With so many triumphs, why haven't vaccines eliminated other common diseases such as malaria and HIV-1 infection (the cause of AIDS)?
One problem is that experimental vaccines often elicit an immune response that does not actually protect against the disease. Most vaccines preferentially induce the formation of antibodies rather than cell-mediated immunity. This is fine for those diseases caused by- toxins (diphtheria, tetanus)
- extracellular bacteria (pneumococci)
- even viruses that must pass through the blood to reach the tissues where they do their damage (polio, rabies)
But viruses are intracellular parasites, out of the reach of antibodies while they reside within their target cells. They must be attacked by the cell-mediated branch of the immune system, such as by cytotoxic T lymphocytes (CTLs). Most vaccines do a poor job of eliciting cell-mediated immunity (CMI).
Example:
Much of the early — and so far unsuccessful — work on anti-HIV-1 vaccines has focused on the antibody response of the test animal. Antibodies may have a role in preventing infection or minimizing its spread, but cell-mediated responses will probably turn out to be far more important. Certainly there are thousands of patients dying of AIDS despite their high levels of anti-HIV-1 antibodies. (The most widespread test for HIV-1 infection does not detect the presence of the virus but the presence of antibodies against the virus.)
