A vaccine to tackle HIV - the AIDS-causing virus - could be available by as soon as 2021, scientists believe.
Trials of three different vaccines are close to entering their final stages, prompting experts to be 'optimistic' about the future.
Results of the vaccine experiments - known as HVTN 702, Imbokodo and Mosaico - will be available as early as next year.
People with HIV can take drugs which makes their viral load undetectable, meaning they cannot transmit it, including through sex.
However, a vaccine does not currently exist to protect against catching the virus, carried by 37million people worldwide.
An HIV vaccine could be available by 2021 as scientists tackling the AIDS-causing virus reveal they are 'optimistic' about three ongoing trials
Chair of two of the trials, Dr Susan Buchbinder, director of the Bridge HIV research program at the San Francisco Department of Public Health, said this is 'perhaps one of the most optimistic moments we have been in'.
Even a partially effective vaccine would be 'a tremendous breakthrough' and 'would really have the power to change the trajectory of the epidemic', she said.
She told NBC: 'We have three vaccines currently being tested in efficacy trials, and it takes quite a bit to actually be promising enough in the earlier stages of trials to move you forward into an efficacy study.'
The oldest ongoing HIV vaccine trial - known as HVTN 702 - was launched in South Africa in 2016.
It's based on a prior candidate, RV144, which lowered the rate of HIV infections by around 30 per cent in an older study.
RV144 remains the only HIV vaccine that has ever demonstrated any efficacy against the virus. However, scientists wanted to make it stronger.
As well as providing better protection than the RV144 regimen, HVTN 702 has been adapted to the HIV subtype that predominates in southern Africa (HIV-1 clade C).
The second trial, of Imbokodo, began in five southern African nations in 2017. Some 2,600 women have been enrolled in the study.
Imbokodo uses 'mosaic' immunogens, which are 'vaccine components designed to induce immune responses against a wide variety of global HIV strains', according to the National Institutes of Health.
The third vaccine, called Mosaico, is also based on this unique mosaic immunogen approach. A trial of the vaccine started in November.
Mosaico will recruit 3,800 gay men and transgender people for its clinical trials at 57 sites in the US, Latin America and Europe.
Imbokodo and Mosaico are very similar in formula and mosaic vaccines are expected to give broader coverage to different strains of HIV.
The two vaccines consist of six injections, with slightly different types administered during the final two clinic