singapore moves to open up its economy.

Said Prof Lim during the webinar The A-Z of the Covid-19 vaccine: "What we want to do is make sure that people around them are vaccinated."

Added Prof Lim, who is also a member of the expert committee on Covid-19 vaccination appointed by the Health Ministry: "So everyone who's eligible should get the vaccine because we want to protect people who either can't get (the vaccine) or those who might not get as much benefit from the vaccine even if they receive one."

She was one of three experts who spoke at the webinar, moderated by ST senior health correspondent Salma Khalik.

The other two experts were Professor Ooi Eng Eong, an expert in emerging infectious diseases at Duke-NUS Medical School and co-developer of a Covid-19 vaccine; and Mr Ashish Pal, managing director of pharmaceutical company Merck Sharp & Dohme in Singapore and Malaysia.

Who should take the vaccine

Those who are at greatest risk will be given first priority, including healthcare workers and front-line personnel, as well as the elderly and the vulnerable.

Even those with other illnesses, such as heart problems, should get one as clinical trials have evaluated safety among this group.

Prof Ooi said the clinical trials were designed to take into account those who are at risk of severe Covid-19, including those with heart problems, cancer, high blood pressure and diabetes, for example. "So in that sense, what the trial managed to include, we know that the data points towards safety," he said.

Singapore had on Monday (Dec 14) approved the use of the Covid-19 vaccine by Pfizer and BioNTech, following similar approvals in Britain and the United States.

The first shipment is expected to arrive in Singapore by the end of this month, and mass inoculations are already under way elsewhere.

Free Covid-19 vaccinations will be offered to all Singaporeans and long-term residents who are currently here, though they will be voluntary.

Who should not take the vaccine yet

1) Those with severe allergies

Singapore's Health Sciences Authority had advised that those with a history of anaphylaxis, or the rapid onset of severe allergic reactions, should not receive the vaccine, as a precautionary measure.

Such reactions have been observed elsewhere. For instance, The New York Times reported that two healthcare workers in Alaska developed reactions after receiving the Pfizer-BioNTech vaccine this week.

The first worker who had no history of allergies had an anaphylactic reaction and experienced a rash over the face and torso, shortness of breath and an elevated heart rate. She was hospitalised.

The second worker experienced eye puffiness, light-headedness and a scratchy throat - although the hospital said the reaction was not considered anaphylaxis. He was back to normal within an hour and released.

Both workers had experienced reactions within 10 minutes of the vaccination.

This reaction is not unique to the coronavirus shot, said Prof Lim.

"All medications can potentially cause allergies, or even anaphylaxis, which is the more serious form with hypersensitivity, (and) it can happen immediately," she said during the webinar.

For instance, such reactions are known to occur when penicillin is administered.

"But we don't stop using penicillin. We just have to know that it can happen and be prepared for it, to manage the patient safely. So the same is going to be true for the (Covid-19) vaccine," she said.

Prof Lim explained that the expert committee looking into Covid-19 vaccines for Singapore had looked at the information coming in from abroad before recommending that people with serious anaphylactic reactions may want to hold off on getting the Covid-19 vaccine.

"That's perfectly valid because we are trying to be safe. But when we give the vaccine, even someone without a (history of developing a) reaction could have a reaction, and we're going to be putting those safety (measures) in place," she said.

2) Pregnant women and children

Determining whether a vaccine is safe for certain groups of people needs to be guided by data from vaccine studies.

And since studies have not been done to evaluate how the Pfizer-BioNTech Covid-19 vaccine could affect fertility or young children, experts are advising pregnant women and those under 16 to wait for more data before getting inoculated.

Prof Ooi said: "The studies to show that the vaccines can be used safely in people who are planning for pregnancy or who are already pregnant have not been completed. But it doesn't mean that it's not safe, that it'll cause sterility. It does not mean that at all."

Moreover, women who fall in this category will likely not be in the first few groups to be offered a Covid-19 vaccine in Singapore.

Said Prof Lim: "It's in my DNA to be sort of kiasu (very cautious). And so, until the data comes, we're probably going to say hold off until we get more data, because we want to do this as safely as possible."

However, based on her past experiences with vaccines - such as those for Hepatitis A, Hepatitis B or tetanus - Prof Lim said she does not think this vaccine would cause any problems with fertility.

Prof Ooi noted that the goal is to also be able to vaccinate children, who may interact with their grandparents - a group that faces higher risk of developing serious illness when infected.

But he added: "The trials have naturally focused on the more vulnerable population first. But the plan would, of course, eventually be to cover the rest of the population not currently covered by the (late-stage) trial."

3) Immunocompromised persons

Prof Lim noted that being immunocompromised - or having a weakened immune system - is a condition that falls on a spectrum.

"So for example, someone with leukaemia, which is a kind of blood cancer, would be clearly immunocompromised," she noted. Those who have undergone an organ transplant would also be considered as such.

However, many questions remain, she said.

"If they were treated for leukaemia, say, a year ago, are they still immunocompromised? Well, it's probably a spectrum as you recover from chemotherapy."

Prof Lim said that to answer these questions, more data is needed for each specific group of patients.

"So you would actually have to look at the data for dialysis patients, for kidney transplants, for leukaemics... So I think we're just going to have to wait for that data to come in. We are certainly as interested as everyone else who's asking us questions. So we will be looking for the information as quickly as we can."