How effective is Avaxim with and without the second shot?

A single dose of Avaxim produces a strong antibody response quickly, but the second shot boosts that level dramatically and likely extends protection.

The one retrieved study on Avaxim directly [1] was a 1997 industry-funded comparison of 60 healthy adults. It measured antibody levels, not actual hepatitis A infections. That is a surrogate endpoint: a lab value, not a real-world outcome like hospitalization or jaundice. The study found that a single dose of Avaxim produced neutralizing antibodies by week 2, with geometric mean titers (GMT) rising to about 100 mfU/mL by week 24 [1]. After the second shot at week 24, the GMT jumped to over 3,000 mfU/mL by week 28 [1]. That is a 30-fold increase.

A separate 1999 study on a different inactivated hepatitis A vaccine [2] compared two-dose and three-dose schedules in healthy adults. It found that a two-dose schedule produced 100% seroconversion by week 28, with a GMT of 1,145 mIU/mL one month after the booster [2]. By week 52, that GMT had fallen to 355 mIU/mL [2]. This shows that while a single dose provides initial protection, antibody levels wane over time, and the booster restores them to a much higher peak.

The evidence is thin. No study in this retrieval compared a single dose of Avaxim to a two-dose schedule using hard clinical endpoints like hepatitis A infection rates. The entire case for the second shot rests on antibody kinetics: it produces a much higher antibody peak, which is assumed to translate into longer protection. That assumption is plausible but unproven by the evidence here.

My call: a single dose of Avaxim likely provides good short-term protection, but the second shot produces a much higher antibody level that probably extends the duration of protection. Confidence: moderate, because the evidence relies on surrogate antibody endpoints and no study in this retrieval tested real-world infection rates.