Some vaccinations require more than one dose to maintain long-term immunity. The influenza vaccine, for example, is reformulated each year in an effort to protect people from the most common circulating strains of flu - a virus that is constantly changing. A tetanus vaccination offers protection for 10 years, which is why adolescents and adults must continue to receive booster doses throughout their lives.
Even though adolescents may have been vaccinated as children, immunity to certain diseases can decrease over time; this is called waning immunity. Hence, booster doses are recommended for pre-teens and adolescents. There are also new vaccines that have been developed, which means there are some recommended vaccinations that were not available when these adolescents received vaccines as infants.
Adolescents are more vulnerable than younger children to exposure to such diseases as Human papillomavirus (HPV). If an adolescent is behind on their vaccinations, there is a catch-up immunisation schedule that can be followed to protect him or her from these diseases.