Understanding the risk of breakthrough infections

Vaccines are an important tool for preventing infectious diseases, illness and death in people of all age groups.1 Globally, it is estimated that vaccination prevents 4-5 million deaths every year.1 They are critical to managing infectious disease outbreaks worldwide and are considered one of the most successful public health interventions.1,2

What does ‘breakthrough infection’ mean?

Although vaccines stop most people from getting seriously ill, no vaccine is 100% effective.3 Some people who have been vaccinated may experience ‘breakthrough infection’ – where they still get infected with the virus despite being fully vaccinated.Importantly, breakthrough infections don’t mean that the vaccine isn’t providing effective protection for the majority of people who have been vaccinated against more serious illness.

When breakthrough infections do occur in vaccinated people, the symptoms are usually milder  and may result in fewer hospitalisations than infections in those who are unvaccinated.4

As more people in a community become vaccinated, this may help reduce the spread of infection.3 However, individuals who have been vaccinated and experience a breakthrough infection may still pass the disease on to others.5

Who is at greatest risk of getting a breakthrough infection?

Breakthrough infections have been reported following vaccination for several diseases, including chicken pox, influenza and COVID-19.4,5,6 Those at greatest risk of breakthrough infection normally include the elderly and people with moderately to severely weakened immune systems. 7

The risk of infection remains much higher for unvaccinated people compared to those who are vaccinated.7

Are breakthrough infections common?

Breakthrough infections become more frequent when there is a lot of illness or virus circulating in a community. When cases increase and virus transmission accelerates, it is more likely that new virus variants will emerge that may be more contagious. These can lead to more infections overall, contributing to an increased frequency of breakthroughs occurring.Read more about virus variants here.

What strategies can help to prevent breakthrough infections?

Ongoing virus surveillance and research is underway to better understand and prevent breakthrough infections and to improve disease management strategies globally.7

Public health measures remain important for reducing the spread of infectious diseases. Strategies to reduce disease transmission generally work well, including handwashing, good ventilation, physical distancing, mask wearing and preventative treatments, such as vaccines.8,9 Vaccinated individuals are advised to take necessary precautions to protect themselves and others, to monitor their health and to seek medical advice if they feel sick.9

Vaccines remain one of our most important and effective defences for protecting against infectious diseases and their consequences.1


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1. World Health Organization. Immunization. Available at https://www.who.int/news-room/facts-in-pictures/detail/immunization. Last accessed September 2021.

2. World Health Organization. Vaccines and Immunization. Available at https://www.who.int/health-topics/vaccines-and-immunization#tab=tab_1. Last accessed September 2021.

3. The Immunisation Advisory Centre. Efficacy and effectiveness. Available at https://www.immune.org.nz/vaccines/efficiency-effectiveness. Last accessed September 2021.

4. Arriola C, Garg S, Anderson EJ, et al. Influenza Vaccination Modifies Disease Severity Among Community-dwelling Adults Hospitalized With Influenza. CID. 2017;65(8):1289–97.

5. Centres for Disease Control and Prevention. Chickenpox (Varicella) For Health Professionals. Available at https://www.cdc.gov/chickenpox/hcp/index.html. Last accessed September 2021.

6. World Health Organization. Vaccine efficacy, effectiveness and protection. Available at https://www.who.int/news-room/feature-stories/detail/vaccine-efficacy-effectiveness-and-protection. Last accessed September 2021.

7. Centres for Disease Control and Prevention. COVID-19 Vaccine Breakthrough Case Investigation and Reporting. Available at https://www.cdc.gov/vaccines/covid-19/health-departments/breakthrough-cases.html. Last accessed September 2021.

8. Ministry of Health – New Zealand. Prevent the spread of infectious disease. Available at: https://www.health.govt.nz/your-health/healthy-living/environmental-health/infectious-disease-prevention-and-control/prevent-spread-infectious-disease. Last accessed September 2021.

9. Centres for Disease Control and Prevention. Prevent Getting Sick – How to Protect Yourself & Others. Available at: https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/prevention.html. Last accessed September 2021.

Veeva ID: Z4-39298
Date of Preparation: October 2021