Our identification of recombinants derived from vaccine strains indicates that birds can be simultaneously infected with the live virus vaccine and other circulating aPMV-1 genotypes. Indeed vaccination is reported to protect poultry from disease but not always from infection with other strains [7, 8, 9] . Suboptimal vaccination strategies could also lead to birds becoming infected with both a vaccine strain and circulating genotype, which can alter viral virulence [16] . This is an important issue for poultry management. Lack of vaccine efficacy has not frequently been reported in the United States but other countries such as Nigeria [52] , Korea [53] , Taiwan [54] , and China [34, 55] have experienced vaccine failures. Recombination between wild type virus and vaccine strains is not unique to aPMV-1; vaccine recombinants of bovine viral diarrhea virus (associated with fatal mucosal disease) [35] , poliovirus (associated with paralytic poliomyelitis) [36, 37] ; and infectious bursal disease virus [38] have all been reported. This raises concerns that modified live virus vaccines, although efficacious, may facilitate emergences of new strains with unpredictable phenotypes through recombination with circulating viruses.