Undervaccinated

Undervaccinated is a public health term used to describe individuals or populations whose vaccination status does not meet the recommended immunization schedule for their age and risk factors. It includes people who have never received certain vaccines, those who have started but not completed a vaccine series, and those who are missing required boosters. In some uses, it is distinguished from being unvaccinated, since partial protection may exist for some vaccines.

Measurement of undervaccination relies on vaccination coverage data. Public health agencies assess what proportion of a population is up to date with recommended vaccines or has completed the necessary series by a given age. Data quality depends on record-keeping, reporting, and population mobility, and estimates can vary by location and subgroups.

Causes and risk factors are varied. Barriers to access, such as cost, transportation, or inconvenient clinic hours, can lead to undervaccination. Medical contraindications or delayed schedules also play a role, as do hesitancy and misinformation. Social, cultural, and logistical factors—such as rural residence, lower health literacy, or limited contact with healthcare systems—can contribute to undervaccination in certain communities.

Implications include increased susceptibility to vaccine-preventable diseases and a higher risk of localized outbreaks. If coverage falls below thresholds needed for herd immunity, transmission can continue, affecting those who cannot be vaccinated or for whom vaccines are less effective.

Prevention and response strategies focus on catch-up vaccination, reminder-recall systems, reducing access barriers, and targeted outreach. School-entry requirements and clear, evidence-based communication are commonly employed to increase vaccination uptake.