Further down the road, the hope is to produce matchstick-sized implants containing slow-release drugs - similar to existing under-the-skin contraceptive devices - that could offer year-long protection.

Companies with drugs involved include GlaxoSmithKline, Gilead Sciences and Merck.

The initiatives build on the success of Gilead's once-daily pill Truvada, which has proved remarkably effective at stopping HIV infection during sex.

Clinical studies show such pre-exposure prophylaxis, or PrEP, can cut the risk of catching the virus by more than 90 percent, as long as people take their pills regularly. The problem is many do not.

Some women in trials in Africa, for example, said they were reluctant to have HIV tablets in the house for fear of what partners or neighbors would think.

An injection given in a clinic, experts argue, would add privacy and ensure steady drug levels. An implant in the arm might even combine contraception and HIV protection in one go.

"The more options there are the better and I think for some individuals injections will be great," said Jean-Michel Molina, professor of infectious diseases at Hospital Saint-Louis in Paris.

"Now that we know antiretrovirals have great potential to prevent HIV infections, it is time to really assess other ways to deliver these drugs."

The need remains acute. Despite treatment advances that have slashed AIDS deaths, around 1.9 million people still catch HIV each year - a number that hasn't budged since 2010. New infections among gay men are actually increasing.

The United Nations AIDS program warned last week that the problem now threatened progress in ending the global epidemic, while the World Health Organization has recommended PrEP for all groups at substantial risk of HIV infection.

STILL NO VACCINE

GlaxoSmithKline's majority-owned ViiV Healthcare unit, working with U.S. government agencies and the Bill & Melinda Gates Foundation, hopes to add the first injectable PrEP.

It plans to start a four-year trial as soon as next month testing its experimental drug cabotegravir in gay men in the Americas and Thailand, with a second trial next year assessing the medicine in African women.

Two separate studies evaluating cabotegravir in combination with another drug for HIV treatment were launched this month.

"The holy grail is a vaccine, but we don't have a vaccine yet," said Myron Cohen of the University of North Carolina at Chapel Hill, who is involved in the ViiV prevention study program and believes new options can help rein in HIV.

He is also working on another prevention trial giving people antibodies via an intravenous drip.

Gilead, meanwhile, is running a late-stage study assessing its next-generation HIV drug Descovy as an alternative oral PrEP, since it has milder side effects than Truvada.

In the long run, Cohen and other HIV experts are especially excited by slow-release drug implants. Implants have yet to prove themselves in human trials, although an experiment in beagles last year with a Gilead drug produced promising results. Products from GSK and Merck are also seen as implant options.

The size of the HIV prevention market remains uncertain and price will be an issue, especially in Africa, but some industry analysts believe it could be substantial.

Truvada, the only medicine so far approved for HIV prevention, is now being used for PrEP by 80,000-90,000 people in the United States, accounting for 35-45 percent of the drug's revenue in the third quarter.

It is starting to gain traction, too, in Europe, with France offering free supplies, while in Britain and other markets, where it is not paid for by the government, people are turning to online "buyers clubs" to get cut-price generics.

There is resistance, however. A French government campaign this month promoting HIV awareness, including PrEP, was attacked by several French mayors.

Some have also criticized PrEP for diluting the "safe sex" message of condom use, especially as it will not prevent other sexually transmitted infections (STIs).

But Sheena McCormack of University College London argues this misses the point. "Sometimes people can't see the wood for the trees. The STI that lasts a lifetime and costs governments a lot of money is HIV."