Immunity to viruses is induced by the host responses to the virus hemagglutinin (HA) and to neuraminidase (NA). Antibodies against HA are the most important component in the protection against AI viruses, followed by antibodies against NA, which not only confer protection against infection, but also reduce the severity of infection and decrease virus spreading in an infected person. Thus an influenza vaccine must contain both HA and NA antigens in a form which will stimulate the production of neutralizing antibodies, local IgA antibodies and possibly cellular immunity. Partial protection has been reported due to the antibodies (serum anti-neuraminidase Ab) and to the surface glycoprotein NA in the chickens (Sylte et al; 2007).
Whole inactivated virus vaccines:
Whole inactivated virus vaccines were the first influenza vaccines to be produced. They are made from whole viruses that have been killed so that they are non-infectious but retain the ability to induce a protective immune response. These vaccines confer 60-90% protection that lasts for one to five years, depending on type of viral strain, but due to antigenic drift the vaccine-induced antibodies will be less effective in conferring protection against new strains.
Subunit virus vaccines:
Subunit vaccines can be produced by identifying and purifying the major antigenic sites (HA & NA) of viral antigen. Examples of purified subunit vaccines include the HA vaccines for influenza A and B. These cause fewer negative reactions and seem to be a good option
Live attenuated vaccines:
Live attenuated vaccines are produced by modifying (attenuating or weakening) a disease-producing ("wild") virus, so that the virus (vaccine) retains the ability to replicate (grow) and produce protective immunity, but usually does not cause illness. Generally, the attenuated form of the virus is obtained by serial passages of the active organism in culture media or cells. These are supposed to induce more solid immunity than inactivated vaccines, but again antigenic drift can interfere. To circumvent antigenic drift, already attenuated strains have been mixed with wild-types of virus to produce recombinants, that code for both attenuated and wild-type HA and NA.
The first DNA vaccine was designed by scientists from the Vaccine Research Center (VRC) at the National Institute of Allergy and Infectious Diseases (NIAID). This candidate vaccine was synthesized by using a modified version of the hemagglutinin gene from the H5N1 Avian Influenza virus. It contains no infectious material from the H5N1 A1 virus and contains only a portion of the DNA. Currently it is under clinical trials. (niaid 2007)
Use of influenza virus-like particles (VLPs) as vaccine:
Pushko et al. (2005), have shown that influenza virus-like particles (VLPs) comprised of HA, NA, and M1 proteins represent a candidate vaccine for avian influenza H9N2 virus Recently Pushko et al. evaluated an H9N2 VLP vaccine and recombinant HA (rH9) vaccine in three animal models. The H9N2 VLP vaccine protected mice and ferrets from challenge with A/Hong Kong/1073/99 (H9N2) virus.
Use of Adjuvants for improving the efficacy of vaccine:
Pushko's group further observed that adjuvant novasome increased the immunogenicity and protection of the rH9 vaccine. Their results have implications for the development of safe and effective vaccines with pandemic potential for avian influenza viruses (Pushko et al., 2007).
Human Monoclonal antibodies:
Recent studies have reported that blood taken from four Vietnamese survivors of the H5N1 bird flu virus protected mice from several strains of the virus. This finding may offer a new way to treat avian influenza. The human monoclonal antibodies were engineered to neutralize H5N1 virus. "We have shown that this technique can work to prevent and neutralize infection by H5N1 bird flu virus in mice," said Dr. Cameron Simmons of the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam. So this technique may offer a weapon to stockpile ahead of a feared pandemic of bird flu. (India 2007)
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