Pediacel vaccine. Syringe and vials on a counter. This combination vaccine protects against diphtheria, tetanus, whooping cough (pertussis), polio and haemophilus influenzae (HIB). Haemophilus influenzae infections can lead to serious diseases such as meningitis. The vaccine is given in one injection to a child in three doses at two, three and four months of age. When injected, the vaccine stimulates the body's immune system to produce antibodies, but without causing infection. The immune system can then respond quickly against future infections by the viruses.