Interference between inactivated and active influenza viruses in the chick embryo: I. A re-evaluation of factors of dosage and timing using infectivity titrations for assay

Abstract

Interference by ultraviolet-inactivated influenza virus with propagation of the active homo- or heterotypic agents in the endoderm of the chick embryo allantois has been re-evaluated with respect to dosage and timing of the two injections, using infectivity titrations for assay. By varying (a) the interfering dose in terms of hemagglutinin units; (b) the intervals between its injection and challenge with serial decimal dilutions of active virus; and (c) the period of incubation after challenge, the following results were obtained. The time required to establish solid interference decreases with the size of the interfering dose; i.e., when increasingly more inactivated virus particles are provided per cell. With 850 HA units no evidence of viral multiplication was obtained upon challenge at 9 hours, whereas with 20–100 HA units interference was extensive, yet not complete in 24 hours. If the interfering and challenge inocula were given simultaneously or at a 2-hour interval the first and part of the second infectious cycles were found to proceed before further spread of the infectious process was arrested, unless the dose of inactive virus was large. In that case the homotypic preparations affected the yields of the initial infectious cycles to significant extents. In all instances, where interference was not as yet fully established, using 20 or more HA units of the interfering agent, the yields of infectious virus were directly proportional to the challenge dose. With a smaller dose of inactivated virus (<20 HA units), a proportion of the cells remained fully susceptible and the yields were no longer proportional to the challenge inoculum, but reached a plateau, presumably in proportion to the number of the remaining susceptible cells. The implications and quantitative aspects of these experiments have been discussed.