A vaccine expert answers your questions about the Covid-19 jab

BC: There are different technologies used to make the vaccines. One of the oldest ways is to make vaccines is to grow the virus and then deactivate it so it is no longer infectious. That is done by using heat and other techniques in a lab. It is then further purified to be safe enough to inject into a person, and your body will generate an immune response to it. This is how Sinovac was made. [This is an inactivated vaccine.]

A more recent technology was used to make AstraZeneca. It uses a common cold virus called adenovirus and modifies it so it has some genetic information from Covid. It then uses that as a way to stimulate people’s immune systems to protect themselves against Covid.

The BioNTech vaccine is clever because it uses that same biological process the virus uses inside a cell to copy itself. When a virus invades our cells, it uses our cells like a factory to make more of the virus – that is the only way it can reproduce. It’s not like bacteria, which has its own cells. This is called an mRNA vaccine. It doesn’t contain the virus itself, but instead contains a single-use “instruction manual” to make a part of the virus which your immune system then responds to.

It's natural to have questions about the vaccine, especially because some of them use new technologies. Photo: EPA

YP : Will the vaccine still be effective against different variants?

BC: In theory, vaccines can be updated to cover those variants. The mRNA vaccine could be updated the quickest by changing the “instruction manual”.

For inactivated vaccines, you first have to grow the virus into large amounts which takes time, and you need to check that the virus you’re growing is growing properly and not changing.

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YP reader Vinky Wu Wing-ki: What are the potential side effects? Is the injection painful, and will it hurt after?

BC: The injection is not painful at all. It’s such a thin needle. After the injection, it’s quite common to feel some minor reaction to vaccination. Maybe your arm will feel sore, you’ll feel a little bit tired, and some people might have a fever, but those are mild.

It’s very unusual to have a major reaction to vaccination. In Hong Kong, we’ve heard of some people going into shock, like some do if they are allergic to peanuts. There might be something in a vaccine that they didn’t realise they were allergic to, or it is a chance event. Those are extremely rare though, 1 in 100,000. You’d have to vaccinate a lot of people to see more of those events.

There’s a very low risk of anything more serious, but there’s always going to be some risk of a mild reaction because you’re injecting your body with something that stimulates your immune system.

Some have reported feeling sore and tired after the second BioNTech injection but other than that, side effects appear to be mild. Photo: SCMP/May Tse

YP: How do you determine the efficacy of different vaccines?

BC: The effectiveness of the vaccine tells you how much your risk is of getting Covid after you’ve been vaccinated. The BioNTech vaccine is 95 per cent effective, meaning your risk of getting Covid is reduced 95 per cent from the initial risk.

Clinical trials determine the efficacy of a vaccine. You must test the vaccine in a large group of volunteers. Half will get the recommended two doses of the vaccine, and the other half will get a saltwater placebo that mimics the process of getting vaccinated. It’s important that those two groups are allocated at random to eliminate differences.

Among those people, they follow up to see who got the virus, and then use a formula to produce a number.

YP: Can you still get Covid after the vaccine?

BC: Yes, you can.

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YP reader Jenny Huang: How long will the vaccine protect someone for? Will you have to get it yearly?

BC: We’ve only been studying Covid for just over a year. We already know that people can be reinfected after their first infection, and we know the vaccines are not 100 per cent [efficient]. As time goes on, we will learn more about how long immunity will last after the recommended two doses of vaccination.

My expectation is that in some people the protection will last for a long time, but for others that immunity will wane a bit more quickly, so it’s possible that we would need booster doses after a year or two.

It’s also possible that there could be a test available to say who should get the booster and who shouldn’t, because their immunity is high.

If there were more variants, and the vaccine from this year doesn’t seem to be that effective, we would get booster doses specifically targeting that variant. There might be different boosters for BioNTech and Sinovac.

The other possibility is that, after the first two doses you get a good level of protection, then you’d get a booster dose after a year, and then other booster doses could be spread out over a few years after.

People queue up for Covid-19 vaccines at the Sha Tin Community Vaccination Centre. Photo: SCMP / Felix Wong

YP reader Noki Wong: Are there any long-term side-effects of the vaccines?

BC: There are very few consequences that are recognised as long-term side effects of this vaccination.

If there were, we wouldn’t choose to use the vaccines because we want them to be safe.

YP : Does the vaccine cause Bell’s palsy?

BC: This is something that’s not fully understood. Bell’s palsy is triggered by an infection, so it’s not necessarily [caused by] the virus or the vaccine, but an immune response to the virus or vaccine. It happens mostly in older people, not children.

It’s quite difficult to disentangle whether it was the kind of thing that would happen anyway, or if it was because of the vaccine, but it is possible to recover from it.

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YP reader Alvin Chan Wai-hei: Is the vaccine suitable for people of all ages – teenagers, young people, middle-aged people and the elderly?

BC: BioNTech is licensed for people aged 16+ [and recently has promising results for people as young as 12], and Sinovac is 18+.

So why are these vaccines not available for children yet? New vaccines are developed and tested in a laboratory first to make sure they are safe, then they are given to a very small number of adults so they can check they are working the way they are supposed to work.

Then, proper clinical trials take place amongst adult volunteers, and then it goes down to teenagers, and after that they will be tested on children.

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YP reader Cheung Hing-yin: Should children get the vaccine?

BC: By the end of this year, we would expect these vaccines to become available for children and teenagers below 16, but we have to see the results of the clinical trials first.

We would expect that the vaccines [will] work better in children and teenagers than they do in adults, because they show a better immune response.

YP reader Angel Zhang: Is there any way to assign a particular vaccine to a specific age group?

BC: Right now in Hong Kong we have a choice of two vaccines. In other places, health authorities have prioritised the most effective vaccines to the most vulnerable people.

There is a case to be made to give the most effective vaccines to people who would benefit the most from that vaccine.