HIV is a very unusual virus and very difficult to vaccinate against.
HIV does not usually produce an immune response that is enough to stop or contain infection. This is one reason HIV infection is life-long. More typical viruses such as flu do: they stimulate an immune response that is sufficient to clear the virus from the body without using medications. A vaccine that imitates them can do the same.
We know it’s possible to produce a strong enough response to HIV, because a few people – so-called ‘elite controllers’ – do manage this feat and can stop HIV replicating damagingly without the help of antiretrovirals.
However, most people need life-long therapy to stay well.
Why? Firstly, HIV disguises itself. It coats its surface proteins – which are the ones that stimulate the main immune response – with a coat of fuzzy sugar molecules that stop immune-system antibodies from recognising and locking on to them.
Secondly, no one individual component of HIV is stimulating enough to the immune system. HIV vaccines need to package inside themselves different sections of HIV that will, combined, hopefully produce a strong enough immune response.
HIV also reproduces rapidly and imperfectly. This means that it is very genetically variable within a population. Vaccines will need to stimulate an immune response that recognises a wide variety of different viral strains.
It also becomes very variable within individuals. This means that it can become resistant, often quickly, to the immune defences created by a vaccine, in the same way that it can develop drug resistance. A vaccine would need to create such a strong response that any virus that has already got inside the body could be neutralised before it starts diversifying.
HIV hijacks the body’s own cellular immune response and turns the very T-cells that direct that response into its breeding ground – which means that the most basic types of vaccines, which use live but weakened versions of the same infection, could make HIV worse.
And finally, it is also a retrovirus, which means that even if suppressed by antiretroviral drugs or an initial immune response, it hides itself away deep inside our own DNA, where it is invisible to the immune system: this is the other reason HIV infection is life-long. In particular, it is invisible to the T-suppressor cells (CD8 cells) that can kill virally infected cells.
This is perhaps an issue for curing HIV more than preventing it getting into our DNA in the first place, but hypothetical cures for HIV may well rely at least in part on a therapeutic vaccine that can neutralise HIV as soon as it comes out of hiding.
One analogy of how vaccines work goes as follows: if the body is a country, then its immune system is like a combination of its immigration and its security forces. A vaccine is like a photo ID of a terrorist, circulated to all members. Then, if the real person ever turns up, they are either refused entry, or, if they slip past security, hunted down and expelled.
However, if the terrorist is HIV, it will be more difficult for the immigration and security forces to do their job.
Firstly, HIV is a master of disguise and, to the Border Force, looks nothing like its photo.
Secondly, it’s more like a terrorist organisation than a single person: the immune system can’t keep up with its new recruits, whether they’re entering the country or are being radicalised there.
Thirdly, if it does get in, it can stay for a long time in ‘sleeper cells’ that are difficult for the security forces to detect and infiltrate.
Full image credit: A man is getting an injection with a syringe at hospital. Image by Marco Verch Professional Photographer. Available at https://foto.wuestenigel.com/a-man-is-getting-an-injection-with-a-syringe-at-hospital/ under a Creative Commons licence CC BY 2.0.