How do vaccines affect immunity?

Key facts

  • Vaccines help your adolescent’s immune system to quickly recognise and clear out germs (bacteria and viruses) that can cause serious illness.

  • Vaccines will strengthen your adolescent’s immune system, in the same way exercise strengthens muscles.

  • Vaccination is the safest way to develop immunity to infectious diseases. Allowing adolescents to develop immunity by catching infectious diseases is not safe.

Last updated on 13 August 2025.

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Can vaccines overwhelm an adolescent’s immune system?

No, vaccines can’t overwhelm an adolescent’s immune system. This is because their immune system protects them from thousands of germs every day. From the moment they are born, children are exposed to countless germs (bacteria and viruses) through their skin, noses, throats and guts. Our immune systems are built to deal with this constant exposure to new things, and they learn to recognise and respond to things that are harmful.  

Vaccination does not get in the way of the ability of your adolescent’s immune system to respond to other germs. Even if all the doses on the immunisation schedule were given to an adolescent at the same time, only a small part of their available immune cells would be occupied.1

Would it be better for adolescents to develop their own immunity?

Allowing adolescents to develop immunity by catching infectious diseases is not safe.

Sometimes catching a vaccine preventable disease can protect an adolescent from catching it again. Other diseases, like influenza (flu) or whooping cough (pertussis), can be caught again. Catching any vaccine preventable disease can also make adolescents as well as vulnerable family members (like grandparents or younger siblings) seriously ill in the process.

Vaccination is recommended because it is the safest way to develop immunity. Vaccines are designed to stimulate immunity without causing disease. The side effects of vaccination are usually mild (e.g. a sore arm) and pass quickly, but the diseases vaccines prevent can cause serious illness that requires hospital treatment.

Are the diseases still around anyway?

Yes. Some of the diseases we vaccinate adolescents against have become very rare in Australia because vaccination has stopped them from spreading. We still vaccinate adolescents against these diseases to stop them from coming back.

Some of the diseases adolescents are vaccinated against are still around, such as human papillomavirus (HPV). HPV infection is linked to almost all cervical cancers detected in Australia. Vaccinating your adolescent against HPV is critical to reducing their risk of cervical cancer and other HPV-related diseases such as genital warts and genital, anal and throat (oropharyngeal) cancers.2,3

Vaccinating your adolescent will protect them from diseases that could be brought into Australia by travellers or which they might catch if they travel overseas now or in the future. In 2022, diphtheria – which is more common in nearby countries in Southeast Asia and the Pacific – was also detected in young children in Australia.4

Please note: In SKAI Adolescent, the phrase ‘your adolescent’ refers to all guardian relationships where health decisions for an adolescent fall under your responsibility.

Drafts of this page were reviewed by members of our Consumer Advisory Group.


  1. Offit PA, Quarles J, Gerber MA, et al. Addressing parents’ concerns: do multiple vaccines overwhelm or weaken the infant’s immune system? Pediatrics 2002;109:124-29
  2. Australian Government Department of Health and Aged Care. HPV vaccine – Fact sheet outlining changes under the National Immunisation Program in 2023. 2023. Available from https://www.health.gov.au/resources/publications/hpv-vaccine-fact-sheet-outlining-changes-under-the-national-immunisation-program-in-2023?language=en
  3. Patel C, Brotherton JM, Pillsbury A et al. The impact of 10 years of human papillomavirus (HPV) vaccination in Australia: what additional disease burden will a nonavalent vaccine prevent? Eurosurveillance 2018;23:1700737
  4. Australian Technical Advisory Group on Immunisation. Diphtheria. Australian Immunisation Handbook. Australian Government Department of Health and Aged Care; Canberra: 2025. Available from https://immunisationhandbook.health.gov.au/contents/vaccine-preventable-diseases/diphtheria