Vaccine confirmed as source of polio outbreak

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Vaccine confirmed as source of polio outbreak

By Amy Norton

NEW YORK, Mar 14 (Reuters Health) - A recent outbreak of the paralyzing

viral infection polio 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 two dozen children in both countries between 2000 and 2001,

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.

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

polio. For this reason, countries such as the US 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.

SOURCE: Sciencexpress 2002;10.1126/science.1068284.

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