A replication defective chimpanzee adenoviral (ChAd) vector comprising a nucleotide sequence derived from an adeonovirus selected from the group of isolates consisting of ECACC accession numbers 03121201 (ChAd4), 03121202 (ChAd5), 03121203 (ChAd7), 03121204 (ChAd9), 03121205 (ChAd10), 03121206 (ChAd11), 03121207 (ChAd16), 03121208 (ChAd17), 03121209 (ChAd19) and 03121210 (ChAd20) and a transgene which encodes at least one immunogen operatively linked to regulatory sequences which direct expression of said transgene in mammalian cells, wherein said vector is at least partially deleted in E1 and devoid of E1 activity.