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Several small animal models, including mice, Syrian hamsters, guinea pigs, and ferrets are used to study the pathogenicity, transmissibility, and antigenicity of seasonal and pandemic influenza viruses. Moreover, animal models are essential for vaccination and challenge studies to evaluate the immunogenicity and protective efficacy of new vaccines. However, authentic human influenza viruses do not always replicate efficiently in these animal models. Previously, we developed a high-yield A/Puerto Rico/8/34 (PR8-HY) vaccine virus backbone that conferred an increased virus yield to several seasonal influenza vaccines in eukaryotic cells and embryonated chicken eggs. Here, we show that this PR8-HY genetic backbone also increases the replication of several seasonal influenza viruses in Syrian hamsters compared to the authentic viruses. Therefore, the PR8-HY backbone is useful for animal studies to assess the biological properties of influenza viral HA and NA. [ABSTRACT FROM AUTHOR]

Updating vaccine strains is essential to control equine influenza. We evaluated the protective efficacy of an inactivated equine influenza vaccine derived from viruses generated by reverse genetics (RG) in horses in an experimental viral challenge study. Wild-type (WT) virus (A/equine/Tipperary/1/2019) and virus generated by RG (consisting of hemagglutinin and neuraminidase genes from A/equine/Tipperary/1/2019 and six other genes from high-growth A/Puerto Rico/8/34) were inactivated by formalin for vaccine use. Twelve 1-year-old naïve horses with no antibodies against equine influenza virus were assigned to three groups (each n = 4): control, WT, and RG. They were vaccinated twice, 4 weeks apart, and were challenged with A/equine/Tipperary/1/2019 2 weeks after the second vaccination. All four horses in the control group and one horse in the WT group had pyrexia for multiple days and respiratory illness, and one horse in the RG group had pyrexia for 2 days without respiratory illness. The mean rectal temperatures and the mean concentrations of serum amyloid A in the WT and RG groups were significantly lower than those in the control group, with no significant differences between them. The WT and RG vaccines significantly reduced viral shedding relative to the control. The protective efficacy of the RG-derived inactivated vaccine against equine influenza virus is comparable to that of the vaccine derived from WT viruses in horses. The RG technique can make it easy to update equine influenza vaccine strains. [ABSTRACT FROM AUTHOR]