Young people aged 15-24 years who have accessed a homeless youth service, refuge, or flexible learning center in Melbourne, Australia.

Young people who have been referred to an immunisation nurse at the Young People's Health Service.

Young people who meet eligibility criteria will be considered up-to-date on referral if their immunisation history statement at the time of referral meets the criteria to be considered up to date as detailed in Appendix II in the Study Protocol.

The percentage of the young people in the population who are up-to-date for their age with each vaccine will be described.

Where available, data collected from the Electronic Medical Records (EMR) will include risk factors for vaccine-preventable diseases and relevant social determinants of health based on the Australian Institute of Health and Welfare, the Australian Government Department of Health and the National Centre for Immunisation Research and Surveillance publications.

Participants will be considered at increased risk of vaccine-preventable diseases if the EMR includes documentation of at least one of: a history of identifying as Aboriginal or Torres Strait Islander, of arriving in Australia as a migrant, refugee or asylum seeker, of incarceration, of men who have sex with men, of alcohol or other drug misuse, of smoking cigarettes, of living with Hepatitis C or HIV, of out of home care, of being a young parent, of having non-professional tattoos, or of sex work.

Electronic medical records will be reviewed to record the percentage of participants who were not up-to-date with scheduled vaccines for their age on referral, and commenced a vaccine catch-up plan following referral to an immunisation nurse at the Young People's Health Service.

Vaccines required to complete a vaccine catch-up plan will depend on the age of the participant and will be determined by an immunisation nurse.

A participant will be considered to have commenced an immunisation catch-up plan if they receive at least one immunisation on the plan.