Weak vaccines 'strengthen disease'

Some new vaccines may not be 100% effective

Scientists have warned that using 'inadequate' vaccines to fight diseases like malaria could lead to the outbreak of more virulent forms.

Researchers at the University of Edinburgh suggest that over time the new strains could kill more people than any vaccination programme would save.

These findings have policy implications for the development and use of vaccines

Andrew Read, University of Edinburgh

Many vaccines save people from dying but are not 100% effective because they do not stop individuals from carrying and transmitting a disease.

The Scottish scientists say that this enables the disease to evolve into a deadlier form which can become resistant to existing vaccines.

Andrew Read and colleagues at the University of Edinburgh used a mathematical model to examine the risks.

Reduced benefits

They concluded that in most cases using inadequate vaccines or those that are not 100% effective could mean that any benefits, such as saving lives, are eroded.

Writing in the journal Nature, they said: "This evolution can erode any population-wide benefits such that overall mortality rates are unaffected, or even increase, with the level of vaccination coverage."

The scientists said their findings could have important implications for future vaccination programmes.

"These findings have policy implications for the development and use of vaccines that are not expected to provide full immunity, such as candidate vaccines for malaria."

Professor Charles Bangham from Imperial College London said vaccines against HIV and hepatitis gave him most concern.

He told Nature that these viruses were difficult for the body to eradicate and were at high risk of evolving and reproducing.

He said that existing vaccines to protect against smallpox, polio and measles were effective and were not in this at risk group.

He suggested that all new vaccination programmes should aim to use only 100% effective vaccines.