One of the main accomplishments in the 20th century is the significant decrease of morbidity-mortality due to infectious diseases in developed countries.

Except for smallpox and rabies vaccines, specific preventive or therapeutic measures did not perform a valuable role in the second half of this century, when diphteria and tetanus toxoids, and when antibiotics and sulfonamides where discovered.

However, it can be said that these measures, especially systematic vaccinations, have been a contributor to the sharp decrease achieved during recent decades in relation to the incidence of diseases against which they immunise, and above all, in relation to its eradication in the world (smallpox) or in countries or regions (diphteria, poliomelitis, measles).

A vaccine is an antigenic preparation used to produce active immunity to a disease, in order to prevent or ameliorate the effects of infection by any natural or ‘wild’ strain of the organism.

Vaccines may be living, weakened strains of viruses or bacteria which intentionally give rise to inapparent-to-trivial infections. Vaccines may also be killed or inactivated organisms or purified products derived from them.

There are three types of traditional vaccines:

• Inactivated - these are previously virulent micro-organisms that have been killed with chemicals or heat. Examples are vaccines against flu, cholera, plague, and hepatitis A. Most such vaccines may have incomplete or short-lived immune responses and are likely to require booster shots.
• Live, attenuated - these are live micro-organisms that have been cultivated under conditions to disable their virulent properties. They typically provoke durable immunological responses. Examples include yellow fever, measles, rubella, and mumps.
• Toxoids - these are inactivated toxic compounds from micro-organisms in cases where these (rather than the micro-organism itself) causes illness. Examples of toxoid-based vaccines include tetanus and diphteria.

Each vaccine immunises against a specific disease. However, there are combined vaccines, which immunise against more than one disease; in fact they are several vaccines in a single shot. We have combinations such as DPT (immunization against diphtheria, tetanus, and pertussis infections) or MMR vaccine (measles, mumps, rubella).

There are currently 25 vaccines of systematic application (vaccination programmes) or non-systematic (individual application) for the prevention of many diseases that are currently transmitted.

In Catalonia there are systematically administered to the whole population: the DPT vaccine (diphtheria, pertussis, and tetanus) against Haemophilus influenzae type B, oral poliomyelitis vaccine, MMR vaccine (measles, mumps, rubella), meningitis C conjugated vaccine, hepatitis B vaccine (in pre-adolescents). During 1998- 99 a pilot programme for heptatitis A vaccination was implemented, which was designed for 12-year adolescents.

Amongst the most relevant vaccines, which in Catalonia are not applied systematically, vaccination against  pneumococcus, flu, hepatitis B and hepatitis A for groups of risk is to be noted.