Ideally, you should have an injection at least two weeks before travel to allow immunity to develop. However, the vaccine may still be advised even if there is less than two weeks before you travel. A second dose of the vaccine 6-12 months after the first gives protection for about 20 years.heck with your practice nurse at least two weeks before you travel to see if you should have this immunisation. Hepatitis A is an illness caused by the hepatitis A virus. The virus mainly causes inflammation of the liver.