The urgency in tackling the Covid-19 pandemic has seen a quickened process in developing and approving a vaccine, but experts say multiple steps have been taken to ensure that any vaccine rolled out in Singapore is safe.

The first shipment of Covid-19 vaccines arrived in Singapore on Monday night, marking the first time the country is buying in large quantities drugs that have been approved for use in a pandemic.

The vaccine developed by Pfizer and its German partner BioNTech was granted authorisation for use here by the Health Sciences Authority (HSA) under the Pandemic Special Access Route.

It is so far the only vaccine approved by the HSA, although others are being studied.

But even during these unprecedented times, steps have been taken to ensure the safety of vaccines that will be used here, said two experts at a press conference on Monday.

Associate Professor Tan Say Beng and Professor Benjamin Seet, both on an expert panel that advises the Government on therapeutics and vaccines, pointed to numerous safety checks along the vaccine development process.

First, the data must show safety in animal studies before the vaccine is tested on humans.

Data from the clinical trials is also reviewed by the HSA when it assesses whether to approve a vaccine for use here.

Prof Tan, who is executive director of the Ministry of Health's National Medical Research Council, said: "During the whole process, there are many vaccine candidates, and many may drop out along the way.

"Some animal studies show that (a vaccine) may not be safe enough, for example, while others make it to human studies but they don't make it through all the different phases of clinical trials."

He added: "So you can think of it as there being multiple safety checks all along the way."

The first two phases of clinical trials, known as early-phase trials, are mainly carried out among smaller groups of up to hundreds of volunteers to test the safety and efficacy of a vaccine.

In these stages, researchers look out for dangerous side effects and analyse patient samples to see how the human immune system is responding to the vaccine.

Phase three trials are much larger, usually involving thousands to tens of thousands of people. These are often held across multiple jurisdictions or countries.

Prof Tan said: "Many agencies in many countries are confident that the clinical trial process was abided by, even though things were quickened in terms of urgency. It was not at risk of compromising the integrity of the study."

For instance, phase three trials must be sized correctly so that the results are representative of a larger group, he said.

The phase three trial of the Pfizer-BioNTech vaccine involved over 43,000 people.

Another Covid-19 vaccine front runner, developed by Moderna, had a phase three trial involving more than 30,000 people.

But responding to a question on why other reactions to the Pfizer-BioNTech vaccine - such as severe allergic reactions reported in the United States, for instance - were picked up only after those trials, Prof Seet, who oversees the expert panel, said some reactions were very rare.

Such reactions are not unique to Covid-19 vaccines, but apply to all types of drugs or shots, he added.

"Because some side effects are very rare, you may need to involve hundreds of thousands, or sometimes even a million or two million people, before you actually see these very, very rare events, and the only way you can pick that up is actually through post-implementation (monitoring)," he said.

That is why the Government will continue to monitor the situation as the vaccine is rolled out here.

But Prof Seet, who is deputy group chief executive for education and research at the National Healthcare Group, noted that not all side effects were dangerous.

Safety is about whether a vaccine or drug will cause actual harm to an individual, he said.

Tolerability, on the other hand, refers to effects like a fever or headache after a shot.

"So that is quite different from safety. Some people will experience some of these side effects - but that doesn't affect overall safety," he said.

Audrey Tan