A vaccine is a treatment that prevents us from getting a disease. It works by teaching our immune system how to protect itself against a disease-causing germ like a virus or bacteria.

Vaccines are one of the world’s most effective tools for preventing diseases. Serious illnesses like polio, smallpox, measles, and mumps have been nearly eliminated from many countries because of vaccines.

Ideally, an AIDS vaccine would prevent people from being infected with HIV (the virus that causes AIDS).

No. Today there are no effective AIDS vaccines.

Yes. There are more than 30 clinical trials of vaccine candidates going on in 25 countries around the world.

No. Each vaccine being tested uses a slightly different design or strategy but none of them can actually cause HIV infection. This is because all the vaccine candidates only use small, synthetic (man-made) pieces of HIV.

The search for an AIDS vaccine has not been easy. A number of vaccine candidates have been studied since the mid-1980s, but few have made it into later-stage, large-scale Phase III trials.

There have been three large Phase III trials. The first two involved a vaccine candidate called AIDSVAX. They were completed in 2003 and did not show any evidence that the vaccine worked.

The third trial took place in Thailand and enrolled 16,000 participants, making it the largest AIDS vaccine trial ever. It tested AIDSVAX with another vaccine called ALVAC.  In 2009, after much debate over the results, researchers concluded that the vaccine only had a modest effect in preventing HIV infection.

There had been high hopes for a fourth trial called Step, which was a smaller study of a vaccine manufactured by Merck. In 2007, Step and another study of the same vaccine in South Africa were called off early due to results showing the vaccine did not work.

Yes. Despite setbacks, the search for an AIDS vaccine has not ended. The disappointing Step trial results and the controversy over the results of the Thai trial caused a lot of debate among researchers and advocates about what to do next. However, research is still moving forward. The focus is on answering basic scientific questions that can help guide vaccine development, while continuing to learn valuable information from previous studies and mapping out future vaccine trials.

A range of vaccine candidates remain in development and evidence from different studies suggests that an effective AIDS vaccine is still possible.

The next vaccine candidate was developed by the National Institutes of Health (NIH) Vaccine Research Center (VRC). An efficacy trial called PAVE 100 was scheduled to begin for this vaccine in 2007, but was put on hold when the Step results were announced.

The trial has since been redesigned and renamed HVTN 505. It is an exploratory study intended to provide information to help guide the next phase of AIDS vaccine development. HVTN 505 is currently enrolling 1,300 HIV-negative men who have sex with me.

The best case is that an AIDS vaccine is found which can protect nearly everyone who receives it from being infected with HIV. But many scientists think that the first-generation AIDS vaccines will provide more limited forms of protection, called partial protection. Partial protection might include:

• Improving overall health and slowing down disease progression in people who receive the vaccine and later become infected
• Protecting some, but not all, of the people who receive the vaccine against becoming infected

Most people think of vaccines in a simple way: once you are vaccinated against a particular disease, you are completely protected for life. In fact, this is not always the case. None of the vaccines that are currently licensed are 100 percent protective in 100 percent of the people who receive them. Some vaccines work better in some people than in others; other vaccines only provide protection for a limited amount of time.

Even a partially-protective AIDS vaccine could be a very important tool for slowing the spread of HIV. It would also help scientists to develop more effective vaccines over time.

The honest answer is that we don’t know. It takes several years to study whether a vaccine candidate is safe and effective. Based on what is now in development and clinical trials, scientists believe it will be six to ten years before the first AIDS vaccine is licensed for use. This first-generation vaccine is not likely to provide complete protection against HIV infection.

Although this sounds discouraging, it is important to remember that vaccine research takes a long time. It has taken decades, with more setbacks than advances, to discover other vaccines. Most importantly, many epidemics in modern times have been ended by an effective vaccine. So it’s crucial to keep moving ahead with AIDS vaccine research.

Like all experimental medical treatments, AIDS vaccine candidates go through a series of safety tests—first in animals and then in small groups of people. These small studies help determine whether or not the vaccine causes any serious side effects. Only vaccines that appear to be completely safe are considered for large-scale efficacy trials that test whether the vaccine protects against HIV infection in HIV-negative people. AIDS vaccines are not tested by deliberately exposing people to HIV.

Before an efficacy trial begins, researchers usually spend two or more years studying communities where trials may take place. They gather many types of information, including how many people get HIV each year. The rate of new infections is called the incidence rate. If 100 HIV-negative people were followed for one year, and five of them got HIV by the end of the year, you would say that there was a 5 percent incidence rate in that group of people.

Once these numbers have been collected, people from the community are asked to enroll in the vaccine trial. The participants are randomly assigned to receive either the vaccine or a placebo (an inactive substance). Neither the researchers nor the trial participants know who has received the vaccine and who has received the placebo.

The participants are followed for a long time—usually two to three years. At the end of the trial, the researchers look to see whether the incidence rate is lower in the group of people who were given the vaccine, as compared to the group of people who were given the placebo.

For example, if there was a 2 percent incidence rate in people who received the vaccine, and a 5 percent incidence rate in people who received the placebo, that might mean that the vaccine was protecting some people against HIV.

Yes. Vaccine trials provide a lot of information to people who are thinking about volunteering and to people who decide to enroll in the trial. One of the key messages is that there is no way of knowing whether the vaccine is effective before the trial. That makes it important for everyone who enrolls to continue protecting themselves by using condoms or clean needles.

This message is repeated to participants every time they come for a study visit. By educating participants, it is possible that the vaccine trial reduces the participant’s risk. All trials also provide free male condoms and counsel participants about other methods like the female condom or safe injection practices.

Right now, most of the candidates that are being developed are designed to prevent HIV infection in HIV-negative people. However there are a few trials that are testing vaccines in people living with HIV (HIV+ people). While these vaccines will not cure AIDS, they may help HIV+ people fight the virus. They are called therapeutic vaccines.

AIDS vaccines are tested first in adults and, perhaps, teenagers. However, an effective vaccine could someday be given to infants born to HIV+ women to help protect the infants from getting HIV through breast milk. This would be most useful in developing countries where feeding with formula is not possible for many HIV+ women.

Over 25 years into the epidemic, we still don’t have ways to protect ourselves against HIV infection during sex that are private, woman-controlled, and do not depend on our partners’ agreement. There is an urgent need for prevention methods that women can choose without asking permission from their partners. An effective AIDS vaccine would give women this option. A woman could decide to be vaccinated against HIV. Later on she might decide to talk about the decision with her partner—or she might not. The choice would be hers.

Women have to participate in large numbers in AIDS vaccine trials in order to find an AIDS vaccine that helps protect us. This is the only way that researchers will be able to find out whether a particular vaccine works equally well in women and men.

Until there is an effective AIDS vaccine, the best way to protect yourself and your loved ones from HIV is by practicing safer sex and not sharing needles.