The development of clinical rabies can be prevented through timely immunization after exposure to the infecting agent: preventive vaccination alone implies no complete protection, but it simplifies the post-exposure procedure considerably, as immunoglobulins are no longer needed and less vaccine administrations are scheduled. Pre-exposure prophylaxis consists of an intramuscular (IM)of intradermal (ID) dose given on days 0, 7 and 21 or 28. The development of immunological memory after this vaccination is therefore critical for the establishment of long lasting immunity against rabies in humans. If a booster dose is given 1 year after pre-exposure prophylaxis, subjects segregate themselves into 'good' and 'poor' responders; the former group, who represent 75% of subjects, may not need further booster vaccination for 10 years, whereas the latter may need more frequent boosters.