My hep a igg was reactive last year, but turned nonreactive this year. shouldn't the immunity last for life? do i need another booster shot before i travel overseas? could this be a lab error?

Natural immunity with Hep A renders a lifelong protection. So if you had positive Hep A IgG last year due to previous exposure, the current test may be falsely negative. However, if you were vaccinated long time ago, the antibody will not persists indefinitely, it will diminish and undetectable at certain time after the vaccination, usually it lasts more than 20 years in healthy people.