Immunity Gaps Adults Often Don't Know They Have

Several vaccines now recommended for adults weren't part of routine childhood schedules until relatively recently. Hepatitis B vaccination became standard for infants in the early 1990s, meaning many adults born before that era were never vaccinated. The human papillomavirus (HPV) vaccine was introduced in 2006. Varicella (chickenpox) vaccine arrived in 1995. Adults who grew up before these vaccines existed may have no protection unless they've since received catch-up doses or acquired immunity through infection — and the latter isn't always confirmed without serologic testing.

Receiving a vaccine in childhood doesn't guarantee lasting protection into adulthood. Tetanus and diphtheria immunity, conferred by the DTaP series in early childhood, fades significantly over time — which is why Td or Tdap boosters are recommended every 10 years. Pertussis immunity from the childhood series also wanes, making adults a common but under-recognized source of transmission to vulnerable infants. Some evidence also suggests measles immunity may be lower among adults who received only one dose of the MMR vaccine, which was standard practice before 1989.

Vaccine series that require multiple doses — such as hepatitis B (three doses), HPV (two or three doses depending on age at initiation), or the meningococcal series — are frequently left incomplete. Healthcare transitions during adolescence and young adulthood, periods of uninsurance, or simply losing track of records can interrupt completion. Incompletely vaccinated adults may assume they're protected when they've only received partial coverage. In many cases, interrupted series can be completed without restarting from scratch, but verification requires accurate records.

Immunization guidelines are not static. The Advisory Committee on Immunization Practices (ACIP) regularly updates recommendations, and new vaccines are periodically approved for adult use. Recent years have brought updated guidance on respiratory syncytial virus (RSV) vaccines for older adults, revised pneumococcal vaccine schedules, and new formulations for influenza and COVID-19. Adults who haven't reviewed their vaccine status in several years may be unaware of recommendations that now apply to them based on their current age, health status, or risk factors.

Certain life circumstances introduce vaccine needs that didn't previously apply. Pregnancy brings specific recommendations for Tdap and influenza to protect both the mother and the newborn. A new chronic condition — such as diabetes, heart disease, or asthma — may qualify an adult for pneumococcal or hepatitis B vaccination that isn't universally recommended. International travel can require vaccines against typhoid, yellow fever, or hepatitis A. Healthcare workers, college students in dormitories, and people in close contact with infants all face elevated exposure risks that warrant targeted immunization review.