The last sixty years have seen the development of the majority of vaccines discussed. These include both the injectable killed polio vaccine developed by Jonas Salk as well as the live, attenuated oral polio vaccine developed by Albert Sabin. (Remember that a killed vaccine has no risk of transmitting the disease to the vaccine recipient, while an attenuated vaccine is merely weakened and rarely can transmit the actual disease.) Together, these two vaccines decreased the incidence of polio in the United States from over fifty thousand cases to fewer than two hundred in just twelve years.
Salk’s vaccine was the first used in mass vaccination campaigns in the 1950s but was replaced by the Sabin vaccine in the 1960s because the oral vaccine was easier to administer and provided longer-lasting immunity. However, because the oral vaccine was a live (attenuated) vaccine, it had a rare risk of causing polio and full-blown paralysis. For this reason, the oral vaccine was replaced with the injectable vaccine in 2000.
The measles, mumps, and rubella vaccines were introduced in the 1960s and combined into one vaccine in the early 1970s. At first, just one dose of the measles vaccine was recommended, but despite a 95 percent protection rate, epidemics still occurred. It appeared that measles was so contagious that the threshold for herd immunity was higher than 95 percent. Since the second MMR was recommended in the 1980s, the level of protection has increased to over 99 percent, and measles has become a very rare disease in the United States. Not only has the incidence of measles decreased, but the incidence of congenital rubella syndrome dropped from twenty thousand cases in the mid-1960s to fewer than fifty cases a year since 2000.
Also during these two decades, the United Nations put forth a heroic effort to eradicate smallpox from the world. Over the twenty years from 1959 to 1979, smallpox vaccination and quarantine efforts were successful in reducing the number of cases. By 1980, the United Nations declared that the first virus in history had been completely contained and no longer caused human disease.
In the 1980s and the 1990s, several vaccines were approved in rapid succession. The meningitis, hepatitis B, and pneumococcal vaccines were approved in the 1980s, while the HiB, DTaP, chickenpox, and hepatitis A vaccines were approved in the 1990s. The first rotavirus vaccine was approved in the mid-
1990s but removed from the market due to side effects just a few years later.
Since 2000, a pneumococcal vaccine for infants, a live, attenuated flu vaccine, two other rotavirus vaccines, and the first human papillomavirus vaccine have all been approved. In addition, several combination vaccines were also approved. The combination vaccines are important because with the large number of recommended vaccines for children, it is possible for a child to receive six injections at one well-child checkup. Even with the combination vaccines, an average two-month-old child will receive three injections and one oral vaccine at his or her well-child checkup.