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NEW THIS WEDNESDAY: ASSESSING THE RISK FOR POLIOVIRUS OUTBREAKS IN POLIO FREE COUNTRIES -- AFRICA, 2012-2013

Monday, 23rd of September 2013

• ASSESSING THE RISK FOR POLIOVIRUS OUTBREAKS IN POLIO FREE COUNTRIES – AFRICA, 2012-2013

Editorial Note below; full text, with tables, is at http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6237a4.htm?s_cid=mm6237a4_w

Countries of the African "WPV importation belt" continue to be at risk for WPV outbreaks, as evident by the outbreak in the Horn of Africa that began April 2013. Assessment of the polio-free countries and subnational areas at higher risk for WPV transmission guides efforts to mitigate the impact of poliovirus transmission after WPV importations or emergence of VDPVs. The qualitative risk assessments allow the WHO Regional Office for Africa and other GPEI partners to prioritize SIA implementation. Risk assessments at the subnational level highlight underperforming areas to prioritize for targeted subnational SIAs. The value of preventive SIAs in risk mitigation in countries in the "WPV importation belt" is recently evident: the only importation-related outbreak identified globally in 2012 was in Niger, in which only a single WPV case was detected. Although risk assessments of countries of the WHO EMR had shown Somalia to be at high risk, security limitations prevented access to a large proportion of the population during the SIAs conducted before the outbreak was detected in May 2013. Additionally, in the 4 months since the outbreak was confirmed, preventive SIAs conducted in Kenya and Ethiopia, along with response SIAs conducted after the outbreak was detected, appear to have limited the WPV cases to certain high-risk border areas.

There are limitations associated with how risks are assessed, because many of the population immunity indicators are imprecise in indicating susceptibility overall and in particular, identifying pockets of underimmunized children; for this latter reason, many of the SIAs are not finely geographically targeted. Also, experience has indicated that SIA quality, assessed through the extent of planning, supervision, and delivery of poliovirus vaccine to a high proportion of children, tends to be lower in countries and areas where WPV has not recently circulated. Therefore, although SIAs enhance population immunity, they do not fully compensate for deficiencies in the delivery of health services and do not remove all risk.

SIA effectiveness has been improved by placing an increased emphasis on supervision and monitoring, which promotes greater accountability at the district and subdistrict levels (7). This has led to the identification and vaccination of children missed during previous SIAs and the formation of more detailed plans to improve subsequent SIAs. Better identification of repeatedly missed subpopulations, such as border populations and nomadic tribes, has led to more inclusive and detailed SIA plans, joint planning sessions between border districts, and improved synchronization of SIAs between countries (7,8).

Strengthening AFP surveillance will not decrease the risk for a WPV importation; however, prompt identification and rapid implementation of appropriate response efforts will limit the size of an outbreak (9). AFP surveillance performance indicators also have limitations in highlighting suboptimal surveillance (9). Although there are plans to extend environmental surveillance for polioviruses to some high-risk polio-free countries to improve the sensitivity of detecting poliovirus transmission and augment AFP surveillance, implementation of environmental surveillance requires substantial investment in personnel, supplies, and equipment to collect, process, and test specimens (9,10).

Mitigation activities are guided by periodic risk assessments. Plans are continually adapted based on the availability of funds, variation in the vaccine production cycle, and the changing epidemiology of WPV. These variables necessitate that GPEI partners make data-driven decisions to prioritize activities.

Throughout the WHO AFR, civil unrest and insecurity pose an increasing challenge for vaccination teams to access and reach children during SIAs. Insecurity has weakened routine immunization programs, hindered preventive SIAs, and limited AFP surveillance in large portions of countries, such as Mali and the Central African Republic during 2012–2013, and in many subnational areas throughout the region, including large parts of northern Nigeria. Partnering with relief organizations and implementing targeted SIAs as areas become accessible can assist in mitigating risks secondary to insecurity.

Going forward, GPEI partners are attempting to engage all development agencies in coordinated efforts to enhance childhood immunization services to optimize population immunity (10). Addressing funding limitations will be integral to ensuring that mitigation activities continue. All efforts to assess and mitigate risks will continue in the WHO AFR as long as endemic circulation of WPV is occurring in areas with low levels of population immunity.