Although public health officials recognize that in order to prevent or, at least, control an avian flu pandemic, vaccine(s) will have to be extensively used. The problem that exists, however, is the unpredictability of pandemics. As noted in an earlier blog, most virologists believe that another pandemic will occur, which means, all things being equal, little or no vaccine will be available for use since it is next to impossible to predict which virus will cause the disease. This and the fact that it takes from 6 to 9 months to produce a new vaccine obviously will create a public health nightmare.
There are two major classes of vaccines that could be used:
(1) Inactivated virus vaccine is the primary means to control an influenza pandemic. This vaccine, which was developed from a killed virus, has been available for over 60 years and stimulates antibodies to hemagglutinin, a target antigen found on the cell surface of the virus. In other words, the greater the serum anti-hemagglutinin antibody, the greater the immunity to influenza.
(2) The intranasal vaccines (FluMist) are live, attenuated to be less virulent, cold-adapted vaccines; they are not considered a candidate for primary vaccine use.
Pandemic vaccines that have been used in the past have led to the following conclusions:
(1) That progress in vaccine development and manufacturing has been made, resulting in increased purity, thereby reducing unwanted reactivity.
(2) That whole virus vaccines may be more immunogenic than sub unit vaccines, but no manufacturer currently produces whole virus vaccines.
(3) That as noted earlier, the higher the level of serum anti-hemagglutinin antibody, the greater the protection.
A number of vaccine candidates are currently being developed, and vaccine priorities have been linked with Avian Influenza based on migratory birds throughout the world. At present, the highest priorities are for Type A Influenza only and include several hemagglutinin(H) and neuraminidase(N) subtypes, both of which are found on the cell surface of the influenza virus. Recent trials incorporating the H5N1 avian flu sub types are briefly summarized below:
University of Rochester
. 451 adults were randomly assigned 2 doses of vaccine containing respectively 90, 45, 15, or 7.5 ug of hemagglutinin antigen or a placebo. These 451 subjects were observed for 56 days. The higher the dose of hemagglutinin antigen, the greater the antibody response, i.e., 58% of adults that received 90ug of hemagglutinin antigen had titers associated with protection. Lesser amounts of antigen lower the titers
Randomized trial in France
. Two doses of one of six inactivated split influenza A vaccines with and without aluminum hydroxide adjuvant were administered. Results were similar to the Rochester study above. The higher the antigen dose (with adjuvant), the greater the antibody response.
Although these preliminary results look promising, much more development and testing needs to be done…………..stay tuned.