It's a quest that's been nearly 40 years in the making: finding a safe and effective vaccine for HIV.
Now scientists are "cautiously optimistic" they're one step closer after a trial of a new vaccine has shown promising results in humans and monkeys.
In a study published in The Lancet, the test treatment was found to produce an anti-HIV immune response in healthy adults, and protect monkeys against infection from a virus similar to HIV.
Lead researcher Dan Barouch from Harvard Medical School said the study was an "important milestone" in HIV research, but that the results should be interpreted with caution.
"We're happy with the current results … but we can't assume that this vaccine will work in humans," Professor Barouch told The Health Report.
To test that theory, researchers will now take the vaccine to 2,600 women in southern Africa who are at risk of acquiring HIV.
"This is only the fifth HIV vaccine concept that will be tested for efficacy in humans in the 35-year history of the global epidemic," Professor Barouch said.
Almost 37 million people worldwide are living with HIV, with an estimated 1.8 million new cases every year.
Vaccine targets multiple strains
One of the major challenges of developing an effective HIV vaccine is the ability of the virus to mutate quickly and evade an attack from our immune systems.
There are many strains of HIV, and previous vaccine trials have typically been limited to specific strains of the virus in particular regions of the world.
For this study, however, researchers used a so-called 'mosaic' vaccine that combines pieces of different HIV virus types in order to elicit an immune response against various HIV strains.
In 2015, Professor Barouch and his colleagues tested various combinations of the mosaic vaccine in healthy adults in east Africa, South Africa, Thailand and the US.
The 393 participants were given four vaccinations over the course of 48 weeks. Each vaccine combination was found to be safe and "well tolerated" and produced an anti-HIV immune response.
At the same time, researchers administered the 'mosaic' vaccine combinations to 72 monkeys to test for resistance against a HIV-like virus that affects monkeys — the simian-human immunodeficiency virus.
They found the vaccine that produced the greatest immune response in humans also provided the best protection in monkeys.
"The vaccine provided 67 per cent protection against the AIDS-like virus," Professor Barouch said.
"Because of the protection in monkeys, and the safety and immune responses in humans, the vaccine has been advanced into a larger scale study … which will determine whether it can prevent HIV infections in humans."
Fifth vaccine trial to make it this far
Since the emergence of the HIV epidemic in the early 1980s, just four experimental vaccines have ever been evaluated for efficacy in humans, and only one has shown evidence of protecting against HIV.
A vaccine tested among 16,000 Thai volunteers in 2009 lowered the rate of human infection by 31 per cent, but that effect was considered too low for the drug to be pursued for widespread use.
Professor Barouch said although the 'mosaic' vaccine had triggered a promising anti-HIV response in humans, there was no guarantee it would be enough to prevent HIV infection.
"The challenges in the development of an HIV vaccine are unprecedented, and the ability to induce HIV specific immune responses does not necessarily indicate that a vaccine will protect humans from HIV infection," he said.
Leading infectious diseases expert Sharon Lewin from the Doherty Institute said the research was "interesting" but that there was still a way to go before an effective HIV vaccine was available.
"There's been a very chequered history here of vaccines looking good in monkeys and then you move them into humans and they don't work," said Professor Lewin.
"That has happened before… but they are using a slightly more sophisticated method in this instance."
Vaccine critical to eradicate HIV
Despite significant advances in our understanding of HIV and its prevention and treatment, the development of a preventative vaccine remains urgent.
The vast majority of people living with HIV are in low- and middle-income countries, particularly in Sub-Saharan Africa, where they (and the people at risk of HIV) often do not have access to prevention, care, or treatment, and there is still no cure.
Professor Lewin said research showed even a "moderately effective" vaccine, together with existing HIV prevention and treatment strategies, would have a "very profound effect" on the global epidemic.
"There are still 1.8 million new infections a year. So, if you had a vaccine that was just 70 per cent effective, the modelling shows you can have a substantial reduction in transmission," she said.
"Hence the significant ongoing investment, which I do think is really needed."
PrEP is effective, but vaccine needed for global epidemic
In Australia and other parts of the world, the relatively recent emergence of Pre-Exposure Prophylaxis, or PrEP, has been hailed as a "game changer" for gay men.
PrEP, which was added to the Pharmaceutical Benefits Scheme in April, is a preventative medicine that is up to 99 per cent effective in stopping the transmission of HIV between men during sex.
"What several countries around the world have shown, including in Australia, but New South Wales in particular, is that if you get treatment and PrEP use up to high levels, you start to see a reduction in new HIV infection," Professor Lewin said.
PrEP, however, must be taken daily, is not a sustainable long-term strategy for tackling HIV on a global scale, she said.
"PrEP is very effective, and while we don't have a vaccine, we definitely need to roll it out … we still have 1,000 new HIV infections a year in Australia," she said.
"But it costs money, you need to be in regular healthcare, and it requires a whole infrastructure to support it … which is why any infectious disease that's ever been effectively tackled has always been associated with a vaccine."
Australia has 'impressive track record' of HIV treatment
In 2016, it was estimated more than 26,000 Australians were living with HIV, according to the Kirby Institute at the University of New South Wales.
Of those people, 95 per cent were accessing care, and more than 90 per cent had what's known as an "undetectable viral load", meaning the HIV virus cannot be detected by standard blood tests or transmitted during sex.
"Australia has an extremely impressive track record of HIV testing, uptake of treatment, and staying on treatment," Professor Lewin said.
That's thanks in large part to the introduction of lifelong virus-suppressing antiretroviral therapy in 1995, which revolutionised the treatment of HIV worldwide.
"So, if you treat people [with antiretroviral drugs], the amount of their virus shrinks down to very low levels. It's always there, but at very low levels, so you block transmission," Professor Lewin said.
The treatment reduces the risk of disease advancement associated with HIV, meaning people on daily treatment can live a long and healthy life.
The International AIDS Conference will be held in Amsterdam from July 23 to 27.