Today, there are more vaccine-preventable diseases than ever before. These include a large number of formerly dread diseases which have been permanently eradicated (at least in developed countries) through universal vaccination programs, as well as a number of rapidly mutating viruses (like influenza, or the flu) against which regularly updated vaccines are prepared and are moderately effective.
In general, all vaccines function by giving the immune system a comparatively harmless target to practice against, allowing it to recognize and learn how to eradicate the threat in advance of exposure to the genuine, harmful pathogen. The first vaccine, discovered by Edward Jenner in the 18th century, was actually a virus: he realized that if people were exposed to the harmless cowpox, they would also become immune to its far deadlier cousin smallpox. Today’s vaccines are more systematically and carefully prepared, and typically involve either dead or inactive (crippled, so they cannot reproduce) copies of the virus which are injected into the bloodstream so that the immune system can learn to recognize them as threats. The immune system begins to manufacture antibodies in reaction to the vaccine, which are then useful against the actual virus in the event of an exposure.
In most cases, this process takes some time, and must be done prior to actual exposure – i.e. preventatively, or prophylactically. There are some exceptions to this general rule, however. The rabies vaccine, for instance, is normally given only after exposure has occurred; it is still effective then, but only prior to the development of symptoms. The tetanus vaccine may also be given after exposure, although all people are recommended to get a tetanus booster shot every ten years.
There are a very large number of pathogens which mutate very slowly, if at all, and therefore provide widespread, long-lasting protection. Many of these vaccines are given according to regular schedules to infants and young children, in order to provide life-long protection. Today, such vaccines exist to provide protection against cholera, diphtheria, Haemophilus influenzae (or Hib, a serious type of pneumonia), Hepatitis A, Hepatitis B, Japanese encephalitis, measles, meningococcal disease, mumps, pertussis (whooping cough), pneumococcal conjugate (Streptococcus pneumoniae), polio (poliomyelitis), rotavirus, rubella, smallpox (no longer distributed, since the disease is extinct), typhoid, tuberculosis, and yellow fever.
In addition to the above, some viruses mutate quickly and therefore a single permanent vaccine is not effective against them. The most important virus in this category is influenza (the flu shot). To keep up with the steadily mutating flu virus, pharmaceutical corporations prepare an annual flu vaccine which is normally distributed in the northern hemisphere’s autumn. If a particularly threatening new strain emerges, a specially targeted vaccine can also be rushed into production, as was the case with the swine (H1N1) flu vaccine in 2009. A similar situation exists with reference to the human papillomavirus (HPV) vaccine, which provides protection only against several particularly dangerous forms of the roughly two hundred strains of HPV.
Vaccine research also continues into a number of diseases which still have no effective preventative measures. These include ebola, HIV, Hepatitis C, malaria, and SARS. Note that this list of diseases prevented by vaccines is lengthy but not comprehensive.
Currently, there is an increasing number of parents who worry about the large number of vaccines given to their children, and worry about the potential health effects of those vaccines. However, it is important to be aware that those vaccines are entirely responsible for a vast drop in a number of what used to be very serious childhood diseases. Smallpox, which once killed millions of people per year, has been entirely eradicated. Polio, which killed and crippled huge numbers of children, has gone from 60,000 new American cases a year in the early 1950s to zero today (although it still survives in a few developing countries). In the first 10 years after the Haemophilus vaccine was introduced in 1985, the number of Hib cases (leading to dangerous meningitis in children) collapsed downward, by 95%. Medically speaking, there is really no question that vaccines save lives.