World Aids Vaccine Day is observed annually on May 18 to educate communities around the world about the importance of HIV vaccine research.

Despite the undeniable success of antiretroviral therapy (ART) in improving the lives and life expectancy of people living with HIV, the virus continues to wreak havoc in our population.

While new infections have gone down from a peak of 16,000 infections per day in 1997 to just over 4,500 per day, worryingly an additional 1.7-million people became newly infected with HIV in 2018, increasing the total number of people living with the virus worldwide to 38-million. This is an untenable situation.

Global efforts have ensured that 25-million people are currently receiving ART. Unfortunately, ART does not eradicate HIV. The virus rebounds soon after treatment is interrupted, regardless of how long a person has taken it.

This is because HIV persists in cells and tissues, even during treatment, in what is called the viral reservoir. Moreover, the reality is that all antiretroviral drugs have potential toxicity, and access to treatment for everyone remains challenging and comes with a huge economic burden.

In addition, infection with HIV is still a cause of stigmatisation and, in many cases, discrimination.

In the absence of a cure, the most effective way to eradicate an infection is through preventive vaccination. The eradication of smallpox is a case in point. Unfortunately, despite considerable optimism in the early stages of the HIV pandemic, a vaccine has remained elusive.

The main reasons are the genetic diversity of the virus - it mutates as it divides, making it difficult for the immune system to find a target to attack - and the difficulty in figuring out what kind of immune response is needed for protection.

Since the discovery of HIV in 1983, there have been a number of vaccine trials that made use of different strategies, all of which have failed.

The only glimmer of hope came from the Thai trial, RV144, which reduced the risk of infection by 31%. Recently, a similar study was undertaken in SA to confirm the results of this trial, called the HVTN702 study. However, in February, the data safety monitoring board recommended that the study be stopped due to lack of efficacy.

The dynamics of transmission are still poorly understood, and the epidemic is clearly out of control.

In the aftermath of this trial, some have started questioning whether we will ever have an effective vaccine.

However, the HIV field is nothing if not robust. There is at least one other efficacy trial that is ongoing, and novel vaccines with new targets and vectors are continuously being developed.

In addition, there has been substantial investment in studying broadly neutralising antibodies (bNAbs) and understanding how they could be used in future vaccine strategies.

However, these antibodies develop only in a minority of people with HIV and, even then, only after years of infection. Fortunately, thanks to very hard work and the willingness of HIV-infected persons to participate in such studies, scientists have found a number of bNAbs that they can now study in the laboratory, and new ones continue to be discovered.

While bNAbs could potentially be used as prophylactic treatment or as part of treatment strategies for people already infected, the ideal scenario would be the development of a vaccine that could induce the body to produce its own bNAbs.

Now is not a time for despondency. Great strides continue to be made. It is, however, critical that communities, volunteers and healthcare workers, as well as social and medical scientists, work together to develop a better understanding of this virus and how we can defeat it.

However, the thousands of volunteers who continue to participate in research projects are the unsung heroes. We salute you.

*Rossouw is a professor in the Department of Immunology at the University of Pretoria (UP)