Immunization with the H5N1 Recombinant Vaccine Candidate Induces High Protection in Chickens against Vietnamese Highly Pathogenic Avian Influenza Virus Strains

Vietnam is one of the countries most affected worldwide by the highly pathogenic avian influenza (HPAI) virus, which caused enormous economic loss and posed threats to public health. Over nearly two decades, with the antigenic changes in the diversified H5Ny viruses, the limited protective efficacy of the available vaccines was encountered. Therefore, it is necessary to approach a technology platform for the country to accelerate vaccine production that enables quick response to new influenza subtypes. This study utilized a powerful reverse genetics technique to successfully generate a recombinant H5N1 vaccine strain (designated as IBT-RG02) containing two surface proteins (haemagglutinin (HA) and neuraminidase (NA)) from the HPAI H5N1 (A/duck/Vietnam/HT2/2014(H5N1)) of the dominant clade 2.3.2.1c in Vietnam during 2012–2014. Importantly, the IBT-RG02 vaccine candidate has elicited high antibody titres in chickens (geometric mean titre (GMT) of 6.42 and 6.92, log(2) on day 14 and day 28 p.i., respectively). To test the efficacy, immunized chickens were challenged with the circulating virulent strains. As results, there was a high protection rate of 91.6% chickens against the virulent A/DK/VN/Bacninh/NCVD-17A384/2017 of the same clade and a cross-protection of 83.3% against A/duck/TG/NAVET(3)/2013 virus of clade 1.1. Our promising results showed that we can independently master the reverse genetics technology for generation of highly immunogenic vaccine candidates, and henceforth, it is a timely manner to reformulate avian influenza virus vaccines against variable H5 clade HPAI viruses in Vietnam.