In the early 1950s, there were four vaccines: diphtheria, tetanus, pertussis and smallpox. Because three of these vaccines were combined into a single shot (DTP), children received five shots by the time they were 2 years old and not more than one shot at a single visit.
By the mid-1980s, there were seven vaccines: diphtheria, tetanus, pertussis, measles, mumps, rubella and polio. Because six of these vaccines were combined into two shots (DTP and MMR), and one, the polio vaccine, was given by mouth, children still received five shots by the time they were 2 years old and not more than one shot at a single visit.
Since the mid-1980s, many vaccines have been added to the schedule. Hib vaccine was added in the late-1980s. In the 1990s, hepatitis B and varicella vaccines were added and the polio vaccine shot (IPV) replaced the oral polio vaccine (OPV). In the year 2000, the pneumococcal vaccine was added and since 2004 the influenza, hepatitis A and rotavirus vaccines were added. Now, children could receive as many as 24 shots by 2 years of age and five shots in a single visit!