FDA Develops Alternative Assay That Could Speed Availability of Pandemic Influenza Vaccines
April 9, 2014 -- A laboratory assay developed by scientists at the U.S. Food and Drug Administration (FDA) could speed the release of pandemic influenza vaccines to the public by shortening the time it takes to test vaccine potency.
The FDA scientists developed the new assay in response to the recommendation of a workshop organized by the World Health Organization (WHO) following the 2009 H1N1 influenza pandemic and an initiative by the U.S. Department of Health and Human Services’ Biomedical Advanced Research and Development Authority (BARDA). WHO and BARDA noted the urgent need for an alternative influenza vaccine potency assay that could expedite the lot release of a monovalent vaccine against pandemic influenza viruses. Such a vaccine could be made available in addition to the seasonal vaccines, which are designed to protect against what scientists identify each year as the most common strains of influenza viruses in circulation.
One advantage of the new potency assay is that it does not require the use of antisera (solutions of antibodies against specific targets on the vaccine being tested) for testing influenza vaccines.
The current standard potency assay for influenza vaccines uses antibodies against hemagglutinin (HA), a protein on the surface of the virus that is a key component of influenza vaccines. In order to make the antibodies, vaccine viruses are treated with enzymes to release HA proteins from their surface. These proteins are injected into several sheep to stimulate production of antibodies for use in the assay. Multiple boosts (additional injections) are often required to obtain large enough amounts of antibodies. In the current standard assay, HA from samples of an influenza vaccine lot as well as a vaccine reference (i.e., one with a known quantity of HA for comparison) diffuse through agar (a wax-like substance) containing sheep anti-HA antibodies. If the anti-HA antibodies recognize and bind to the HA, the two molecules form tiny visible rings, demonstrating that the HA is suitable for use in influenza vaccines. The diameters of the rings correlate with the amount of HA in each vaccine lot. The amount of HA can be calculated by comparing those rings with the rings formed by the reference vaccine, which has a known amount of HA.
Drawbacks to the current assay are that the use of enzymes to release HA from influenza viruses does not reliably produce sufficient amounts of HA and the resulting proteins are difficult to purify before injecting into sheep. Moreover, scientists must inject several sheep multiple times, creating another potential delay in the process of making antibodies for the vaccine potency assay (2-3 months).
The new assay does not use antibodies that attach to HA; instead it is based on the natural interaction between the influenza virus envelope protein, HA, and the receptor molecule on the surface of target cells lining the human respiratory tract. This receptor is a specifically-shaped docking area on the cell that the HA protein of the influenza virus attaches to so the virus can inject its genetic material into the cell.
The FDA researchers used artificial versions of the HA receptors (sialic acid [ SA]-glycans) known to be targeted by pandemic avian influenza or human influenza and tested whether the HA proteins from influenza vaccines recognized and attached to them. Only those HA proteins that were identical to the viral protein—and thus able to act as a vaccine--were able to fit into the appropriate SA-glycan receptor sites. The scientists used a technique called surface plasmon resonance (SPR) to measure the amount of HA proteins attached to the artificial receptors that were coated on special chips. In this way they were able to determine both how much HA was in vaccine samples as well as the stability of those proteins—key measurements for evaluating vaccine potency. This assay was also able to measure the amount of HA protein in vaccines that were mixed with substances called adjuvants—substances that are added to some vaccines to increase their ability to trigger immune responses.
Importantly, the SPR-based assay measurements of HA content in vaccines agreed closely with those obtained by the standard antibody-based ring-forming assay. This suggests that the newer assay might be reliable enough to replace the slower, standard assay. This and other alternative potency assays are undergoing further evaluation by influenza scientists.
Scientists in the Office of Vaccines Research and Review conducted this study.
Posted: April 2014
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