In the past, vaccines have been known to be substances that teach the body to recognise and defend itself against infections from harmful pathogens, such as bacteria, viruses and parasites.

This is because they contained a harmless element of the infectious agent that directly stimulates the immune system to mount a response, beginning with the production of antibodies. Vaccines seek to provide active acquired immunity to particular infectious diseases.

They stimulate the body's immune system to prepare itself in the event that infection occurs. Cells that are responsive to the vaccine proliferate in order to manufacture antibodies specific to the provoking agent and also to form 'memory cells'.

Upon encountering the infectious agent a second time, these memory cells are quickly able to deal with the threat by producing sufficient quantities of antibody. Pathogens inside the body are eventually destroyed, thereby thwarting further infection.

Several infectious diseases including smallpox, measles, mumps, rubella, diphtheria, tetanus, whooping cough, tuberculosis and polio are no longer a threat in Europe due to the successful application of vaccines.

This is not different in the case of Ghana and Africa. Low-cost vaccines against preventable diseases have contributed to the reduction in childhood mortality.