Covid-19 vaccines: Difference between Pfizer & Sinovac, explained

MS Explains: Countries around the world are racing to get their hands on Covid-19 vaccines. How do they work and how safe are they? We find out.

Joshua Lee | February 25, 2021, 12:25 PM

Singapore has received the first shipment of Sinovac's Covid-19 vaccine, CoronaVac.

As of February 24, the vaccine is not yet approved for pandemic use by the Health Sciences Authority (HSA).

There are currently two vaccines that have been approved in Singapore: Pfizer-BioNTech's BNT162b2 and Moderna's mRNA-1273.

All three vaccines — by Sinovac, Pfizer-BioNTech, and Moderna — were secured by the Singapore government last year via Advance Purchase Agreements.

Covid-19 vaccines generally work by triggering the body's immune response to the virus that causes the disease — SARS-CoV-2.

The body then remembers it and activates the same response when it is infected.

However, different vaccines trigger the body's immune response differently.

The Covid-19 vaccine by Sinovac uses an inactivated virus, while those by Pfizer-BioNTech and Moderna utilise mRNA technology.

What is an inactivated vaccine?

Inactivated vaccines use a "killed" version of the virus, which is treated with UV light or chemicals so that it cannot cause disease.

The killed virus is then introduced into the body so that its antigens (the components that stimulate the immune system) will trigger an immune response.

Here's an analogy to explain how inactivated vaccines work.

A group of gangsters (SARS-CoV-2, the virus that causes Covid-19) has been terrorising your neighbourhood.

You want to protect your house (your body) from these intruders, all of whom sport the same spiky hairstyle, which, you come to learn, is the trademark of this particular gang.

The police finds one of the gang members, clubs him on the head, and drags him all the way into your house.

You bring this unconscious intruder to your dog (your immune system), and train it to bite people with the same spiky hairstyle.

The next time one of these gang members visits your house, the hope is that your dog will attack him with gusto automatically.

This method is also used in vaccines that combat polio and rabies.

Inactivated vaccines are typically not as strong as live vaccines, so typically, several doses are needed over time.

This is why some vaccines are administered with an initial dose, followed by booster shots later on.

A 3D print of SARS-CoV-2. The entire virus — albeit not a live one — is used in inactivated vaccines. Image via.

CoronaVac: Mild symptoms, but efficacy inconsistent

CoronaVac by Sinovac is an example of an inactivated vaccine.

One of CoronaVac's advantages is that it can be stored in a normal refrigerator at 2-8°C without the need for for super-cool storage like those used for mRNA vaccines (elaborated more below).

While the Ministry of Health (MOH) has said that the HSA is currently waiting to assess the safety and efficacy of CoronaVac, different trials for the vaccine have produced inconsistent efficacy results.

CoronaVac's clinical trial in Bandung, the capital of West Java province, yielded an efficacy rate of 65.3 per cent.

The clinical trial in Turkey, however, produced an efficacy rate of 91.25 per cent.

Data from a Brazilian clinical trial pegged CoronaVac's effectiveness at 50.4 per cent.

Sinovac has defended the efficacy of its vaccine, saying that the two-dose regime has to be administered within a long time frame — about three weeks apart.

The side effects of the CoronaVac are reportedly mild. A study conducted on volunteers in Jiangsu, China from April 2020 to May 2020 found that most participants reported mild side effects, with the most common being pain at the injection site.

One participant in the study developed hives after the first dose. The researchers opined that it could be related to the vaccine.

However, this participant did not develop a similar reaction after the second dose.

What is an mRNA vaccine?

Covid-19 mRNA vaccines (or, messenger RNA vaccines) use material that is genetically coded to instruct our cells to produce the spike protein unique to SARS-CoV-2.

Once the vaccine is injected, our muscle cells produce the protein, triggering our immune system to launch an attack.

If we get infected with SARS-CoV-2 in future, our body will remember how to fight the virus.

With an mRNA vaccine, the immune system is able to have a "preview" of what the Covid-19 virus would look like, without actually getting the disease.

Continuing the same analogy from above:

Let's say you don't have the time to go out and find a gangster to train your dog (a.k.a your immune system).

You decide to train your dog on your own. So, you get hold of the particular brand of hair wax that these gangsters use, and learn how to style your hair in the same way.

You then (carefully) train your dog to attack you, and in the process, learn to attack people with spiky hair — all without bringing any gangster into your house.

Hopefully the dog will attack the next gangster who visits your house.

Pfizer & Moderna: High efficacy

One advantage of mRNA vaccines is that they can be produced in less time compared to the more traditional vector vaccines.

However, mRNA vaccines are notoriously unstable.

They have to be kept at super-cool temperatures (Moderna's vaccine needs to be stored at -20°C and Pfizer's vaccine at -70°C) and once in the body, they can be easily broken down by the immune system before they reach their target.

If you're interested, here's a video by Moderna on how mRNA technology works.

Of course, the more important question is: Are mRNA vaccines effective?

The Pfizer-BioNTech vaccine has an efficacy rate of 95 per cent. It has been approved for individuals aged 16 and above.

An Israeli study has also found that the Pfizer-BioNTech vaccine can reach efficacy levels of 85 per cent just after one dose.

The Moderna vaccine, on the other hand, is 94 per cent. It has been approved for use in individuals age 18 and above.

A few severe reactions experienced

At least three people in Singapore have experienced anaphylaxis (a severe allergic reaction) after receiving the Pfizer-BioNTech vaccine.

The three individuals — all in their 20s and 30s — experienced multiple symptoms including rash, breathlessness, lip swelling, throat tightness and giddiness.

They were tended to quickly and all three recovered.

Mild symptoms like injection site pain and swelling, fever, headache, fatigue, body aches, giddiness, nausea have also been reported.

Generally, these are common symptoms with all vaccines (not just Covid-19 vaccines) and they reflect the body's immune system responding to the vaccine dose.

Are the vaccines safe to use?

Vaccines typically take eight to 10 years to go through development and rigorous testing before they reach the market.

The entire development and testing process for Covid-19 vaccines has been compressed to under a year because we are now dealing with a global health emergency.

There is not much info on these vaccines' effect on vulnerable sub-groups like pregnant women, and young children, and those with compromised immune systems (for instance, those living with HIV).

It is also impossible to tell whether these vaccines will have long-term effects, since none of them are over a year old.

But for many countries now, the potential benefits outweigh the possible risks because there is an urgent need for mass vaccination.

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Mothership Explains is a series where we dig deep into the important, interesting, and confusing going-ons in our world and try to, well, explain them.

This series aims to provide in-depth, easy-to-understand explanations to keep our readers up to date on not just what is going on in the world, but also the "why's".

Top image credit: Bioworld and Wikipedia.