There are some people who should wait to receive a Covid-19 vaccine, including those with a history of severe allergic reactions, said experts yesterday.

Serious allergies occur in 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, 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.

But the experts speaking at a webinar hosted by The Straits Times yesterday noted that barring these groups, those here offered a Covid-19 vaccine should take it.

Said Prof Lim: "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: "Everyone who is 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. The others 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 for the pharmaceutical company Merck Sharp & Dohme in Singapore and Malaysia.

Those at greatest risk will be given priority, including healthcare workers and front-line personnel, as well as the elderly and vulnerable. Even those with other illnesses, such as heart problems, should get vaccinated 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.

But other groups should wait for more information. They include:

THOSE WITH SEVERE ALLERGIES

Singapore's Health Sciences Authority has 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. But 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 at 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 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.

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."

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 had 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.