In the past few years the science of HIV prevention has come on a long way, identifying new ways in which people can reduce their risk of HIV transmission to themselves or to others. We now know that antiretroviral drugs (ARVs), taken either by people living with HIV or by HIV-negative people, substantially reduce the risk of HIV transmission.
Just as couples can choose from a range of contraceptive options if they wish to prevent pregnancy, people should be able to have more than one option for HIV prevention.
A vaccine to prevent HIV infection remains an important, but challenging, priority for researchers. The history of smallpox and polio shows that when highly effective, affordable vaccines are available, mass vaccination programmes can virtually eradicate infectious diseases.
A vaccine to prevent HIV infection would be a substance which teaches the immune system how to create protective immune responses against HIV. While scientists have discovered a great deal about the immune system’s response to HIV in over 30 years of research, they have not yet identified a fully effective substance.
The development of a vaccine to prevent HIV infection is especially challenging because:
- the diversity of HIV subtypes and the frequency with which the virus mutates means that a vaccine would need to prevent infection with multiple strains. But in general, antibodies are not effective against multiple strains
- HIV attacks and uses the same CD4 cells that are needed for an effective immune response.
The most exciting recent progress in vaccine research concerns the Thai RV144 trial, which reported its results in 2009.1 Individuals who received a vaccine were 31% less likely to become infected with HIV than people who received a placebo.
This was the very first time that an HIV vaccine had been shown to have any impact on the rate of new infections.
Individuals received two vaccines with different modes of action; a ‘prime’ vaccine called ALVAC and a ‘boost’ vaccine that was modelled on the gp120 protein on the outer surface of HIV. Several groups of researchers are now trying to refine and improve this approach.
Vaccine researchers are also excited by the discovery that some individuals living with HIV produce antibodies that work against multiple strains of HIV. These are called broadly neutralising antibodies (bnAbs). Researchers are attempting to manufacture bnAbs to see whether they could protect HIV-negative people from a new infection.2
Microbicide research is at a more advanced stage than vaccine research, but faces different challenges. A microbicide is a gel or cream, applied either to the vagina or the rectum, which helps prevent HIV infection.
Vaginal microbicides containing antiretroviral drugs are effective, so long as they are used consistently and correctly. One study observed 39% fewer infections, but its findings have not been replicated.3
The challenge is adherence – in other words, creating a product which women who are at high risk of HIV infection are able to use regularly.
In this respect, the issues for microbicides and oral pre-exposure prophylaxis (PrEP) are comparable. In fact, a microbicide gel is essentially a different way to deliver PrEP and is sometimes referred to as ‘topical PrEP’.
The focus of research is now on developing alternative delivery mechanisms which may be easier for people to use. For example, vaginal rings may only need to be applied once a month and could also provide contraceptive protection. In the PrEP field, long-acting injections may be easier to adhere to than daily tablets.
There is also work on developing a rectal microbicide, suitable for use during anal sex.
Photo credit: ©iStock.com/nicolas_
- 1. Rerks-Ngarm, S. et al (2009) ‘Vaccination with ALVAC and AIDSVAX to prevent HIV-1 infection in Thailand’ New England Journal of Medicine 361(23):2209-2220
- 2. Van Gils, M.J. & Sanders, R.W. (2013) ‘Broadly neutralizing antibodies against HIV-1: templates for a vaccine’ Virology 435(1):46-56
- 3. Abdool Karim, S. et al (2010) ‘Effectiveness and Safety of Tenofovir Gel, an Antiretroviral Microbicide, for the Prevention of HIV Infection in Women’ Science 329(5996):1168-1174