The promise of a vaccine against HIV has got one step closer. Results from the largest vaccine trial ever conducted show a modest but encouraging 31 percent efficacy in preventing new HIV infections in Thailand. This has vindicated thousands of scientists and volunteers who have been hoping that a safe and highly effective HIV vaccine is possible. This news comes at a time when the movement to achieve universal access to HIV prevention and treatment is gaining momentum. Today more than 4 million people living with HIV are receiving antiretroviral treatment and fewer babies are being born with HIV. With less than half the people who need treatment having access and with each day more people becoming infected with HIV than are started on treatment, we are mortgaging our future. But we are also exposing a fundamental social injustice – between the privileged and the forsaken – a divide we can bridge.

An acceptable vaccine is not yet ready, but let us prepare today for tomorrow. Let us learn from the lessons of the AIDS response thus far.

The first challenge is access and affordability. Antiretroviral treatment has been around since 1996, but real access to treatment began only when public pressure was put on world leaders and the prices of medicines came down. Today, AIDS activists are repeating these efforts to reduce prices, this time for second line antiretroviral medicines. It is unacceptable that 98 percent of pregnant women in developed countries are able to access HIV prophylaxis to stop transmission to their babies when little more than 33 percent in developing countries can do so.

The news coincides with this week's United Nations General Assembly. During the meeting the Secretary-General reminded us of "our commitment to equity" and I appealed to many Heads of State who are committed to placing equity in the AIDS response high on their list. We must not allow cost to deter people from access to a vaccine.

The second challenge is creating the conditions for massive uptake of an effective vaccine. Time and again, women and girls are unable to make independent decisions about their health and education. Many men and women do not come forward to take an HIV test for fear of stigma and discrimination. People without a voice – sex workers and their clients, injecting drug users and men who have sex with men – are often excluded from health and social welfare programmes. We look to civil society to continue to break down the barriers to vaccine uptake.

The third challenge is in creating health systems capable of delivering the vaccine. Currently clinics are geared towards immunizing infants and young children. The largest benefits of an HIV vaccine will likely accrue from vaccinating the present cohort of young people and those at higher risk of HIV exposure. A failure to reach adolescents will represent another failure to break the back of the epidemic.

There is no time for complacency in our efforts to stop new HIV infections. The world needs a strong HIV prevention campaign that is evidence-informed and grounded in human rights. It is high time to end discrimination, bad laws, and harmful social norms that fuel HIV transmission.

As scientists and world leaders absorb the implications of the Thai study results in the coming weeks they must be mindful of these challenges. A "ready to use" vaccine is years – perhaps decades away, but when it does become available, it ought to be financed as a public good that is accessible for all. How else can we reasonably expect to put an end to this epidemic?

Michel Sidibé is Executive Director, Joint United Nations Programme on HIV/AIDS (UNAIDS)