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Comparison of different vaccine strategies. (1) Release of antigenic material, which can be viral proteins, empty particles (SVPs) or infectious particles. (2) Released infectious particles can undergo the first round of replication, (3) this produces either more infectious particles (as is the case with live-attenuated vaccines) or empty particles called SRIPs. Sub-viral particles (SVPs); single-round infectious particles (SRIPs).
