My father was tested for hbv in the year 1999 and the result came out to be Hbsag negative.So he took vaccines(3 doses at intervals of 1 and 6 months after the 1st dose) as prescribed by a doctor at that time.But he forgot to take the booster dose which he was asked to take after 5 yrs.Now my question is whether the vaccine is still active in his body or he has to go for any booster dose/fresh set of vaccines?