Based on all the above, the investigators must stress the concept of "boostability" after a risk exposure: the main target of travel medicine today is to get a sufficient serological response on day 7 after a risk in prevaccinated persons (accelerated immune response through memory cells) and after two post-exposure vaccinations (day 0 and 3). It should also be noted that a schedule of 1 week would be preferable to the current schedule, because it would be less time consuming, would improve compliance and gives less interference with the intake of other prophylaxis measures, e.g. mefloquine. Two recent but small studies from Thailand suggest that an accelerated schedule of three intradermal injections within 1 week interval is as effective and immunogenic as administered within 4 weeks.