vaccine for polio devised 50 years ago has crippled the disease worldwide, fueling hopes of stopping its transmission in every nation by 2005.

Polio was endemic in countries throughout the world in the 1940s and early 50s, paralyzing 20,000 children annually in the United States alone. In their quest for a vaccine, scientists hit a wall: They could not grow the virus outside of nerve tissue, which would provoke dangerous levels of inflammation in the brain if used in a vaccine.

But 1954 marked a turning point. Jonas Salk emerged from his laboratory with a promising vaccine, thanks to three Boston scientists whose work was recognized that year with a Nobel Prize: Harvard bacteriologist-virologist John Enders and two young associates, Frederick Robbins and HSPH professor Thomas Weller.

The trio's breakthrough, Weller later wrote, was "almost an afterthought." While working to isolate the chicken pox virus, they hit upon a technique for culturing large quantities of poliovirus in non-nervous tissues, opening the door to making vaccines for polio and myriad other diseases.

Salk's vaccine, an injectable one made from killed poliovirus, passed muster in large trials. But it was an oral form created with weakened virus, introduced in the 1960s by Albert Sabin, that made vaccination on a global scale possible. Besides being cheaper than Salk's, Sabin's vaccine could be administered in poor and remote areas by volunteers. Moreover, the live virus could pass through the gut from one person to another, imparting immunity not only to vaccine recipients, but to their close contacts as well.

So successful were polio control efforts that, in 1988, the World Health Organization, UNICEF, the U.S. Centers for Disease Control and Prevention, and Rotary International launched the Global Polio Eradication Initiative. Polio paralyzed 350,000 children in 125 countries that year. By 2003, fewer than 700 cases were found in just six nations: India, Pakistan, Egypt, Afghanistan, Niger, and Nigeria.

Periodic outbreaks have disrupted the initiative's forward march. In 2003, the Nigerian state of Kano halted vaccinations in response to some Muslim leaders' fears that the campaign masked a mass-sterilization effort. As a result, poliovirus spread quickly through Nigeria and several neighboring nations.

"While the goal of wiping out the disease may be met in the near future, many obstacles remain to ridding the world of the virus itself," says HSPH Dean Barry R. Bloom. Poliovirus can linger asymptomatically in the vaccinated host, then spread and cause disease in unvaccinated individuals. Rarely, the Sabin-vaccine-derived virus has been known to mutate into a virulent form. (To prevent this, many experts advise a switch to the killed-virus Salk vaccine once nations become polio-free, despite the cost.) No one yet knows when it might be possible for countries to cease surveillance or stop vaccinating children, since doing so would leave them defenseless against circulating virus as well as vaccine-derived strains. Yet another challenge will be to inventory and control all viral strains in laboratories worldwide to prevent their release, either accidentally or deliberately.

Despite the hurdles, Bloom says, hopes run high for eliminating polio-related deaths and disability. With $3 billion and 20 million volunteers behind it to date, he says, "The Global Polio Eradication Initiative is the most ambitious public health campaign the world has ever seen."

 


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