Cross-posted from: "[health-vn discussion group]" <health-vn@cairo.anu.edu.au>
Effort to Defeat Polio Faces Unique Challenges
Unclear When Vaccination Can Be Halted
By David Brown
Washington Post Staff Writer
The troubled 19-year-old campaign to eradicate polio is celebrating recent progress and an unexpected infusion of cash, but experts are coming to realize that they will not be able to end the expensive and laborious efforts to control the virus anytime soon.
Ridding the world of polio will be a far messier business than the 1977 eradication of smallpox, which remains a unique achievement in medicine. That is because it is now clear that the virus that causes polio could reemerge years, and possibly even decades, after the last case is found.
The reason involves peculiarities of poliovirus and the oral (Sabin) vaccine being used to eradicate the disease.
The vaccine contains a weakened poliovirus that stimulates immunity against the "wild" virus, which can cause paralysis. On rare occasions, however, the vaccine virus can mutate to a more dangerous form, spread from person to person and cause a paralytic infection.
This phenomenon, only recognized in the past decade, has caused outbreaks in 10 countries since 2000. This year, 7 percent of all polio cases worldwide were caused by mutant, vaccine-derived virus.
The only way to prevent this from happening is to keep the world's infants and toddlers fully immunized -- in other words, to keep up the exhausting, expensive full-court press that has now gone on seven years longer than was anticipated.
To rid the planet of polio, people will eventually have to stop using the oral vaccine. Whether they should forgo immunization altogether or use the alternative vaccine -- the Salk "polio shot" that does not contain live virus and cannot cause infection -- is now under debate.
Switching to the Salk vaccine, however, will be hard.
It's more expensive: roughly $2.70 per dose, compared with 15 cents for the oral vaccine. About 135 million infants are born each year, and each will need at least two shots. While the two companies that make most of the world's supply can ramp up production to that level, it will take them at least five years, and possibly a lot longer.
Consequently, even after eradication, many countries will have to continue using the oral vaccine -- and be prepared to suppress mini-epidemics caused by it.
Before the eradication campaign began, polio paralyzed more than 350,000 people a year in 125 countries. Stopping that suffering is the chief goal of eradication.
Health officials also hoped the end of polio would also mean the end of Polio vaccination, with its trouble and expense. That was the case with smallpox. No country routinely vaccinates against that disease anymore, a move that has saved more than $17 billion in the United States alone.
But an unambiguous transition to a "polio-free" world is unlikely. It will be hard to know when, if ever, it is safe to forgo vaccination.
Part of the reason is a fear that terrorists might intentionally reintroduce the virus -- a possibility countries did not worry about much when they decided to stop smallpox vaccination. But there is also the chance of accidental or unrecognized release.
Work is underway to identify laboratories holding poliovirus, reduce their number and increase their security. But unlike with smallpox, there may be places that have the pathogen and do not know it.
Poliovirus replicates in the intestine. Thousands of labs around the world have frozen samples of feces that may contain the virus -- either the wild type or the vaccine virus -- that could theoretically escape.
Further complicating matters is the fact that most polio cases are "silent." In only 1 in 200 infections from the wild virus is there the characteristic muscle weakness that lets physicians make the diagnosis. Polio virus can travel far before anyone knows it.
The campaign threatened to unravel in 2003 when several populous, largely Muslim states of northern Nigeria stopped immunizing because of rumors that the polio vaccine contained the AIDS virus or an antifertility drug. The disease roared back. By the time vaccination resumed a year later, the virus was out of the barn. Over the next three years, 20 countries were reinfected with virus traced via genetic fingerprinting to strains from northern Nigeria. (Seven other polio-free countries were reinfected at the same time after a resurgence of the disease in India.) These included not only many African countries but also such faraway places as Saudi Arabia and Indonesia. Worldwide, polio's annual toll, which had fallen to 784 in 2003, climbed to 2,000 last year.
Responding to the Nigerian outbreak took hundreds of millions of dollars and a huge amount of work.
The main tool in eradication is National Immunization Day, a coordinated effort in which workers go door to door over one or two days and put drops of vaccine in the mouths of every child under age 5. Some reinfected countries had to hold four or five of these days in a year, rather than just two, to become polio-free once more. (Outbreaks caused by vaccine-derived virus are handled the same way.)
The effort is paying off. The number of reinfected countries is down to seven, and the number of countries where the disease is still "endemic," meaning it has never been stopped, is now four, the lowest in history. (They are Nigeria, India, Afghanistan and Pakistan.) Through last Tuesday, only 857 cases of the disease had occurred this year.
But in the northern Indian states of Uttar Pradesh and Bihar, which have a combined population of 270 million, polio is holding on tenaciously. Astonishingly, one-third of polio cases in northern India last year were in children who had gotten the vaccine at least 10 times and still had not developed immunity.
To address the problem, polio campaigners recently switched to a vaccine with only one type of polio virus in it. (Normally, there are three.) This cuts down on the competition and targets the specific type of wild virus in the area. The campaign is gaining traction.
The initiative, supposed to be completed by 2000, has cost $5.3 billion so far. Rotary International, which has 32,000 civic clubs around the world, has contributed $675 million. Last month, the organization got a $100 million donation from the Bill and Melinda Gates Foundation, contingent on the clubs' matching that sum.
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Vern Weitzel
mailto:vern@coombs.anu.edu.au