Implications of circulating vaccine-derived poliovirus

A circulating vaccine-derived poliovirus is a rare strain of poliovirus, genetically changed from its original strain contained in Oral Polio Vaccine (OPV).

The emergence of a vaccine-derived poliovirus that can circulate in the population shows that too many children remain under-immunized. A fully-immunized population with OPV will be protected from all strains of poliovirus, whether wild or VDPVs.

Although quite rare, cVDPVs are not a new phenomenon and have occurred in various parts of the world. In the past 10 years worldwide:

• over 10 billion doses of OPV have been administered to more than 2 billion children;

• 9 cVDPV outbreaks have occurred in 9 countries, in communities with low OPV coverage, resulting in under 200 polio cases;

• during that period, more than 33,000 children were paralyzed by wild poliovirus while over 3.5 million polio cases were prevented by OPV.

cVDPVs in the past have been rapidly stopped with 2-3 rounds of high-quality immunization campaigns with OPV.  The solution is the same for all polio outbreaks: immunize every child several times with OPV to stop polio transmission, regardless as to the origin.

The benefits of OPV far outweigh the risk of a cVDPV: 

• OPV has been the vaccine of choice for the more than 190 countries which have eliminated polio.

• OPV remains the only vaccine used by the Global Polio Eradication Initiative to interrupt all wild poliovirus transmission, globally.

cVDPVs in Nigeria

The emergence of a circulating vaccine-derived poliovirus in Nigeria reaffirms that not enough children are protected from poliovirus (wild or vaccine-derived) and that much more must be done to reach all children with vaccine.  Of the 69 children with cVDPV in Nigeria, 40% had never been vaccinated; 87% were under-vaccinated (three or fewer doses).

Consistent with global recommendations, three rounds of trivalent OPV (the recommended vaccine for the type of cVDPV in Nigeria) were conducted in northern Nigeria after the first case was confirmed in 2006. The first round was conducted in November 2006 another in January 2007 and a further round in March 2007. These three rounds of immunization have reduced by more than half the number of cVDPV transmission strains and the geographical extent of the virus. In September 2007, an additional dose of trivalent vaccine was administered to children in the 13 high risk northern states, including those where the cVDPV continued to circulate.

Information on all cVDPVs in 2006-2007, including the cases in Nigeria have been available publicly since April 2007 (report in PDF), and have been included in presentations at various polio eradication and global laboratory network meetings.  Reports on both the work of the global lab network and on VDPVs in general have been issued as standard every year.

Since introduction of monovalent oral polio vaccine against type 1 (mOPV1) in Nigeria , wild poliovirus type 1 has declined: 58 cases have been reported this year as compared to 846 last year. Type 1 polio, which has caused international outbreaks, has a higher paralytic attack rate than the two other types and is the eradication effort's primary target.

Wild poliovirus remains a greater threat to children in Nigeria than vaccine-derived virus. Since 2005, Nigeria has reported over 2000 polio cases due to wild poliovirus. In that same period, there have been 69 cases due to circulating vaccine-derived poliovirus.

Nigeria continues to improve its polio immunization activities, both supplementary and routine to stop all polio transmission, including the cVDPV. The critical issue is to achieve high coverage during these activities by reaching all children.

The cVDPVs in Nigeria are due to type 2 poliovirus, which was eliminated in the wild in 1999. It is the most responsive of the 3 types of poliovirus to OPV.  Previous type 2 cVDPVs were detected in Madagascar in 2002 and 2005 and in Egypt in the 1980s-90s. 

Enhancing routine immunization with trivalent OPV (targeting all 3 types of polio) in the northern states is the key to maintaining immunity against type 2 polio, as monovalent OPVs are increasingly used to eradicate type 1 and type 3 wild polioviruses.