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To understand why an HIV vaccine has not yet been developed, it may help to spell out what a vaccine is – and is not.

A vaccine is, essentially, a fake infection.

Edward Jenner discovered in 1757 that if you gave people cowpox, they didn’t get smallpox. Even before then it had been known in some cultures that a mild form of a disease could protect people from a severe form. This is because if people have a mild infection – or are given a substance that is not an infection but looks like one to the immune system – then the immune system will remember it.

This remarkable ability of our immune system protects us from repeated severe illnesses that would otherwise eventually kill us.

Once the B-cells and T-cells that are the body’s disease fighters have reacted to an infection and helped to expel it from the body, then most die. But a small proportion of long-lived memory cells withdraw deep into the immune system’s centres in the bone marrow and lymph nodes. They are primed to wake up and start reproducing in huge numbers as soon as the infection they remember, or something that looks like it, comes along.

To use a metaphor: 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. (HIV, however, is a master of disguise, and can subvert the security force.)

Is PrEP a vaccine?

Sometimes other ways of preventing HIV are spoken of as if they were a vaccine. This is particularly the case with pre-exposure prophylaxis (PrEP). It’s a medicine, and it stops you getting HIV – so isn’t it a sort of vaccine?

The difference between a vaccine and PrEP is that a vaccine, because it mobilises our body’s own defences, may only have to be renewed occasionally or, in the case of childhood vaccines for things like polio or measles, never.

Glossary

vaccine

A substance that contains antigenic components from an infectious organism. By stimulating an immune response (but not disease), it protects against subsequent infection by that organism, or may direct an immune response against an established infection or cancer.

 

pre-exposure prophylaxis (PrEP)

Antiretroviral drugs used by a person who does not have HIV to be taken before possible exposure to HIV in order to reduce the risk of acquiring HIV infection. PrEP may either be taken daily or according to an ‘event based’ or ‘on demand’ regimen. 

immune system

The body's mechanisms for fighting infections and eradicating dysfunctional cells.

prophylaxis

Taking a drug to prevent an illness. Primary prophylaxis is the use of drugs to prevent a first occurrence of illness. Secondary prophylaxis is the use of drugs to prevent re-occurrence of illness. Pre-exposure prophylaxis (PrEP) and post-exposure prophylaxis (PEP) for HIV are both forms of primary prophylaxis.

lymph nodes

Bean-sized structures throughout the body's lymphatic system, where immune cells congregate to fight infections. Clusters of lymph nodes are found in the underarms, the groin, and the neck.

But PrEP stops working when you stop taking it.

In the near future, long-lasting formulations of HIV drugs will be available that may only have to be given once or twice a year as both treatment and PrEP – and then as injections or implants, so they may feel like a vaccine.

The power of a vaccine, however, lies in the fact that it strengthens your body’s own defences against viruses and bacteria, instead of simply killing or inactivating them with a drug.

Getting it to do this, though, especially with a foe as tricky as HIV, is not straightforward, and has already had to involve some groundbreaking science.

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