Comparison of the effectiveness of rHVT-H5, inactivated H5 and rHVT-H5 with inactivated H5 prime/boost vaccination regimes in commercial broiler chickens carrying MDAs against HPAI H5N1 clade 2.2.1 virus.

ABSTRACT

Vaccination is the main tool implemented in Egypt since 2007 to control H5N1 avian influenza. The present study aimed at comparing the effectiveness of three avian influenza vaccination regimes in commercial broiler chickens carrying high levels of maternally derived antibodies (MDAs). Day-old chicks were divided into four experimental groups. Group I received only the rHVT-H5 vaccine (recombinant turkey herpesvirus (HVT) which carries a H5 clade 2.2 insert) administered at D1. Group II received only the KV-H5 (an oil emulsion killed vaccine prepared from reassortant HPAI virus (A/duck/Anhui/1/06)) vaccine (inactivated reverse genetic H5N1 clade 2.3.4 virus) administered at D8. Group III received rHVT-H5 and KV-H5 as prime/boost. Group IV served as unvaccinated control. Weekly serological monitoring was conducted using the haemagglutination inhibition test. Two challenge experiments were conducted at D28 and D35 using HPAI H5N1 clade 2.2.1 virus. Birds were monitored daily 14 days post-challenge for morbidity and mortality, and oropharyngeal swabs were collected for virological monitoring. Initially, day-old chicks had high mean MDA titres (9 + 0.9 log2). The MDA half-life was >7 and <7 days, respectively, for unvaccinated and vaccinated birds. Group III showed the highest post-vaccination humoral immune response and seroconversion rate. The highest protection rate against morbidity (80-90%) and mortality (90-90%) was obtained in Group III after challenge at D28 and D35, respectively, as compared to Group I (70-70%) and (80-90%) and Group II (0-0%) and (30-30%). Groups I and III had lower number of shedder birds. The vaccination regime with prime/boost conferred the highest and earliest protection, and can hence be recommended for the broiler production sector in endemic and high HPAI H5N1 challenge areas.