Clinically relevant vaccine-vaccine interactions: a guide for practitioners.

The need for combination vaccines has been recognised for many years. Many children must have 9 or 12 injections in their first year, which places a considerable burden on the child and the health service. Combination vaccines or simultaneously administered vaccines need to generate a protective immune response to all vaccine components that is equivalent to the response when administered separately. This is not always the situation. Many vaccines should not be administered together because of adverse reactions known as vaccine-vaccine interactions, a phenomenon where one vaccine affects another vaccine, thus potentially causing loss of immunogenicity, loss of protective efficacy or induction of adverse reactions. It is important to remember that most vaccine-vaccine interactions are asymptomatic and may only be discovered when the immune status of the vaccine recipient is analysed or when the individual is challenged by the microbe. The interactions may occur because of physical or chemical interactions within the vaccine formulation, interactions between live vaccines or immunological interference. This review summarises known vaccine-vaccine interactions that have been critically analysed and categorised based on their clinical importance.