Even with the plethora of vaccines, it is obvious that there are still many infections that can afflict you and your children. Not every disease can be prevented with a vaccine. HIV is an example where the vaccine development effort has stalled and scientists have failed to come up with a working vaccine despite decades of research (see Chapter 20 for a more detailed discussion on the HIV vaccine). As mentioned before, no vaccine works 100 percent of the time. Furthermore, the immunity generated after vaccination wanes after long period of time.

How long do vaccines work? This is a tricky question, because each person's immune system reacts to even the same vaccine differently. Going back to the police department analogy, the record keeping of the immune system is not perfect. After a long time (five to ten years, perhaps), the immune system's record on each particular germ becomes faded and outdated. Just as the police's profile of criminals needs to be updated on a regular basis to keep it relevant because the criminal could have been captured, deceased, or moved to a different location, the immune system needs a reminder every now and then to keep the relevant information on germs up-to-date.

The fact that the immune system's germ archive needs regular updates is the rationale behind booster shots. Not all vaccines require booster shots, but many do. Those vaccines that do not require lifelong boosting include vaccines against hepatitis B, hepatitis A, rotavirus, pneumococcus, and Hib. There are two reasons why these vaccines do not require additional booster after completing the initial series. For hepatitis A and B, the immune system's memory after the initial injections is quite long-lasting. Evidence shows that the vaccines remain effective between fifteen to twenty years after the completion of the series of vaccines. The effect may last longer, but the evidence is unavailable at this time because these vaccines have not been around for that long yet.

With the vaccines for rotavirus, pneumococcal, and Hib infections, booster vaccines are not routinely recommended because infections caused by these two bacteria pose very little threat to anyone older than five years of age. It would not matter much if older children get sick from these infections because the immune system is generally stronger for older children and adults.

Other vaccines, including shots for tetanus, whooping cough, measles, mumps, rubella, and polio, require booster injections to update the immune system's record on these germs. The vaccines for tetanus and whooping cough are especially unique because a regular booster is needed every ten years, even into adulthood. You are never too old to get the tetanus booster vaccine.

The bottom line is that very few vaccines can protect a person indefinitely. But the same is true for an infection that occurs naturally. If your child survived a whooping cough infection, it is still possible to get whooping cough again later on in life. The immunity after a whooping cough infection lasts four to twenty years after the infection. It does not protect your child forever. The same thing goes for chickenpox. It is definitely possible to get chickenpox twice, or even three times.

The immune system's record of germs does not work perfectly, and the immune system's memory does not last a lifetime. This is one of the reasons why your child can get sick from cold and stomach viruses year after year, regardless of how many times he has already gotten sick before.