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Dear Dr. Roach: I am confused about immunity from vaccines versus immunity gained from getting a disease. Protection from whooping cough can wane in as little as four years (both disease and vaccine), but the tetanus vaccine can last up to 30 years. However, the disease gives no natural immunity. And measles disease gives lifelong immunity. What causes the differences?

C.B.B.

Dear C.C.B.: Some diseases are able to escape the body’s defense systems. The immune system works better if it has “seen” a germ (virus, bacteria, parasite or fungus) before. Unfortunately, there are diseases to which the body never gains immunity. Although there are exceptions, in general, diseases that give lifelong immunity are easier to make vaccines for. Take measles, for example. You are right that most people will have lifelong immunity after two doses of vaccine or after having the disease. (There are serious complications of the disease, which is why it’s much better to get the vaccine.) However, most people will never get natural immunity from malaria, even after heavy exposure and numerous bouts of the disease.

Some diseases, like influenza, are constantly changing, requiring new vaccines every year. It’s a challenge for immunologists to create effective vaccines, but science is making progress.

In the case of pertussis (whooping cough), infection does not prevent recurrences. In the pre-vaccine era, nearly everyone had pertussis as a child, and it is estimated that an adult had pertussis between two and three times in a lifetime. Also in the pre-vaccine era, there were about 9,000 deaths per year from pertussis, compared with about 20 deaths per year now. Even though immunity from the current pertussis vaccine does wane, booster doses are not currently recommended.

In the case of tetanus, surviving the infection (even now, 10 to 20 percent of those infected with tetanus die) does not confer immunity. Survivors should be immunized as soon as the condition stabilizes. A full series of tetanus vaccines gives nearly 100 percent protection, which is why tetanus is called “the unforgivable disease”: Nobody should get it, and if the omission of the vaccine was the physician’s fault, he or she can’t be forgiven for it.

The mechanisms by which infections escape our immune system are complicated: Over millions of years, humans have evolved to better fight off infections, but infections have learned to evade our defenses.

Email questions to ToYourGoodHealth@med.cornell.edu.

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