Efficacy of a plant‐produced virus‐like particle vaccine in chickens challenged with Influenza A H6N2 virus

Abstract

SummaryThe efficacy, safety, speed, scalability and cost‐effectiveness of producing hemagglutinin‐based virus‐like particle (VLP) vaccines in plants are well‐established for human influenza, but untested for the massive poultry influenza vaccine market that remains dominated by traditional egg‐grown oil‐emulsion whole inactivated virus vaccines. For optimal efficacy, a vaccine should be closely antigenically matched to the field strain, requiring that influenza A vaccines be updated regularly. In this study, an H6 subtype VLP transiently expressed in Nicotiana benthamiana was formulated into a vaccine and evaluated for efficacy in chickens against challenge with a heterologous H6N2 virus. A single dose of the plant‐produced H6 VLP vaccine elicited an immune response comparable to two doses of a commercial inactivated H6N2 vaccine, with mean hemagglutination inhibition titres of 9.3 log2 and 8.8 log2, respectively. Compared to the non‐vaccinated control, the H6 VLP vaccine significantly reduced the proportion of shedders and the magnitude of viral shedding by >100‐fold in the oropharynx and >6‐fold in the cloaca, and shortened oropharyngeal viral shedding by at least a week. Despite its potency, the cost of the antigenic mismatch between the inactivated H6N2 vaccine and challenge strain was evident not only in this vaccine's failure to reduce viral shedding compared to the non‐vaccinated group, but its apparent exacerbation of oropharyngeal viral shedding until 21 days post‐challenge. We estimate that a kilogram of plant leaf material can produce H6 VLP vaccines sufficient for between 5000 and 30 000 chickens, depending on the effective dose and whether one or two immunizations are administered.

Continuing evolution of H6N2 influenza A virus in South African chickens and the implications for diagnosis and control

“…Because a reservoir of infection has never been identified, for a long time it was thought that H6N2 infections have a short window period of viral shedding of about a week to ten days. A recent vaccine-challenge study with strain H44954/2016 [44] however showed that excessively high levels of viruses (>8.92 log 10 viral copies/ml) were shed from the oropharynx in the first four days of infection and that 36% of non-vaccinated chickens were still shedding virus three weeks after exposure. In comparison, >5.7 million-fold fewer viral particles were shed from the cloaca and cloacal shedding had ceased completely by two weeks post infection.…”

“…We included the homologous antigen used for challenge, H44954/2016, for experimental purposes (Table 3). is common for inactivated whole-virus vaccines to induce anti-NP antibody responses of up to 10 log 2 with a single vaccination [44]. This makes it impossible to use the ELISA tests to reliably distinguish whole-virus vaccine from field challenge antibody responses.…”

Continuing evolution of H6N2 influenza A virus in South African chickens and the implications for diagnosis and control

“…We included H44954/2016, the homologous antigen used for challenge, for experimental purposes (Table 3). [44]. This makes it impossible to use the ELISA tests to reliably distinguish whole-virus vaccine from field challenge antibody responses.…”

“…Tracheal swabs, cloacal swabs or tissue samples (tracheas or organ pools) from flocks that showed typical signs of H6N2 infection such as respiratory symptoms or drops in egg production were submitted by veterinarians to diagnostic laboratories for PCR confirmation. The only flock that had been vaccinated was that from which H44954/2016 was isolated (44). RNAs were extracted with the Quick-RNA miniprep kit (Zymo Research, Irvine, USA).…”

“…infections have a short window period of viral shedding of about a week to ten days. A recent vaccinechallenge study with strain H44954/2016 [44] however showed that excessively high levels of viruses (>8.92 log 10 viral copies/ml) were shed from the oropharynx in the first four days of infection and that 36% of non-vaccinated chickens were still shedding virus three weeks after exposure. In comparison, >5.7 million-fold fewer viral particles were shed from the cloaca and cloacal shedding had ceased completely by two weeks post infection.…”