The importance of optimizing outbreak response

From case investigation to mass campaigns

In April 2010, a wild poliovirus type 1 outbreak was confirmed in Tajikistan, representing the first importation of a wild poliovirus into Europe since the Region was certified polio-free in 2002. This outbreak underscores the risk ongoing indigenous will poliovirus transmission continues to pose to polio-free areas everywhere, and of the need to conduct effective outbreak response activities in the event of an importation.

First and foremost, in any outbreak, an effective initial case investigation must be conducted, to enable the subsequent implementation of comprehensive and effective control measures. Following reports of any case or cluster of cases of acute flaccid paralysis (AFP), an investigation should be launched to determine if the AFP case(s) display signs and symptoms consistent with polio. This will be followed by a virological investigation, ensuring collected specimens are processed in a WHO-accredited laboratory, and prioritizing any isolated poliovirus for intra-typic differentiation and genetic sequencing. If polio is confirmed, surveillance needs to be sensitized through active searches for additional AFP cases in the community, and the state of routine immunization coverage assessed to determine overall population immunity levels. Travel information should be collected to determine whether the case(s) (or their close contacts) had any connections with polio-endemic countries or areas. Finally, the descriptive epidemiology of the case or cluster being investigated should include information on geographical and temporal clustering, and age, gender and ethnicity.



Surveillance sensitivity should subsequently further assessed, including laboratory quality indicators (non-polio AFP rates, timeliness of stool collection, processing of stool specimens in a WHO-accredited laboratory, proportion of cases pending, geographical distribution of AFP cases, etc) for the area involved during the previous 12 months, to determine the possibility of transmission that might have previously been missed. Retrospective record reviews should be conducted in health facilities in the area of the outbreak and surrounding areas. Provincial surveillance units across the country should be instructed to notify, by telephone, of the possibility of cases in other provinces.

All of these activities will help lay the ground-work for an effective outbreak response. While the response activities are comprehensively conducted on a large-scale, resources and technical support can be prioritized, as appropriate, to those areas and populations identified during the case investigation to be at highest-risk. The outbreak response should follow the international outbreak response guidelines adopted by the World Health Assembly in 2006 (Resolution WHA59.1), with at least three large-scale immunization campaigns with the respective monovalent oral polio vaccine.

New approaches are now being evaluated to more rapidly and comprehensively build population immunity levels, as part of the outbreak response. The Short Interval Additional Dose (SIAD) approach, to more rapidly boost population immunity levels by administering subsequent doses of monovalent OPV at intervals of two weeks (rather than the traditional interview of four weeks necessary with trivalent OPV), could prove to be an important new tool in outbreak response. This approach has been successfully employed in outbreak settings in the past (eg Kenya, Somalia) and is now being applied in Tajikistan, and to further validate this approach in outbreak settings, a clinical trial in Egypt later in 2010 will assess SIAD in young infants.

These activities will provide invaluable insight into optimizing outbreak response, and may potentially lead to a revision of internationally-agreed outbreak response guidelines, if appropriate. In the meantime, to minimise the risk and consequences of an importation, it is critical that all polio-free countries maintain high population immunity levels through high vaccination coverage and strong AFP surveillance.