Safety and immunogenicity of a novel cold‐adapted modified‐live equine influenza virus vaccine

Objective To design and evaluate the safety and immunogenicity of a modified-live vaccine to prevent equine influenza virus (EIV) infection based on the novel reassortant cold-adapted strain A/HK/Otar/6:2/2010. Methods Surface proteins (HA, NA) from the wild-type strain A/equine/Otar/764/2007 (H3N8) and internal proteins (PB2, PB1, PA, NP, M, NS) from the attenuated cold-adapted donor strain A/Hong Kong/1/68/162/35CA (H3N2) were included in the vaccine. Horses were administered 109.2 EID50/mL of the modified-live vaccine or saline solution using a nasal spray. The clinical condition of the animals was assessed throughout the study and nasopharyngeal swabs were collected for virus titration. Two yearlings in each group were euthanased on day 5 post vaccination (PV) for histological examination and measurement of viral titres in the organs. Serum samples and nasal secretions were collected to evaluate serological response. Lymphoproliferation after restimulation in vitro was determined to evaluate cell-mediated immunity. To evaluate the protective capacity of the vaccine, the yearlings in both groups were challenged with the wild-type virus at 28 days PV and their clinical condition and serological response was evaluated. Nasal swabs were collected to assess viral shedding from the upper respiratory tract. Results Single intranasal administration of a modified-live EIV vaccine caused no adverse effects and vaccinated yearlings and pregnant mares did not form detectable levels of antibodies by days 7, 14 and 28 PV, as indicated by the HI reaction and ELISA. Secretory antibodies could be detected on day 7 and reached maximal levels on day 14 PV. In vitro studies showed that the yearlings and pregnant mares both formed a cell-mediated immune response by day 14 PV. The vaccine protected yearlings against challenge with wild-type virus. We conclude that single intranasal administration of the modified-live EIV vaccine was safe in the yearlings and pregnant mares that we treated, and was immunogenic and protective in the yearlings.