SCIENTISTS and medical researchers across the globe have continued to explore various interventions aimed at addressing the global HIV pandemic, which now stands at about 34.4 million people living with the virus.

One of the interventions currently under scientific studies and scrutiny is the possibility of administering anti-retroviral therapy (ART) to people who may be at high risk of contracting the virus, for the sole purpose of prevention.

But then one of the biggest challenges hindering the use of ART as a prevention intervention for people, who are HIV negative, is the question as to whether or not African governments have the luxury to spare the already scarce ART drugs, for purposes of prevention.

This is so because the most Sub-Saharan African states are still battling with meeting the increasing demands for HIV positive people who are in dire need of treatment.

Globally, there is an estimated 34.4 million people living with HIV, out of which about 2.4 million of these are children, with 15.3 million men and 15.7 million women.

The administration of ART as a prevention intervention would not only prove expensive, but would ultimately translate to more people needing ART in the long term.

Another factor that may require careful consideration is adherence. It has been an up-hill battle for health practitioners to emphasis the importance of adherence to ART for people already infected, and the next question would be how much more difficult would it be to ensure that people who are negative, will adhere to ART, for the sole purposes of HIV prevention.

Under the Millennium Development Goals (MDGs) on the reduction of HIV is intended to ensure zero new HIV infections, zero discrimination against those infected, and zero AIDS related deaths, by 2015.

But, how far is the African continent from achieving this goal?

Currently, there about 7,400 new HIV infections on a daily basis, 15 million people in need of ART, out of which 5.2 million were placed on ART by 2010, and statistics further indicate that for every two people placed on ART, there are about five others who get infected.

In this regard, some of the prevention interventions for HIV include the consistent use of condoms, embracing medical male circumcision, effective management of genital infections (STIs), the use of microbicides and HIV vaccines, the prevent of mother to child transmission (PMTCT) and chemoprophylaxis (both pre-exposure prophylaxis (PrEP), and post exposure prophylaxis (PEP).

PrEP dosing is said to increase efficacy, while its administration may be difficult because people have difficulties in recognising exposure because of issues such as denial, substance abuse and imperfect communication between partners.

ART also inhibits HIV directly, and prevents mother to child transmission (PMTCT).

The concept of prophylaxis is not unique to HIV alone. Prophylaxis has been used and is still being used as a proven concept for malaria, tuberculosis (TB), pneumonia and meningitis.

The attributes of an effective antiretroviral prophylactic include it being safe, well tolerated, cost effective, low risk of resistance and affordable.

An ART drug known as Tenofovir, has become a first generation PrEP agent because it is protective in animals, licensed for treatment, has an excellent safety record, long half life (the period between the time of the effectiveness of the drug) and does not interfere with TB treatment or any hormonal contraception.

Tenofovir is also known to have a relatively high barrier to resistance mutations. Without ART, about 15 to 30 per cent of children become infected with HIV from their mothers, and an addition of between 5 to 20 per cent becomes infected during breast feeding.

In 2010, about 390,000 children in the Sub-Saharan region become infected with HIV through mother to child transmission (MTCT), meaning that there is still a lot of work to be done in order to achieve zero infection of HIV.

In 2009, United Nations (UN) Secretary General Ban Ki Moon, was quoted as having said "We have effective drugs. There is no reason why a mother should die of AIDS. There is no cause for any child to be born with HIV, and if we work hard enough, we can virtually eliminate MTCT", he said.

The Centre for the AIDS Programme Research in South Africa (CAPRISA) study 004, findings revealed no substantive safety concerns, no tenofovir resistance identified, safe for use by Hepatitis B virus infected women, and no evidence of risk compensation.

The study also revealed that tenofovir gel can prevent herpes simplex virus-2 (HSV-2) infection in women, with a reduction rate of up to 51 per cent.

From the CAPRISA 004, the proof of the concept that Tenofovir gel can prevent HIV infection in women was a 39 per cent protection against HIV overall, with a 50 per cent reduction in HIV after one year of tenofovir gel use, with a 54 per cent effectiveness in women with high adherence.

The HIV Prevention Trials Network (HPTN) study 052 randomised study, confirmed earlier observational data that if the HIV positive partner in a discordant couple took ARVs, the transmission to the HIV negative partner was virtually eliminated, at least for more than two years.

The randomised studies found that taking ARVs by the HIV negative partner in a discordant couple (Pre-exposure prophylaxis-PrEP), would also reduce the transmission substantially.

These results have ignited enthusiasm for ARVs as a break-through for HIV prevention.

A joint United Nations Programme on HIV/AIDS (UNAIDS) executive director Michel Sidebe has described it as 'game changing'.

However, the biggest question will remain, are ARVs a magic bullet to stop the global epidemic in its tracks?

HIV is an elusive enemy, and a variety of major logistical, cost, biological behaviour impediments stand in the way of broad impact at scale.

Global statistics indicate that there are approximately 6.6 million people on ART globally.

But 2.7 million become infected each year. Simply identifying and reaching a major proportion of the infected but untreated is a herculean task.

Many people are difficult to reach and/ or resistant to testing. For example, Lesotho, a relatively advanced compact country with the world's third highest HIV prevalence, launched a national campaign in 2004 to test everyone.

Yet by 2009, only just over half of the adult population had been tested even once, while the perpetual stream of the newly infected increased the testing burden.

Even an exceptionally optimistic model (that assumed universal testing every year, nearly complete adherence, and 40 per cent of impact from other prevention programming) projected it would take a decade to bring new infections in generalised epidemics (where the epidemic affects a substantial portion of the general population) close to zero, and 50 years for virtual infection elimination .

The challenge remains identifying those potentially at substantial risk for HIV acquisition.

For some, it may be relatively straight forward, such as identified negative partners in discordant couple, but beyond those the potential for PrEP is unclear, even some at higher risk, such as those with multiple sexual partners.

Acceptance and long-term adherence; for ART as prevention to have a substantial impact, very large numbers of those persons testing positive-most symptom-free would need to take them (ART) voluntarily and consistently for a lifetime.

Even now, adherence is far from perfect, and some patients discontinue for a variety of reasons, including drug side effects.

Adherence amongst symptom-free people is even more problematic, especially when they experience side effects.

These issues are greater for an HIV negative person, who might choose PrEP, because ARVs have no immediate clinical benefit to them.

There is also compelling evidence showing that oral antiretroviral drugs (ARVs) can prevent heterosexual transmission of HIV.

According to a download from www.sciencemag.org, volume 334 of December 2011, poor adherence was one of the major reasons for failure to show impact in the two recent oral PrEP trials.

According to the website, there is what is referred to ARVs benefit, whose findings argue on not relying on ARVs alone, but also reinforcing behaviour risk reduction, as occurred in each successful clinical trial.

For PrEP, a particular worry is that some individuals would take ARVs sporadically and engage in risky sexual behaviour.

Recent global estimates indicate nearly two-thirds of the $7 billion current basic programme funding supports treatment and care-related services, despite less than half of the treatment eligible people are receiving ARVs.

In a world currently beset with economic stress and with increasing emphasis on other compelling global health priorities, the prospect of increasing provision of ARVs several fold, may seem unrealistic.

Furthermore, the costs would be cumulative as survival improves and progressively more people take ARVs for treatment and prevention.