How COVID-19 vaccines work

Find out how mRNA vaccines work, and what the Pfizer vaccine is made up of.

Last updated: 4 May 2022

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How COVID-19 vaccines protect you

The three approved COVID-19 vaccines in New Zealand – Pfizer, AstraZeneca, and Novavax, stimulate your body’s immune system to produce antibodies that will fight the virus if you’re exposed to it. 

The vaccines help prevent you from getting infected and having COVID-19 symptoms, or severe illness. This means you could have no COVID-19 symptoms or will have much fewer, milder symptoms and recover faster. 

As with any vaccine, being up-to-date with your COVID-19 vaccinations may not mean you’ll be fully protected. However, it is highly effective if people have both doses (and a booster if you’re eligible). That means, if you do catch COVID-19, you’re far less likely to fall seriously ill and end up in hospital.  

Pfizer is the preferred COVID-19 vaccine for use in New Zealand because of its safety and effectiveness profile. 

Why boosters are needed 

While two doses are likely to provide a good degree of protection against severe disease from Omicron for some time, a booster dose offers greater protection. 

Current evidence shows your protection against infection after the primary vaccination course decreases over time. Giving a ‘top up’ vaccine after a primary course (for most people this is 2 doses) helps boost your immunity against COVID-19. 

About boosters

How the Pfizer vaccine works: mRNA vaccines

The Pfizer vaccine (Comirnaty) is an mRNA vaccine that contains the genetic code for an important part of the SARS-CoV-2 (COVID-19) virus called the ‘spike protein’. Spike proteins are the little projections on the surface of the virus.

  1. Once you’ve had the vaccine, your body reads the genetic code and makes copies of the spike protein.
  2. Your immune system detects these spike proteins and learns how to recognise and fight against COVID-19. It knows it needs to attack the virus to protect it from spreading in your body.
  3. The genetic code then gets broken down and removed very quickly and easily by our body. 

The vaccine cannot give you COVID-19

mRNA vaccines do not contain any of the virus that causes COVID-19, or any other live, dead or deactivated viruses.

The vaccine does not affect your DNA

It does not affect or interact with your DNA or genes. mRNA vaccines never enter the nucleus of the cell which is where our DNA is kept.


The Pfizer vaccine is an mRNA vaccine that protects us against COVID-19.

How does it work? Let’s start by looking at the coronavirus.

This is the virus that causes COVID-19.

The virus uses the spikes on its surface to enter our cells.

The spikes of the virus also help our body to identify the virus as an intruder.

The vaccine works by showing our body the spike protein, so our immune system can prepare to quickly spot and attack the virus.

To create the vaccine, scientists created mRNA (or messenger RNA) that contains the recipe for building the spike protein.

This acts as a messenger carrying instructions to our cells.

When we get the vaccine, the mRNA instructs our cells to build copies of the spike protein. They only build the spike protein, not the whole virus.

As our bodies build these copies, our immune systems kick in and create antibodies to fight off the intruders.

We get two doses of the vaccine because the first dose starts to build our immune response and the second dose acts as a booster so our immune system can remember and mount a stronger response when it encounters the virus.

Once the immune system is primed, it will remember the virus for months or even years. If we encounter the virus in the future, the immune system will launch an antibody attack immediately.

Nothing is left behind from the vaccine (our body breaks down the mRNA) and there is no possibility of the vaccine affecting our body’s DNA.

Te reo Māori

He rongoā ārai mate mRNA te rongoā ārai mate Pfizer ka tiaki i a tātou i te Kowheori-19.

He pēhea tana mahi? Me tīmata tātou i te tirotiro ki te mate korona.

E ahu mai ana te KOWHEORI-19 i tēnei huaketo. Ka whakamahi te huaketo i ngā tara o tōna mata ki te kōkuhu atu ki ō tātou pūtau.

Mā ngā tara o te huaketo e āwhina hoki tō tātou tīnana ki te tautohu i te huaketo hei kaiwhakaeke. Ka mahi te rongoā ārai mate mā te whakaatu ki tō tātou tinana te pūmua o te tara (pūmua tara), kia pai ai te whakariterite o tō tātou pūnaha awhikiri ki te tautohu me te patu tere i te huaketo.

Hei waihanga i te rongoā ārai mate, i waihangaia e ngā kaimātai pūtaiao te mRNA (arā, he messenger RNA) he mea pupuri tohutohu mō te hanga i te pūmua tara.

Ka noho tēnei hei kaikarere e kawe tohutohu ana ki ō tātou pūtau.

Kia whiwhi tātou i te rongoā ārai mate, ko tā te mRNA he tohutohu i ō tātou pūtau ki te hanga tārua o te pūmua tara.

Ka hanga ērā i te pūmua tara anake, kaua te katoa o te huaketo.

Nō te hanganga o ēnei tārua i ō tātou tinana, ka whana mai ō tātou pūnaha awhikiri ki te waihanga paturopi hei whawhai i ngā kaiwhakaeke.

Ka whiwhi tātou e rua ngā tukunga o te rongoā ārai mate nā te mea, ko tā te tukunga tuatahi he tīmata noa iho ki te whakapakari i tō tātou ahwikiri ārai mate, ā, ko te tukunga tuarua hei whakakaha ake kia mahara ai tō tātou pūnaha awhikiri me kaha tonu tana urupare i te tūtākinga ki te huaketo.

Kia rite mai te pūnaha awhikiri, e kore e wareware te huaketo mō ētahi marama, ētahi tau rawa rānei.

Ki te tūtaki tātou ki te huaketo hei ngā rā e tū mai nei, ka tere tonu te huaki ā-paturopi a te pūnaha awhikir.

Kāore e whakarērea he paku aha i te rongoā ārai mate (ka whakapopo tō tātou tinana i te mRNA) nō reira e kore rawa e pāngia te pītau ira o tō tātou tinana e te rongoā ārai mate.

mRNA vaccines have been in development for decades

mRNA vaccines have been developed through major international collaboration.

Researchers have studied and worked with mRNA vaccines for decades. This includes studies for vaccines against flu, Zika, rabies and cytomegalovirus (CMV).

Scientists have also researched past coronavirus infections (SARS and MERS). Once scientists identified the coronavirus that causes COVID-19, they could quickly adapt the technology for COVID-19.

Although it’s relatively new technology, this vaccine has gone through all the usual safety checks and regulations.

This includes international clinical trials to help demonstrate the efficacy and safety of the vaccine. The Pfizer vaccine is being used worldwide and continually and closely monitored for effectiveness and safety.

How the COVID-19 vaccines were developed so quickly

How efficacy of the Pfizer vaccine was measured

The efficacy of the Pfizer vaccine (Comirnaty) was measured in two ways.

Phase 1 clinical trial – level of antibodies

The immune response to the vaccine was measured by looking at the level of antibodies in the bloodstream and how well they worked to neutralise the COVID-19 virus in laboratory tests.

Phase two and three clinical trials – vaccine and placebo

The efficacy of the Pfizer vaccine was tested in about 44,000 participants aged 16 years and over where COVID-19 was already circulating in communities. About half of these participants were randomised to receive the vaccine and the other half received a saline placebo.

The trial looked at how many people got COVID-19 symptoms after they were vaccinated compared to how many got COVID-19 after getting the placebo.

Participants had two doses of the vaccine or placebo, getting their second dose within 19 to 42 days after their first dose. They were then closely monitored and evaluated for at least 2 months after their second dose.

Vaccine clinical trials and testing

What’s in the Pfizer vaccine

The Pfizer vaccine (Comirnaty) is a mRNA-based (messenger ribonucleic acid) vaccine.

It does not contain any live, dead or deactivated viruses. There are no animal products in this vaccine.

Summary of ingredients

The Pfizer vaccine contains:

  • messenger RNA encoding SARS-CoV-2 spike protein
  • lipid nanoparticle – a stabilised fat-based bubble to protect and carry the mRNA into our cells
  • salt buffers – to maintain the pH of the vaccine
  • sucrose – to protect the vaccine while in storage.
Full list of ingredients

Active ingredient

30µg of a nucleoside modified messenger RNA encoding the viral spike (S) glycoprotein of SARS-CoV-2


These ingredients make up the lipid nanoparticle which is the transport mechanism for the active ingredient to make it inside a cell without being broken down.

  • (4-hydroxybutyl)azanediyl)bis(hexane-6,1-diyl)bis(2-hexyldecanoate)
  • 2[(polyethylene glycol)-2000]-N,N-ditetradecylacetamide
  • 1,2-distearoyl-sn-glycero-3- phosphocholine
  • cholesterol.


These ingredients help make sure the vaccine pH is close to that of human cells.

  • potassium chloride
  • monobasic potassium phosphate
  • sodium chloride
  • dibasic sodium phosphate dihydrate


This ingredient protects the lipid nanoparticle at very cold temperatures (-80 degrees celsius that the vaccine is stored at).


What the Pfizer vaccine does not contain

The Pfizer vaccine does not contain:

  • animal products
  • antibiotics
  • blood products
  • DNA
  • egg proteins
  • fetal material
  • gluten
  • microchips
  • pork products
  • preservatives
  • soy
  • latex (the vial stopper is made with synthetic rubber – bromobutyl).
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