Vaccines are the best tool we have against COVID-19. But how do they protect us? We take a closer look at how your immune system is the hero in COVID-19 protection.

Vaccination is the best protection we have against COVID-19 right now. COVID-19 vaccines are effective against virus variants including Delta. Vaccines are the best tool we have to protect our own health, and that of our friends, families and communities.

But some people may still feel uncertain about getting vaccinated, and that’s understandable. You may not understand how vaccines work. And we’re living through a pandemic – it’s a lot to take in.

We want to arm you with the science you need to make an informed decision about vaccination. So, how do vaccines work? We spoke to our COVID-19 expert, immunologist Dr Daniel Layton, about how vaccines work with our immune systems. And why we still need them even if our immune systems are strong.

What does your immune system do when you’re infected by a virus?

Most people know we have an immune system, and that it’s our immune system that ‘fights off’ infection. But do you know just how complex that immune response is?

SARS-CoV-2 is the virus that causes COVID-19. First, your immune system recognises a virus has entered your body. Then a complex cascade of cells and proteins work together to kill the virus and infected cells.

There are two main weapons in this fight. There are B cells, which produce antibodies that bind to the virus and stop it from entering your cells. And there are cytotoxic T cells, which can kill infected cells and the virus in them.

Your body needs to be able to recognise a virus for the B and T cells to kick into action. If your immune system can’t already recognise a virus, it can take a while longer to respond to the threat.

Vaccines work with our immune systems to fights viruses like SARS-CoV-2. This image shows the two weapons in the fight: T cells and antibodies (produced by B cells).

How do I become immune to a virus?

Once your body has recognised and fought off a virus, it stores some of its arsenal away for the long term. This is in case you are exposed to the virus again.

This is thanks to memory cells. They lie in wait, remembering what the virus looks like. So if you get infected again, your memory cells rapidly start searching out and killing the virus.

This is why researchers found in a peer-reviewed study of more than 9000 patients that only 0.7% of people already infected with COVID-19 got re-infected. This is also how vaccines work to trigger your immune response to recognise and fight the virus.

It was actually Melbourne researchers who published the first description of the immune response to COVID-19 in a patient at the very beginning of the pandemic. 

Why do I need a vaccine if I have a healthy immune system? 

Being healthy and having a healthy immune system is very important. It strengthens your body’s ability to fight off invaders.

But no matter how healthy your immune system is, it still takes a relatively long time for your body to recognise a new threat such as SARS-CoV-2. And then it needs to make enough antibodies and T cells to fight the virus.

It’s in the first two weeks, while you’re building your defences, the virus takes hold and becomes much harder to clear. This increases the severity of the COVID-19 disease.

If you’ve been vaccinated, your memory cells are already raring to go. They can recognise the virus straight away, and quickly whip up an army of antibodies and T cells. They don’t have to start from scratch like an unvaccinated person does. So, you begin to fight the virus far more quickly. This reduces its ability to multiply and cause disease.

How do vaccines work? Why do I need two doses?

It typically takes about two weeks for your immune system to generate strong immunity after vaccination. This is because vaccination mimics your natural immune response. But it doesn’t give you the actual disease.

After your first dose, research shows that your protection fades quickly. It’s only after your second dose that you have long-lasting protection.

So, we need a second dose to boost the numbers of those memory cells. This builds enough of an army to prevent the virus from taking hold.

Do I still need a vaccine if I’ve already had COVID-19?

Yes, it’s a good idea to get vaccinated even if you have already had COVID-19.

Immunity from vaccines or COVID-19 infection are both driven by your body’s natural processes to eliminate a threat. But there are some important differences. A recent study showed vaccine-induced immunity can provide better protection from new COVID-19 variants than immunity from infection can. 

Antibody responses after COVID-19 infection are highly variable. Some people’s immune systems do not mount a great response to the virus on their own. This often opens the door to more variants and repeated infection cycles in the population. 

Getting the vaccine can provide a much more reliable and robust protection.  

Why can I still get COVID-19 after I’ve been vaccinated? 

The memory B and T cells that we store away, waiting to fight SARS-CoV-2, need to be told that you’re under attack. Once this happens, they launch into action.

This means the virus often needs to infect you before your immune system sounds the alert, even if you’ve been vaccinated. But your defences start fighting the virus very quickly, protecting you from severe disease.

So, while you can still contract COVID-19, you won’t get as sick. We’ve seen this in people contracting COVID-19 a second time. This is similar to the flu vaccine – it stops you getting severe disease, but you might still get a mild episode of the flu.

Once you have developed this immune memory, there is a much lower chance you will catch or spread the virus.

How long does COVID-19 immunity last? Will I need a booster shot? 

Unfortunately, we don’t really know yet. Research has shown antibody levels can last months in people who’ve recovered from COVID-19 infection. But some people have lost immunity, and some have caught the virus again. As studies continue, researchers will monitor if there is a need for a booster shot.

Another reason you may need an additional shot would be if the virus changes. It’s normal for viruses like SARS-CoV-2 to mutate. Sometimes these mutations include changes to their protein sequence that hide them from your immune system.

A variant with this change can sometimes ‘escape’ your immunity, because your antibodies and T cells are looking for the old version. Think of it like a lock and key: if you change the lock, the key doesn’t work any more.

So if ‘escape variants’ emerge, our protection could start to diminish, and we will need a variant booster shot. These boosters mimic the new variants and update our protection. While studies and discussions are well underway, it’s too early to know when these might be available in Australia.

What about boosters for immunocompromised people?

When our immune system isn’t fully functional, we often don’t generate a great immune response. ‘Immunocompromised’ refers to people with weakened or damaged immune systems.

People can have a compromised immune system for several reasons. These include having certain autoimmune diseases, or taking certain medicines such as immunosuppressants. Our immune systems even start to slow down with age.

It’s likely immunocompromised people may need a third vaccine dose to give them a protective level of immunity. Official advice is expected on this in the next few weeks.

Why are there different intervals between first and second vaccine doses?

In Australia, the Therapeutic Goods Administration has approved Pfizer vaccine doses at an interval of three weeks. AstraZeneca is approved for between four and 12 weeks. 

Both vaccines were proven to be highly effective in clinical trials. Additional clinical studies of the AstraZeneca showed efficacy increased when there was a longer interval between doses.

In an ongoing COVID-19 outbreak, like Australia is experiencing, this increase may be outweighed by the importance of receiving a second dose as soon as possible to get some protection. That’s why the official advice from the Australian Technical Advisory Group on Immunisation (ATAGI) recommends a shorter four-week interval for the AstraZeneca vaccine for people living in outbreak situations.   


  1. Why do we still use AZ which has a death risk when the other vaccines don’t?

    1. Hi Michael, the likelihood of developing the blood clot condition TTS (thrombosis with thrombocytopenia syndrome) after receiving the AstraZeneca vaccine is so low that cases of blood clots were not observed in the preclinical and phased clinical trials.

      The rate of blood clots in the normal population is almost 100 times greater than the rate of clots related to the AstraZeneca vaccine in Australians.

      More information on blood clots and COVID-19 can be found in our blog:

      Team CSIRO

  2. In the first section of this article (“What does your immune system do when you’re infected by a virus?”) it would have been judicious to explain the devastation caused by the patient’s body triggering a “Cytokine storm” (CS) – a critical life-threating condition requiring intensive care admission and having a quite high mortality. CS is characterized by a clinical presentation of overwhelming systemic inflammation, hyperferritinemia, hemodynamic instability, and multi-organ failure, and if left untreated, it leads to death. The trigger for CS is an uncontrolled immune response resulting in continuous activation and expansion of immune cells, lymphocytes, and macrophages, which produce immense amounts of cytokines, resulting in a cytokine storm.

    Thus, the body acts as its worst enemy via a hyper-active immune response.

  3. Great article, simple to understand. Thanks!

  4. Excellent article. My only criticism is that when talking about why you can still get Covid after vaccination, the numbers of vaccinated people that do get breakthrough infections is not mentioned.
    I believe that figure is only in the region of 30%, but they have good protection against serious illness as you say in the paragraph .
    And that 70% of the immunised remain free of infection as WELL.
    It is a common reason the ‘wary’ use to talk themselves out of having immunisation. ‘” What’s the point ?” “Why take the risk?” Because these wary folk are almost always under 50 yrs, they figure that they would live through a Covid infection anyway.

    One of the other big worries is the speed of development and lack of testing.
    How much time is taken developing seasonal influenza vaccine each year?? Comparisons to that might be useful. I know that in the Southern Hemisphere, we have the advantage there, because the new strains usually appear in Northern Hemisphere.
    One person was relieved to hear that the vaccine was developed and manufactured for Influenza vaccine between the end of Northern winter and May in Southern hemisphere.

  5. Excellent article, I’m sharing it with my family and friends.

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