CSU 5/2011: THE EPIDEMIOLOGY OF POLIOMYELITIS DECONSTRUCTED

Monday, 3rd of January 2011

CSU 5/2011: THE EPIDEMIOLOGY OF POLIOMYELITIS DECONSTRUCTED

A tour d’horizon by two well known writers on poliomyelitis. From their conclusions:

If polioviruses are to be eradicated globally, several requirements must be met. First, in countries where wild polioviruses still circulate, there must be a massive effort to immunize a high proportion of very young infants with OPV—perhaps followed by 1 or more doses of IPV. Second, in countries where wild polioviruses are no longer circulating, there must be a transition from OPV to IPV to eliminate VAPP and reduce the dangers of outbreaks associated with cVDPVs. Finally, there must be continuation of rigorous surveillance efforts to inform these programs, with a plan for emergency intervention whenever polio outbreaks occur. This strategy will require the coordinated effort of international and national public health programs, supported with sufficient resources to produce and administer both OPV and IPV. Only time will tell whether this strategy will be implemented and, if implemented, whether it will succeed.

The authors' discussion of persistent transmission, starting on page 1221, deserves a close reading. 'Why has it been so difficult to complete the eradication of poliomyelitis, considering past successes in so many countries, including tropical nations, where all of the risk factors prevail and resources are often limited? Explanations fall into 3 main categories: failure to vaccinate, failure of the vaccine, and viral epidemiology. Each mechanism appears to play a role to varying degrees in different polio-endemic countries.'

Their discussion of the contentious 'endgame' issues linked to polio eradication:

'Let us assume for purposes of discussion that it is feasible to eliminate wild polioviruses from those countries where it is still endemic. There has been extensive discussion among experts regarding alternative strategies for the posteradication endgame (109–116). Indefinite continuation of OPV carries several liabilities: continued cases of VAPP including new prolonged excretion of immunodeficiencyassociated VDPVs by immunodeficient vaccinees); the potential for new outbreaks caused by cVDPVs; and public health ‘‘fatigue,’’ leading to reduced OPV coverage and its attendant dangers. Termination of OPV without transition to IPV carries the risk of initiating new outbreaks caused by cVDPVs, because of either spread of cVDPV during the termination phase or unregulated importation of OPV from other countries. Additionally, this option carries the ethical liability of a double standard for high- and low income countries. For these reasons, there is an evolving (but not necessarily total) consensus that, in principle, the optimal choice is a transition from OPV to IPV (109, 113–115, 117).

The most important concerns about the IPV option have focused on practical issues regarding the cost and global coverage levels attainable with an injected vaccine as compared with an oral vaccine (118). However, most middle and low-income countries are already using injected vaccines for other diseases, so they could adopt IPVas either an additional or incorporated product. IPV is more expensive than OPV, and it has been estimated that it would cost at least $1 billion to produce sufficient IPV for developing countries. In addition, there are issues pertaining to formulation (a single product vs. incorporation of IPV with other immunogens), dosage regimens, and recruitment of collaborating manufacturers with sufficient production capacity (119). Recently, the Global Polio Eradication Initiative undertook to fund a research program for creation of an affordable IPV for developing countries (21, 118, 120), including exploration of the efficacy of fractional intradermal IPV doses (121, 122). A key research question, which is the subject of several ongoing and proposed studies, is the efficacy of IPV in preventing poliovirus circulation in the highest risk settings (21, 123).

Full text is at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2991634/?tool=pubmed

Students of cVDPV should look at its recent history in China, as set forth in a recent article in the Journal of Infectious Diseases. None of the five cVDPV in China achieved sustained transmission in a country with high routine OPV coverage and excellent AFP surveillance.

If these Chinese findings, accessible online at http://www.ncbi.nlm.nih.gov/pubmed/21050127 , were generalizable across other developing countries, the need for IPV to put paid to cVDPV would be less pressing than it is. Even in China, one autonomous region had sustained local transmission in a remote area; see details at http://www.ncbi.nlm.nih.gov/pubmed/20631095 The cVDPV circulated for about 12 months after the initiating OPV dose in mountain communities of Guangxi where only 27 percent of children aged 5 to 10 had received 3 or more doses of OPV. If this can happen in China, it can, a fortiori, happen elsewhere.

Good reading.