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Immunocompromised subjects have a higher risk of infection. Some infectious risks could be controlled by vaccination, carried out according to normal schedules, in the absence of effective curative treatment. According to the type of immunodeficiency and its severity, certain vaccines may be recommended, others should be avoided and still others may be used with no particular risk. Thus, in immuno-compromised subjects, vaccines consisting of inactivated, inert or dead microorganisms are indicated. In contrast, those consisting of live microbes are not recommended for several reasons: 1) there is a risk that the immunocompromised host will be unable to control infection with the vaccine; 2) there is a risk of an increase in viral replication, particularly in HIV-infected subjects and 3) there is a risk that the response of the immune system will be poor. HIV-infected individuals now account for most of the cases of secondary immuno-deficiency, following the emergence of AIDS in 1980. They are increasing in number, especially in developing countries, where antiretroviral treatment is not widely available. In this context, vaccinations against transmissible childhood viral and bacterial diseases (e.g. measles and polio) with a high prevalence is advisable, with modifications according to the risk of contagion and the degree of immunodeficiency. However, these vaccines do not target opportunist infections, the prognosis of which is poor and against which we have no vaccines. Other vaccines are recommended only for immunocompromised subjects visiting countries with specific epidemiological situations (endemic or epidemic diseases) or if vaccination is required by the country visited.

More and more immunocompromised people travel abroad especially in tropical countries where infectious risks are high. Before leaving, these subjects must consult their general practitioner who will determine their fitness in function of type of immunodeficiency, travel destination, availability of medical care at the destination, and possibility of medical evacuation. Counseling should also be provided concerning the precautions necessary to avoid the hazards of exposure to fecal material, venereal disease, insect bites, and sun. Antimalarial drug prophylaxis is the same as for uncompromised subjects. Advising immunocompromised subjects about vaccinations is difficult since there is no consensus on the subject. Administration of inert vaccines is usually recommended but their effectiveness is often diminished and harmful effects have been observed in HIV-infected subjects. Administration of live vaccines is always contraindicated in severely immunocompromised subjects but some live vaccines can be used in moderately immunocompromised subjects. The guidelines for vaccination differ depending on the underlying cause of immunodeficiency: congenital defects, cancer, hemopathy, treatment with immunosuppressors or corticosteroids (transplant patients and patients with systemic disease), HIV-infection, or spleen dysfunction. If there is a high risk of contracting a disease for which vaccination is contraindicated, drug prophylaxis or administration of immunoglobulins can be an alternative. If not, travel should either be postponed or the destination should be changed.