ACCORDING to the World Health Organisation, malaria kills about 3,000 people a day, as many as 70% of them children under the age of five. Many groups of researchers are working on vaccines against the disease, but most agree that any vaccine that results will be imperfect. Nobody is expecting to confer full immunity with a vaccine, because the organisms that cause malaria are not viruses or bacteria (the traditional targets of vaccination) but single-celled animal-like creatures. These are a lot more complex and diverse than traditional vaccine targets. It is therefore hard to prime the immune system against all the strains of them that may cause the disease.
That might not be thought to matter much, on the basis that some protection is better than none. But a paper by Sylvain Gandon, Margaret Mackinnon and their colleagues at the University of Edinburgh, published in this week's Nature, shows that this ain't necessarily so. Partially effective vaccines may end up doing more harm than good. The researchers' mathematical models suggest that such vaccines may provoke the evolution of particularly virulent strains of the pathogen that causes the disease.
To understand why an imperfect vaccine might increase a disease's virulence, consider the matter from the pathogen's point of view. The main cost of increased virulence is that it will shorten the lifespan of the host, reducing the chances of the disease being transmitted to new hosts. On the other hand, a pathogen benefits from increased virulence because pathogenic organisms that are more virulent are less easily defeated by a host's immune system. That means that once a pathogen gets into a new host, it has a better chance of establishing itself.
In nature, the balance between these two forces is what governs the virulence of a given disease. The effect of a vaccine that confers full immunity, from the pathogen's point of view, is to reduce the size of its host population, since only unprotected individuals can then be infected. If anything, that will tend to reduce virulence, since the pathogen will have to hang on longer between transmission opportunities, and so will “want” its host to survive. But a vaccine that confers only partial immunity will increase host survival anyway, allowing pathogens that are not affected to “bank” this increased survival by becoming more virulent themselves.
This means two things. First, in the long run, the vaccinated will be no better off than they would otherwise have been. Second, the unvaccinated are actually worse off, since the newly virulent strain will spread at the expense of the older, less virulent ones. That is something that policymakers need to consider carefully if and when they are presented with a vaccine against malaria.