SUNDAY, SEPTEMBER 25, 2016

Study hints at wrinkle in polio eradication effort

This is a molecular surface diagram of the three-dimensional structure of type 2 wild poliovirus strain.
Eradication of polio is a very real possibility, with Pakistan and Afghanistan being the only two countries that have yet to be declared polio-free.

However, a study published Thursday in PLOS Pathogens suggests a possible complication to efforts to eradicate the viral disease.

Currently, there are three strains of wild polio virus (1, 2, and 3) and two different types of polio vaccine.

Inactivated polio vaccine (IPV) works by inducing neutralizing antibodies to protect against paralytic polio; oral polio vaccine (OPV, which contains weakened, live virus) neutralizes antibodies as well as inducing mucosal immunity.

On rare occasions, OPV has caused vaccine-associated paralytic polio; side effects can also include circulating vaccine-derived polioviruses (cVDPV).

Over 90 percent of cVDPV cases are caused by the strain 2 component, which has not been seen "wild" since 1999 and is believed to be eradicated. Strain 2 OPV is also responsible for as much as 38 percent of vaccine-associated paralytic polio cases.

Because of this, the World Health Organization plans to implement a switch from trivalent OPV (containing strains 1,2,3) to bivalent OPV (containing only strains 1 and 3) in routine immunization programs.

Once eradication occurs, the use of all OPV in routine immunizations will cease.

But in the recent study, Javier Martin of the National Institute for Biological Standards and Control, Potters Bar, United Kingdom, and his colleagues analyzed more than 100 stool samples collected between 1995 and 2015 from a white male who had gotten a full course of childhood immunizations, including OPV at 5, 7, and 12 months old, with a booster at about 7 years of age.

The man was later diagnosed with an immunodeficiency. Researchers found high levels of strain 2 polio virus in all stool samples analyzed. Further analysis showed that the excreted viruses were different from the weakened vaccine strain, and that they had started to diverge from it an estimated 28 years ago, around the time of this individual's last known vaccination with OPV.

The study also showed that "of the total of 73 iVDPV cases that have been described between 1962 and 2014, only seven involved infections lasting more than five years."

Further, a number of highly mutated VDPV strains that showed molecular properties typical of iVDPVs had recently been isolated from sewage samples in Slovakia, Finland, Estonia and Israel, indicating an unknown number of chronic excreters exist elsewhere.

The conclusion by the researchers was "enhanced surveillance including sewage sampling and stool surveys to search for the presence of iVDPV strains and the development of efficient anti-viral treatments to interrupt virus replication in immune-deficient individuals are needed to be able to identify and manage the possible risks of iVDPV strains spreading and causing disease in patients and the general population, particularly in the light of changes in vaccination strategies as part of the polio eradication endgame and the absence of an established outbreak response strategy."

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