Hepatitis is an infection of the liver. The two types we mostly worry about, Hepatitis A and Hepatitis B are both caused by viruses. Both are present throughout the world, developed and undeveloped, but the developed countries have more mechanisms in place to prevent their spread.
Hepatitis A, which used to be called "infectious hepatitis" is usually contracted through drinking contaminated water. It often causes fairly mild disease in children so a lot of older adults may have had it as a child and never known it. In adults it tends to be more severe. Hepatitis B is blood borne. It is spread through sexual intercourse, blood transfusions, contaminated needles, etc. It is usually considered a more serious disease because it can stay in the liver for a lifetime constantly causing disease and finally death via a number of mechanisms.
There are various immunizations now available to prevent these diseases.
For Hepatitis A there are two types--passive immunization and active immunization. Active immunization means that one receives an injection of a weakened or somehow altered strain of the virus and then produces their own antibodies. There are two brands currently available in the U.S. Each requires two doses six months apart to be fully effective but is actually quite protective 2-4 weeks after the first dose. And therein lies the rub. Immunity, once established, lasts for a lifetime. But it takes at least 2 weeks for one's body to produce the necessary antibodies so you should not wait until the last minute to get the shot.
For those who wait until the last minute, the usual vaccination very likely will still be preventive, but some experts believe that passive immunization is preferable. This means using someone else's antibodies. The old gamma globulin shot, also known as immune globulin. It is immediately effective, but often hurts like the dickens when injected and the benefit lasts only 3-5 months until the injectee's body destroys all the circulating antibodies. In this country the shot is quite safe, but since it is made from blood products its safety cannot be guaranteed in some other countries. Therefore if one is going to be abroad more than 5 months most experts recommend that the traveler either get the passive immunization and the active one at the same time (so that the active one kicks in about the time the passive one is wearing off) or take along one's own syringe and needle if expecting to get a booster overseas. Either way, it is not as effective as simply getting the active dose in a timely fashion.
There are two brands of hepatitis A vaccine currently available in the U.S. which may be used interchangeably. (This is NOT true of the hepatitis B vaccines; i.e. the second dose must be given with the same brand as the first.)
After a single dose, 85% of recipients will be immune after 2 weeks and 96% after 4 weeks. A second dose, 6-12 months later, will boost immunity to over 99%. The second dose is also believed to be necessary for long-lasting protection.
As neither of these vaccines has been available for very long, how long the immunity lasts is unknown. Manufacturers recommend boosters after 7 years but experts feel that probably the immunity is good for over 20 years.
Some confusion exists among international travelers because a lower strength, 3-dose regimen of Havrix is marketed in Europe (and used to be in Canada) and re-vaccination after 5 years is recommended. So if someone had their first dose in the U.S. and then gets their second in Europe, they may be confused when they are told they need a third; and the reverse happens when they start in Europe and then move to the U.S.
Older adults may want to simply be tested first to see if they are already immune. Chances are fairly good that they are and then they can avoid the whole debate.
Much the same story holds for Hepatitis B. There are both active and passive forms of immunization, with the same pros and cons. In addition, active immunization requires three shots over a six month period in order to be effective and even then doesn't work for everyone.
Two further concerns are the other types of hepatitis and the people for whom the immunization is ineffective. For people on chemotherapy or other immunosuppressives or people with HIV or other immune deficiencies, the active immunization is less effective. They may have to have a higher dose or simply use passive immunization. Also, standard advice is to not give the active vaccines to pregnant women or nursing mothers. Most people, however, will go ahead and give it if the risk of getting hepatitis is high enough--remembering that in small children hepatitis is often a mild disease anyway.
But there are other forms of hepatitis--C, E, non-A/non-B, etc. For these there are no immunizations. Which is why even people who have had all the necessary shots need to be compulsive about not drinking the water, not having unprotected intercourse, not getting blood transfusions or shots with contaminated needles, etc. Hepatitis E, rapidly spreading throughout the world, is a particular threat to pregnant women. Pregnant travelers, therefore, should be extremely careful to drink only beverages which have been thoroughly boiled, filtered or chlorinated.