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A recent outbreak of the paralyzing viral infection poliomyelitis in Haiti and the Dominican Republic has been traced to a strain of oral polio vaccine (OPV) that mutated back to virulence, according to international health officials.

Based on genetic analysis of viral samples, they believe the outbreak, which struck nearly 2 dozen children in both countries between 2000 and 2001 [see refs. above] arose from OPV given to one child in 1998-1999. Health officials had already cited the vaccine as the probable source of the outbreak, but this new analysis now makes it "unequivocally clear," research leader Dr. Olen Kew, of the US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, told Reuters Health. Their findings were published Thursday in Sciencexpress, the online edition of the journal Science [see: Sciencexpress 2002;10.1126/science.1068284].

Poliovirus spreads from person to person, usually going unrecognized because it produces mild symptoms or none at all. When it attacks nerve cells, however, the infection can cause crippling, sometimes deadly, disease. OPV contains a live, weakened polio virus that is very effective at conferring immunity to the infection. But it can, in rare cases, cause poliomyelitis. For this reason, countries such as the USA now use only inactivated polio vaccine (IPV), which is injected. OPV continues to be the standard in developing countries, however, because it is cheaper and easier to administer, doesn't require supplies of sterile needles and is more effective than IPV at preventing outbreaks.

But the cases in Haiti and the Dominican Republic illustrate a potential risk with OPV when it is given in a population where many people are unvaccinated. After a person receives OPV, virus from the vaccine is  shed in the stools for a short period of time. In this outbreak, shed virus from a single OPV dose spread and mutated back to a virulent state, causing paralytic disease in a group of children who had either not been vaccinated or had not received a complete course of OPV.  This same scenario has been blamed in a number of cases of paralytic disease in Egypt and the Philippines. According to the researchers, one of the critical factors in all of these outbreaks was the large number of unvaccinated, vulnerable people in the population.

In fact, Kew explained, it is common for OPV strains to "back-mutate."  It only becomes a potential problem in populations with low vaccination rates, which lack a "wall of immunity." He said the findings underscore the importance of improving vaccine coverage in areas such as Hispaniola, the island that comprises Haiti and the Dominican Republic. They also emphasize the need to eradicate wild-type poliovirus "as soon as possible," Kew and his colleagues note.  If eradication happens, public health officials could phase out the use of OPV, in favor of IPV. Officials are currently trying to develop an "end-game strategy" for going about this, said the CDC's Dr. Mark Pallansch, a co-author on the study. However, he noted, an "emergency stockpile" of OPV would be kept on hand in the event of a polio outbreak.
(By Amy Norton)
ProMED-mail<promed@promedmail.org

[Readers are referred to the Sciencexpress article referenced above
2002;10.1126/science.1068284
http://www.sciencemag.org/sciencexpress/recent.shtml

Through the use of nucleotide sequencing Kew et al were able to demonstrate that the viruses responsible for the paralytic poliomyelitis outbreak in the Dominican Republic and Haiti were derivatives from an attenuated poliovirus vaccine (OPV) in use during 1998-1999. The reversion of the vaccine derived poliovirus (VDPV) to a virus with neurovirulence that was able to circulate and lead to an epidemic of paralytic disease in an under-vaccinated population raises significant concerns for the future in terms of continued use of the attenuated poliovirus vaccine in areas where wild virus is no longer circulating.

In the article above, Kew is mentioned as saying that it is common for vaccine viruses to "back mutate" (revert to wild poliovirus-like neurovirulence), but the concerns arise in areas with low vaccination coverage and therefore low herd immunity, so that these reverted viruses are in an environment favorable for circulation and propagation of an outbreak of paralytic disease. As OPV has been in use since 1960, one wonders if this is a new occurrence or just a new recognition of the problem. In the USA in the 70s, vaccination coverages with OPV had fallen below the 80% rate in many areas (presumed to be due to complacency rather than a strong objection to vaccination), while background wild poliovirus circulation had been interrupted. Is the cut-off point for vaccination coverage which will support this occurrence much lower than that necessary to interrupt transmission of wild virus? What are the implications for the polio eradication efforts in regions that have interrupted circulation of the wild virus and have been declared as polio free, and therefore are no longer doing national immunization days (NIDs)? Can these countries sustain vaccination
coverages high enough to prevent this occurrence? And what are the
cost implications of this "emerging" infectious disease problem?
- - Mod. MPP]

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