NEW ORLEANS The Centers for Disease Control and Prevention (CDC) officially recommends a sequential polio vaccination schedule.
The CDC now recommends two doses of inactivated polio vaccine (IPV) at 2 and 4 months of age, followed by two doses of oral polio vaccine (OPV) at 12 to 18 months and 4 to 6 years. Although this schedule is preferred, physicians and parents may choose to follow an all-IPV or an all-OPV schedule, depending on each child's situation.
The debate over whether the schedule should be changed may not yet be over. At the recent Infectious Diseases Society of America 34th Annual Meeting here, Samuel Katz, MD, and D.A. Henderson, MD, explained why they stand on opposite sides of the issue.
Katz, a pediatrician at Duke University, Durham, N.C., favored the change to a sequential IPV-OPV schedule.
Wild poliovirus has been eliminated from North and South America since 1991, and in 1995, 150 countries worldwide reported no cases of wild poliomyelitis, Katz said. The World Health Organization (WHO) estimated that 83% of all children received at least three doses of polio vaccine before their first birthday. Polio remains endemic in only 62 countries.
As polio nears eradication, the risk-benefit ratio has changed, Katz said. The risk of acquiring vaccine-associated paralytic poliomyelitis (VAPP) from the live oral vaccine outweighs the benefits offered by OPV immunization: stronger mucosal immunity and ease of administration.
Since 1979, only vaccine-associated polio has occurred in the United States. An average of eight cases occur each year, almost equally divided between vaccine recipients and their close contacts. Most cases in vaccine recipients occur after the first dose of OPV.
"There are complexities to any change in policy; no one likes to change," Katz said. "We have been [vaccinating with OPV] in this country for over 30 years and it is very difficult to convince people that a major change should be made because six or eight individuals may be paralyzed."
Henderson, who for the past decade chaired the technical advisory group for polio eradication in the Western Hemisphere, remains unconvinced. He called the switch a "halfway policy": a costly process that will produce unintended consequences without addressing the underlying problem.
"What is the rationale for change? It is VAPP," Henderson said. "We saw a larger number of cases in the pre-1985 period, when the type 3 virus was changed to one that is less reactogenic. We see diminishing cases since that time, and in the last four to five years we've seen approximately six cases per year. ... This is a very rare event, indeed. If we change to this new schedule, the thought is we would prevent the cases among those receiving the vaccine. ... We are looking at a policy to prevent two, maybe three cases at a cost of somewhere between $25 and $80 million."
Although Katz favored using IPV for the first two doses in the United States, he firmly stated that it is not a feasible option for countries where polio remains endemic. But he disagreed with critics who fear that countries with endemic polio would follow the United States and switch to IPV.
"In 1971, this country discontinued smallpox immunization, and that did not in any way detract from the conclusion six years later of smallpox eradication globally. And if in 1996 we discontinue the primary use of OPV, I am not concerned that will alter the year 2000 goal or the program of the World Health Organization."
Henderson, chief medical officer of the 1966-1977 WHO smallpox eradication campaign, was less optimistic.
"At this time, polio is endemic in countries that roughly correspond with the countries that had endemic smallpox when we began the smallpox eradication program. It took us 10 years to eradicate smallpox from those countries. Why? Because they are the poorest countries with the poorest infrastructure. ... I am more optimistic than most I have been called too optimistic but I hope that we eradicate polio by 2005, maybe in 2010 more realistically. ... The bottom line is we are going to have polio in the world for many years to come. It is not going to be gone by 2000, and that you can take to the bank," Henderson said.
As long as polio remains in the world, the risk of importation is a real threat. In its draft recommendations, the CDC stated that the risk of wild poliovirus being imported into the United States was "exceedingly remote," Henderson said. However, the CDC also estimated that last year alone, 40 to 200 undetected importations into the U.S. occurred. Wild polio was imported into Canada in 1988, 1993 and March 1996.
"Is Canada so different from the U.S.? I don't think so," Henderson said. "Am I willing to say that it is exceedingly remote that we are going to have cases imported into the United States? I have trouble with that."
In a 1988 review of the polio policy, the Institute of Medicine (IOM) concluded that the polio vaccination policy should not be changed until a combination vaccine incorporating IPV and other antigens is available, according to Henderson, who was a member of that IOM committee.
"We have a more complex immunization schedule than we had before Hib, varicella and hepatitis B vaccines were added. Now there is even more reason not to add two more inoculations. We clearly have fewer vaccine-associated cases than in 1988. If there was cause then to reject this change in policy, there is even more cause now."
The two additional injections required by the sequential schedule is an important consideration, especially with the recent licensure of DTaP vaccines for the infant series, which are not yet combined with other antigens.
"This change in polio policy was basically decided alone without considering the implications it would have on the broader immunization policy program," Henderson said. "We have not looked at the overall issues with the increasing number of vaccines that are available. We cannot go along approving one vaccine after another, looking at one schedule after another in a vacuum as if nothing else existed. This is what has been done here, and it has created complete chaos."
Combination vaccines are essential, but the ones that will be most useful are still in development. Licensure of the first DTaP combination vaccine (DTaP-Hib) is anticipated by the end of this year, Katz said, as is a combination of Hib with hepatitis B vaccine. The ultimate combination DTaP-Hib-IPV-hepatitis B vaccine is anticipated in 1998.
"Our success is one of our enemies: the more we succeed, the more complex the system becomes," Katz said. "But that should not deter us from trying to protect children safely against vaccine-preventable disease."
|% Cases Related to OPV Dose|
The table shows the percentage of people stricken with vaccine-associated paralytic polio in the United States from 1980 to 1991. The risk appears to decrease with the number of doses.
You can express your views on this article, or other relevant themes, in the Infectious Diseases in Children Specialty Forums.