aVDPV

Ambiguous vaccine-derived poliovirus (aVDPV) is a designation used for a vaccine-derived poliovirus lineage in which there is no clear evidence of sustained transmission in a population. aVDPVs originate from the attenuated poliovirus used in oral polio vaccines, which can mutate as it replicates in the gut. When a virus isolated from a person or an environmental sample shows vaccine-derived characteristics but data are insufficient to determine whether transmission is ongoing, it is labeled an aVDPV.

Detection and classification rely on laboratory genetic analysis and epidemiological information. Sequencing of the virus helps determine that it is derived from a Sabin vaccine strain, and investigators assess whether there is epidemiological or environmental evidence of circulation. If transmission is demonstrated, the isolate may be reclassified as a circulating vaccine-derived poliovirus (cVDPV); if not, it remains an aVDPV until more data are available. Immunodeficiency-related vaccine-derived polioviruses (iVDPVs) are a related category that arises in people with certain immune disorders, but those are distinguished from ambiguous cases.

Public health response to an aVDPV typically involves enhanced surveillance, further laboratory testing, and investigations to identify potential links to other cases or environmental sources. Immunization activities may be intensified to raise population immunity and interrupt any possible transmission. The appearance of an aVDPV underscores the ongoing importance of high routine immunization coverage and careful environmental and clinical surveillance in polio eradication efforts.

See also: circulating vaccine-derived poliovirus (cVDPV), immunodeficiency-associated vaccine-derived poliovirus (iVDPV).