The next steps: COVID-19 vaccine boosters
3 March 2022
Whether it be a cup of coffee in the morning, or relying on the power of an electric bicycle to get up that hill, there are times we all benefit from an extra boost. When it comes to COVID-19 vaccines, staying up to date with your vaccination is an effective way to provide an extra layer of longer lasting protection from the worst effects of COVID-19. These can come in the form of boosters.
COVID-19 vaccines are proven to be very effective at reducing severe illness and death. Waning immunity is normal and expected over time in all vaccines and after infection. Because this is a new virus for our immune system to learn to respond to, immunity wanes by five to six months after the first course of most COVID-19 vaccines (a course usually consists of two doses, but three if someone has a severely weakened immune system鈥攌nown as immunocompromised). Waning immunity, and the appearance of more infectious variants like Omicron, can make breakthrough infections (COVID-19 infection despite being vaccinated) more common, even though vaccines still protect against severe disease and death. When you get a booster vaccine it acts to provide an extra level of immunity to fight the virus. It also acts as a reminder for your immune system on how to fight the virus when you are exposed to it. Booster doses are particularly important for of severe disease if they get COVID-19, such as people with underlying health conditions, and those such as healthcare workers with an increased risk of being exposed to the virus and transmitting it to vulnerable people.
Booster shots are for Australians aged 16 years and older who had their last COVID-19 vaccine at least three months ago. Those who have had COVID-19 itself will have unpredictable levels of immunity so should also get a booster shot once they have recovered. People who are very immunocompromised (for example with cancer or organ transplant) routinely need an additional (third dose) when they are vaccinated, and then are also recommended a booster dose from three months after their third dose. If you鈥檙e unsure how or if this affects you, your health professional can help you to understand how the from Australia鈥檚 Technical Advisory Group on Immunisation (ATAGI) fit your circumstances.
What鈥檚 the difference between a booster dose and a third primary dose?
If someone is severely immunocompromised (that is, has a weakened or malfunctioning immune system), the two doses that everyone else receives may not be quite enough to reach the level of immunity that the general population experiences. So, a third dose is necessary to get immunity closer to a good level. These third vaccine doses are part of an immunocompromised person鈥檚 primary course. This differs from a booster dose, which is considered as an extra top-up dose to extend protection as immunity wanes. Boosters are recommended around three to six months after the primary course of vaccination, whereas third primary doses are required from two months after the second vaccination. Boosters (in this case a fourth dose) are also recommended for those who have had three doses in their primary course. If you鈥檙e immunocompromised, this distinction is important, so that you know if you鈥檒l need three or four doses.What do we mean by an 鈥榠mmune response鈥� to a vaccine?
Vaccination, whether it be for COVID-19 or another disease, works by stimulating what鈥檚 called the to fight an antigen. An antigen is a specific substance that the body recognises as foreign, such as a virus. In the case of COVID-19, vaccination relies on exposing the immune system to a non-harmful version or part of the SARS-CoV-2 virus (the virus causing COVID-19).
Regardless of whether it鈥檚 from a vaccine or an actual virus, your body recognises the antigen as a threat, and triggers the adaptive immune system to get into gear. This involves two important activities by white blood cells called lymphocytes (B and T cells):
- B-cells create that will constantly circulate in the bloodstream and lock onto antigens, neutralising them so they cannot cause damage
- T-cells seek out the antigen and attack it.
The immune system also has a 鈥榤emory鈥�, so when the lymphocytes are activated by an infection or a vaccine they make special memory cells that then will create another strong frontline of defence when they see the antigen (such as SARS-CoV-2) again.
When immunity wanes, that simply means there are fewer circulating antibodies and T-cells present over time. A acts to further stimulate your immune system to produce more lymphocytes and immune memory that is adapted to be even better at the job, providing stronger immunity against COVID-19.
How effective are boosters and third doses?
Boosters are very effective at providing better protection from the harms of COVID-19. , a study of nearly 4.7 million people who received booster doses showed those who had boosters were ten times less likely to get infected with COVID-19 (mostly the Delta variant) and 18 times less likely to have a severe infection than those without a booster (but who had been initially vaccinated). As time passes after a primary course of COVID-19 vaccination it may become more common to get infected with the virus, however, even without getting a booster, vaccines remain . Getting a booster dose makes your immunity to COVID-19 both stronger and longer lasting.
But what about the effectiveness of third doses? If someone is immunocompromised, from a chronic condition or an illness, for example cancer, their immune response may not be able to reach the levels we hope for after vaccination. Third doses aim to create an immune response against COVID-19 as close as possible to the level that the general population will get from two doses. The term 鈥榠mmunocompromised鈥� covers a broad range of people with a broad range of conditions, so the effectiveness of third doses will vary. Research on shows that even if someone has a poor immune response after the first two doses, a third dose may be the trigger to create some antibodies and provide a greater level of immunity. Physical distancing, wearing masks and avoiding high-risk COVID-19 settings become more important for immunocompromised people because their immune response to the virus is weaker.
What is the benefit of using mRNA vaccines for booster shots?
In Australia, the Pfizer (Comirnaty) and Moderna (Spikevax) vaccines are recommended for booster and third doses. The AstraZeneca (Vaxzevria) COVID-19 vaccine is recommended as a booster only in select circumstances.
Most Australians (unless vaccinated overseas) have had either the Pfizer, AstraZeneca or Moderna COVID-19 vaccines for their primary course of vaccination. Studies show that boosting with either Pfizer and Moderna vaccines is safe and stimulates a very strong immune response in people who had Pfizer, AstraZeneca or Moderna for their primary vaccination course. A of seven different vaccines as mix-and-match boosters provided further evidence that these vaccines are highly safe and effective as boosters. As the pandemic evolves, the recommendations for who is eligible for boosters, and when, may change. The vaccine advisory body, ATAGI, posts updates on when there are changes.
There is still more to learn about how long COVID-19 vaccine effectiveness lasts, and how well these vaccines will protect against different variants that emerge. However, it is expected that the more highly vaccinated the global population is, the less likely it is that new variants will emerge. Vaccination plays a major part in providing improved protection against COVID-19 both for individuals and across populations.
The vaccine rollout will continue to develop and adapt as the virus does. Getting vaccinated and following the recommended vaccine schedule is the optimal and safest way to protect against COVID-19.