Sir

David L. Heymann and colleagues, from the World Health Organization (WHO) issued a global call for new poliovirus vaccines in their recent Commentary article (Nature 434, 699; 2005). This represents a welcome change in the WHO poliovirus eradication strategy; however, some important issues remain to be addressed.

The global struggle against poliomyelitis has been a huge success and, ultimately, is expected to lead to eradication of the disease. However, several risk factors associated with the principal vaccine used today (oral polio vaccine, OPV), suggest that new vaccines will still be needed to accomplish eradication.

As Heymann and colleagues note, circulating vaccine-derived polioviruses have caused poliomyelitis outbreaks in five countries. Moreover, some people continue to excrete virulent poliovirus more than ten years after being vaccinated with OPV. Ending OPV immunization, therefore, can lead to increased risks from OPV derivatives, and Heymann and his colleagues are justified in calling for the development of new vaccines.

At present there are no adequate alternatives to trivalent OPV, but the WHO proposes to replace it with monovalent OPV (mOPV). Although mOPV vaccination may be useful under some circumstances, it poses the same risk factors as OPV. These risks vastly increase if mOPV is used in the post-vaccination era, because of diminishing population immunity.

Switching from OPV to the more costly Salk inactivated vaccine (IPV) eliminates these risks, and this has occurred in most industrialized countries. However, accidental release of the wild-type virus during IPV manufacture threatens to restart epidemics. Therefore, the WHO suggests using ‘Sabin IPV’ (sIPV) produced by inactivation of OPV. However, questions about the effectiveness of sIPV require further research and extensive clinical trials.

We propose instead that time should be given to conducting research for truly new poliovirus vaccines, and for the development of anti-polio drugs.

We also urge the WHO and public-health authorities around the world not to stop OPV vaccination until an efficacious and low-cost IPV has been developed.

We must also consider whether we should ever terminate poliovirus vaccination. In a world threatened with terrorism, we should remember that polioviruses could be synthesized rapidly at very low cost. The polio outbreak that happened in the 1940s in an Eskimo village in arctic Canada, with 25% poliomyelitis and high mortality, provides a sobering example of the devastation that can occur in unvaccinated communities.