Polio eradication: Not over for a while yet and why

The news on polio eradication — the almost 30-year up-and-down struggle to make polio the second human disease, after smallpox, to be eradicated from the planet — recently has been wonderful. The case numbers are seesawing — fewer in the four countries where the disease remains endemic, more in the countries where has recently crept back — but overall much lower than just a few years ago. The game-changer has been the entry into the eradication fight of the Gates Foundation, which has declared polio one of its major priorities. That infuses a huge bolus of cash into an effort that was always short of funds, but even more it shines a spotlight of international attention on a mostly neglected disease. Where Gates goes, governments and the media follow.

But before expectations of success rise too high, it is important to say that there is a largely unexplored and difficult end-game to polio eradication. It is smartly captured in an essay by Debora MacKenzie of New Scientist, who reminds us that ending all cases of wild-type polio will only be the end of the beginning — because we will still have to protect the world from the virus that we have been using all these years in the vaccine.

Here’s the risk, briefly stated (I explained it at more length here): The oral vaccine formulation used in the developing world (as in the photo above) uses a weakened but live form of the polio virus — deliberately, because the weakened virus takes hold in the gut, reproduces and produces immunity in the recipient, and then sheds in feces and indirectly immunizes close contacts. But once in a while, that virus reverts back to the virulence of wild-type polio, and then can cause full-strength polio infection — known as VDPV, for vaccine-derived polio virus — in anyone who is not currently protected. The risk that someone will not be protected is significant: Historically, once interruption of transmission is achieved, poorer countries cease vaccinating, turning the money they were spending on polio campaigns to other critical health needs. That leaves any child born after the end of vaccination vulnerable to infection if VDPV surges, as it did in the Caribbean in 2000 after the Americas were declared polio-free, and in Nigeria in 2006.

VDPV is not a risk in industrialized countries, because we no longer use the oral vaccine. Instead, we use the killed-virus injectable, which carries no risk of reversion to wild-type, but is more expensive and complex to administer because it requires health-care personnel, and sharps that must be disposed of. (For those keeping historical score, the injectable or IPV is the Salk vaccine and the oral, OPV, is the Sabin version.)

The only way to defeat polio for good is to switch any country that used OPV to IPV — that is, to do a final, global round of vaccination after the current campaigns have ended. That would be a formidable challenge just finding the personnel, distribution networks, and cash to pay for it. But as MacKenzie makes clear, there’s a bigger challenge: There may not be enough vaccine to go around.

Quoting a report she unearthed that was prepared for the Gates Foundation, she says:

The vaccine industry can make 120 million doses a year, but … if the world switched to IPV tomorrow, we’d need 450 million. Yet vaccine companies have little incentive to build expensive new factories to make a product that will mostly be used once wild polio is gone – maybe in three years, maybe 10 – and then only for another five years or so.

This is reminiscent of the problems the world faces in obtaining updated influenza vaccines: To immunize the planet against a truly dangerous pandemic strain, we would need not only updated vaccine technology, but much larger manufacturing capacity. Yet disease eradication is a public good, and it is not in the financial interest of private-sector companies to lead the way, given the uncertainty that they will see any return on their investments. (Here’s an analysis of flu-vaccine manufacturing shortcomings that I wrote several years ago, before the H1N1 “swine flu” epidemic made people think flu was no big deal.)

As in the case of flu, MacKenzie says, there are bright spots on the horizon, including some new formulations and dosing schedules that could make IPV more affordable. But she is right to say that insufficient attention to deploying IPV remains the major vulnerability for the long-embattled, now almost-successful campaign:

More research on IPV will help solve these problems, and that has been increasing since 2008. But research budgets everywhere are being slashed, and even with Bill Gates’s deep pockets the World Health Organization has barely enough money to do its main job of wiping out wild polio.

Right now, despite past setbacks, that is really looking likely. It will be a crime if we get the endgame wrong because making IPV wasn’t profitable enough, because the research money ran out, or because impoverished governments couldn’t justify the expense. Polio eradication is in sight. We must not drop the ball just as we reach the final, agonising inch.

Cite: MacKenzie D. Don’t let polio eradication slip away again. New Scientist, 29 March 2011.



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