The present invention relates to an alphaviral replicase, especially Semliki Forest Virus replicase, or an expression vector encoding an alphaviral replicase, said alphaviral replicase comprising RNA dependent RNA polymerase activity, for use as an immune system modulating adjuvant. The alphaviral replicase may be used in the combination with a vaccine providing an adjuvant function therein, which when present therein will generate an additional boost to the immune response in the subject to whom this combination is administered as compared to when the vaccine alone is administered to a subject in need thereof. The aim of the present invention is to provide an efficient and easy to administer, species-independent adjuvant which will provide advantages to the adjuvants used together with vaccines today.