Avian influenza: genetic evolution under vaccination pressure

Abstract

<p>Abstract</p> <p>Antigenic drift of avian influenza viruses (AIVs) has been observed in chickens after extended vaccination program, similar to those observed with human influenza viruses. To evaluate the evolutionary properties of endemic AIV under high vaccination pressure (around 2 billion doses used in the last 12 years), we performed a pilot phylogenic analysis of the hemagglutinin (HA) gene of AIVs isolated from 1994 to 2006. This study demonstrates that Mexican low pathogenicity (LP) H5N2-AIVs are constantly undergoing genetic drifts. Recent AIV isolates (2002–2006) show significant molecular drifts when compared with the H5N2 vaccine-strain or other field isolates (1994–2000). This study also demonstrates that molecular drifts in the HA gene lineages follow a yearly trend, suggesting gradually cumulative sequence mutations. These findings might explain the increasing incidence of LP H5N2 AIV isolated from commercial avian farms. These findings support recent concerns about the challenge of AIV antigenic drift and influenza epidemics.</p

Role of vaccination-induced immunity and antigenic distance in the transmission dynamics of highly pathogenic avian influenza H5N1

“…Results from countries where vaccination is in place show that if the vaccine coverage and the HI titres of vaccinated animals are high, then vaccination leads to a substantial decrease in transmissibility and infection of animals [17,34,43,53,54]. More studies emerging from the field show that ineffective vaccination has often led to selection of antigenic variants, which spread rapidly because there is in effect no protective immunity [4,5,7,8,14,15,17,55]. We argue that instead of this evidence making a case against vaccination it should provide even more reasons for vaccination to be applied in such a way that high HI titres against the challenge strains can be obtained, since if it is, a sufficient level of herd immunity can be achieved irrespective of the antigenic distance between vaccine and challenge strains that may result from initial selection in vaccinated chickens.…”

Highly Pathogenic Avian Influenza Virus Subtype H5N1 Escaping Neutralization: More than HA Variation

“…These escape mutants are less susceptible to vaccine-induced neutralizing antibodies. Related to vaccination programs and sometimes deficiencies in the programs, the occurrence of escape variants in poultry has been described for Central America, Indonesia, China, and Egypt (9,11,15,19,24,31,32).…”

Evidence of Antibodies to H5 Subtype Avian Influenza Virus in Commercial Layer Farms in Jos, Nigeria

“…Indication that inactivated oil emulsion AI vaccines were used in commercial layer farms in Plateau State could have a negative implication due to the fact that some scientists suggested that vaccinating flocks might pose a risk of transmitting AI virus to other flocks (Cardona et al, 2006). Longterm circulation of AI virus in vaccinated population may result in both antigenic and genetic changes in the virus as was reported to have occurred in Mexico (Escorcia et al, 2008). The presence of antibody against AIV could pose serious consequences as LPAI can easily mutate to HPAI as reported by Capua and Marangon (2000); Capua and Alexander (2004) in some parts of the world.…”