Here us what you’ll find on the Centers for Disease Control website in response to the question How long does protection from Hepatitis A vaccine last?
A recent review by an expert panel, which evaluated the projected duration of immunity from vaccination, concluded that protective levels of antibody to HAV could be present for at least 25 years in adults and at least 14–20 years in children.
Regarding questions as to whether someone previously vaccinated with Hep A series (two dose or three dose) needs a booster after a certain number of years since being vaccinated:
There are several studies that have been conducted over the years to determine exactly how long antibodies or “titers” (the things we measure to determine if a vaccination is providing immunity or protection) remain in the blood, post vaccination.
Most data show that the Hepatitis A vaccination series provides immunity and protection for at least 25 years and as many as 40 years in some adults. Based on the data and recommendations, which vary, we are suggesting that people who received a Hepatitis-A vaccination more than 20 years ago get a booster of one dose of Hep-A vaccine to ensure they are protected.
Though some of those vaccinated under this guidance will certainly already be protected by the vaccination they received earlier in life there is simple no way to be certain other than to be tested for the presence of the HAV antibody and titers. The Hepatitis A vaccine is safe and getting a booster, even in the case that a person has the HAV antibodies present from a prior vaccination presents no risk or downside. Therefore the safest and surest way to ensure protection from Hepatitis A infection in those vaccinated more than 20 years prior is to get a booster.
Resources and information can be found at the US Centers for Disease Control website.
Looking at non US sources for Hepatitis A can also be helpful for a greater understanding of disease surveillance and methods of containment and prevention. Below is one such resource from the UK: