Millions of people around the world are affected by serious diseases that are difficult to prevent or cure some old and some new. For example, AIDS has no cure. After the discovery of the first antibiotics, scientists hoped that “wonder drugs” could wipe out the infectious diseases that had always plagued humans. But microbes that cause disease will probably be with us forever.
They multiply rapidly, and they mutate, or change, easily. When they mutate, they may produce drug-resistant forms, which can reproduce and quickly outnumber those that the drug kills. The drugs are then no longer effective. Although new drugs are constantly being developed, new vaccines are very important to prevent infection with some diseases, especially those that are difficult or impossible to treat with drugs or other methods.
Scientists are working to discover vaccines to protect people from HIV and AIDS but so far is no cure for HIV. AIDS may be the most frightening disease of the twentieth century. HIV, the causal virus, attacks the immune system and gradually destroys it, leaving the affected person defenseless against a variety of other infections, one of which is eventually fatal.
As almost everyone knows, the search for a vaccine to protect against AIDS is a high priority in the world of medicine. Almost two decades after AIDS first appeared, there is still no magic cure or vaccine to prevent the disease. Medicines that seem to hold it in check are available, and they allow affected people to live longer. However, these drugs are very expensive, they do not work for everyone, and they are not a cure for AIDS.
Like other microbes, HIV can mutate and produce forms that are resistant to these medicines. HIV infects more than 30 million people worldwide, 90 per- cent of them in Africa and Asia.
How do you get HIV?
The virus is spread by the exchange of blood or body fluids. You cannot get HIV from casual contact. Sexual contact is one means of spreading the virus, and the use of needles contaminated with HIV is another. The only way to prevent it is by avoiding risky behavior.
Scientists who are trying to develop effective vaccines against HIV disease face some very tough challenges. In other viral diseases such as polio, for which successful vaccines have been made, some people who develop the disease recover. But no one has ever recovered from HIV disease, because the immune system cannot fight HIV in the same way it fights other viral diseases.
So researchers do not know what kind of antibodies might exist in the immune system of a person who recovers. They do not even know whether someone’s immune system can protect itself by natural means.
What makes HIV especially hard to combat?
First, its main target is the immune system itself. It invades the key cells of the immune system, the lymphocytes known as helper T cells, which direct the various complicated immune responses. The virus “hijacks” the host’s cellular machinery, reprogramming it to make more virus particles. In the process, it destroys the ability of host cells to function. HIV can remain hidden in the T cells for along time.Other parts of the body may also act as holding tanks for HIV, hiding the virus so that the immune system cannot detect it.
Second, unlike some other viruses, HIV can exist in the body outside of cells as free virus as well as inside infected cells. So, to be effective against HIV, a vaccine would need to stimulate the two main types of immunity—humoral immunity and cellular immunity.
Humoral immunity is the protection provided by antibodies in the defense against the free virus. Cellular immune system, the antibodies don’t work against HIV taken from infected patients. Studies of several other strategies are underway. Some researchers have used protein fragments from inside HIV to make vaccines. Others have produced and tested several DNA vaccines made with naked DNA from HIV in both animals and humans.
In some studies, scientists found that the vaccine protected the animals against HIV infection. Researchers are also experimenting with artificial copies of viral particles called pseudovirions. Another experimental approach involves the insertion of some HIV genes into a weakened form of canarypox virus—a bird virus that is not harmful to people. The canarypox virus delivers the HIV genes to the body cells, where they produce HIV proteins. The proteins gain the attention of the body’s immune system, causing it to make antibodies and killer T cells. Several of these vaccines are being tested in human volunteers.
Some researchers believe that using vaccines containing live weakened HIV would provide the best protection, but these vaccines have sparked the most controversy. Some scientists want to make a vaccine that protects people from HIV infection and does not cause AIDS. A group of physicians have volunteered to be the first human guinea pigs for this kind of experiment. But most researchers believe that more animal studies are needed before a live HIV vaccine is tested in humans.
Chimpanzees can be infected with HIV, but only one chimpanzee is known to have developed full-blown AIDS. Full-blown disease develops in humans with HIV infection when the immune system can no longer fight back. Other infections invade the body and overwhelm the damaged immune system. However, no one uses chimps for AIDS-related studies because they are an endangered species.
Researchers sometimes use macaque monkeys because they can be infected with simian immunodeficiency virus (SIV), a virus that is similar to HIV and causes a disease that resembles AIDS. In fact, some scientists believe that SIV mutated and became HIV. Experiments have shown that live, weakened SIV vaccine has protected some monkeys from infection by naturally occurring SIV. But after observing the vaccinated monkeys for a long time, the researchers discovered that the vaccine itself infected some monkeys. They worry that the same thing might happen if humans are vaccinated with a similar HIV vaccine.
Another finding has forced scientists to conclude that a live AIDS vaccine, even a weakened form that is missing one or more genes, might be too dangerous to give to humans. A group of people infected for more than 10 years with such a weakened form of HIV had been healthy for many years, and their immune systems seemed able to resist the progression of HIV disease. However, they eventually started to show signs of AIDS. Even if no vaccine can protect against AIDS completely, investigators hope that they can make a partially effective vaccine that would help a person’s own immune system keep HIV in check so that he or she could remain healthy and never develop AIDS.