[ksaj]

To be fair, children have to get combined booster tetanus, diphtheria, pertussis and polio shots and combined booster measles, mumps and rubella (MMR) shots for exactly the same reason.

The same holds true for the Twinrix hepatitis A and B vaccine. The first shot gives you partial immunity (measurable but not very effective), and the booster brings it up to a more statistically significant risk level and permanence, which is why two shots are needed a week apart before travelling to certain countries.

A lot of vaccines need boosters.