Timeliness of childhood vaccination in the Federated States of Micronesia

Background: Vaccination coverage is typically measured as the proportion of individuals who have received recommended vaccine doses by the date of assessment. This approach does not provide information about receipt of vaccines by the recommended age, which is critical for ensuring optimal protection from vaccine-preventable diseases (VPDs).

Objective: To assess vaccination timeliness in the Federated States of Micronesia (FSM), and the projected impact of suboptimal vaccination in the event of an outbreak.

Methods: Timeliness of the 4^th dose of diphtheria, tetanus, and acellular pertussis vaccine (DTaP) and 1^st dose of measles, mumps, and rubella vaccine (MMR) among children 24-35 months was assessed in FSM. Both doses are defined as on time if administered from 361 through 395 days in age. Timeliness was calculated by one-way frequency analysis, and dose delays, measured in months after recommended age, were described using inverse Kaplan-Meier analysis. A time-series susceptible-exposed-infected-recovery (TSEIR) model simulated measles outbreaks in populations with on time and late vaccination.

Results: Total coverage for the 4^th dose of DTaP ranged from 36.6% to 98.8%, and for the 1^st dose of MMR ranged from 80.9% to 100.0% across FSM states. On time coverage for the 4^th dose of DTaP ranged from 3.2% to 52.3%, and for the 1^st dose of MMR ranged from 21.1% to 66.9%. Maximum and median dose delays beyond the recommended age varied by state. TSEIR models predicted 10.8-13.7% increases in measles cases during an outbreak based on these delays.

Conclusions: In each of the FSM states, a substantial proportion of children received DTaP and MMR doses outside the recommended timeframe. Children who receive vaccinations later than recommended remain susceptible to VPDs during the period they remain unvaccinated, which may have a substantial impact on health systems during an outbreak. Immunization programs should consider vaccination timeliness in addition to coverage as a measure of susceptibility to VPDs in young children.