There are a number of indications from human observations and animal studies that the immune system can thwart immunodeficiency viruses. This implies that a vaccine against human immunodeficiency virus (HIV) is possible.
The Evidence from People
In the first few weeks after initial infection with HIV, the immune system deploys T cells that suppress the amount of the virus circulating in the body. This response controls viral proliferation enough to create a symptom-free steady state, which is called the viral set-point. In many people, the steady state of infection can last for eight to 10 years before the virus overwhelms their immune defenses and they develop AIDS. Antibodies against HIV are also generated within weeks of infection, but HIV mutates rapidly, and in the vast majority of people these antibodies cannot completely neutralize the many mutant versions of the virus present in any single person. Most researchers think a vaccine that both strengthens suppressive T-cell responses and induces antibodies would prove effective.
A minority of HIV-infected individuals, called elite controllers, suppress the amount of virus in their bodies indefinitely, without needing drug therapy. Scientists are trying to determine the precise immune responses that protect elite controllers, in order to devise a vaccine that would replicate those responses.
There are documented cases of individuals who have been repeatedly exposed to HIV but have not become infected. These individuals, known as exposed seronegatives, were first described among female sex workers in Nairobi, Kenya. Some research has indicated that some of these exposed seronegatives have immune responses against HIV, suggesting the immune system is at least partly responsible for their protection. If it is, they may hold the key to an effective vaccine.
The Evidence from Animals
Simian immunodeficiency virus (SIV) is a cousin of HIV that causes an AIDS like disease in non-human primates. The best animal model we have of HIV infection in humans is SIV infection in rhesus macaques, a kind of monkey.
Some licensed vaccines, called live-attenuated vaccines, use a crippled but living version of the targeted pathogen—or disease-causing organism—to teach the immune system what it must attack to prevent infection. Such vaccines, however, cannot be developed against HIV because of concern that the crippled HIV might revert to its disease-causing form. But live-attenuated SIV vaccines have proven effective in protecting rhesus macaques from infection with SIV, providing a proof of concept for an AIDS vaccine.
SIV vaccines modeled after HIV vaccines being developed for humans have significantly suppressed the amount of virus in the blood of nonhuman primates that were later exposed to and infected with SIV. This is evidence that it’s possible to control the progression of immunodeficiency virus disease by means of a vaccine, even if the vaccine does not prevent infection by the virus.
A handful of human antibodies that effectively neutralize many of the different types of HIV in circulation have been isolated from HIV-infected individuals over the years. When non-human primates are injected with large amounts of these neutralizing antibodies and then exposed to immunodeficiency virus (in this instance, SIV with some HIV genes), the non-human primates are completely protected from infection. This implies that if a human vaccine is able to elicit neutralizing antibodies, it will protect people from HIV infection.
Taken together, these lines of evidence suggest that control of HIV infection by the immune system can be improved. A vaccine would be a means to that end.