By shortening the time it takes to test vaccine potency, a new laboratory assay developed by U.S. Food and Drug Administration (FDA) scientists may speed the release of pandemic influenza vaccines to the public. The new assay was developed in response to 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).
According to the FDA, WHO and BARDA recognized the urgent need for an alternative influenza vaccine potency assay that could expedite the lot release of a monovalent vaccine against pandemic influenza viruses. With a faster assay, single-strain vaccines could be made available as the strains appear in addition to the traditional multivalent seasonal vaccines.
Surender Khurana, Lisa R. King, Jody Manischewitz, Elizabeth M. Coyle, Hana Golding, Division of Viral Products, Office of Vaccines Research and Review, Center for Biologics Evaluation and Research Scientists, conducted the assay development study titled “Novel antibody-independent receptor-binding SPR-based assay for rapid measurement of influenza vaccine potency.”
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. “Drawbacks to the current assay,” says FDA, “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, notes the FDA, 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.
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, explains FDA, was also able to measure the amount of HA protein in vaccines that were mixed with adjuvants. 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.
