1

IMMUNE CORRELATES OF PROTECTION AGAINST INFLUENZA A VIRUSES IN SUPPORT OF PANDEMIC

VACCINE DEVELOPMENT

FDA/NIH/WHO Public Workshop, December 10-11, 2007Bethesda, MD

Workshop Organizers:OVRR/CBER: Jerry Weir, Hana Golding, Maryna Eichelberger Maureen HessNIAID/NIH: Kanta Subbarao, Catherine LukeWHO: David Wood, Martin Friede

Hana Golding, Ph.D., DVP/OVRR/CBER VRBPAC February 21, 2008

2

Current State of Efficacy Evaluation of Pandemic Influenza Vaccines

• Current situation (limited human-to-human transmission of avian influenza) is not conducive to traditional vaccine clinical efficacy trials. Therefore, evaluation of pandemic influenza vaccines is relying on immunological measures “borrowed” from seasonal influenza vaccines.

• The principal correlate of influenza vaccines efficacy is Haemagglutination inhibition (HI) antibody titer as a read-out

3

Challenges in Efficacy Evaluation of Pandemic Influenza Vaccines

• Is it appropriate to extrapolate what we know from seasonal influenza vaccination to pandemic influenza vaccines, when most of the populations are lacking pre-existing immunity? Higher pathogenicity of H5N1?

• Is an HI titer (or any antibody measurement) appropriate to predict clinical benefit from new types of influenza vaccines such as:– Live attenuated vaccines– Plasmid DNA vaccines– VLP and Vector vaccines

• How do we establish the protective levels associated with newly defined immunological endpoints and accurately quantify the responses following vaccination?

4

GOALS OF THE PUBLIC WORKSHOP

• Identify the gaps in our knowledge and abilities in addressing the unique challenges encountered in the development and evaluation of vaccines intended to protect against pandemic influenza

• Facilitate implementation of global research agenda to improve efficacy assessment of pandemic influenza vaccines

5

Session I: Correlates of protection against seasonal influenza (Chair: Robert Couch)

• This session included descriptions of humoral and cell-mediated responses to influenza, with an emphasis on immune mechanisms that contribute to protection against influenza infection or disease.

• Both antibody and T cell responses contribute to protection against seasonal influenza. Meta analysis of human challenge studies support the conclusion that HI antibody titer of 1:40 is associated with >50% reduction in the risk of contracting influenza infection or influenza disease.

6

Session II: Immune responses to avian influenza infections and vaccines against novel influenza viruses in humans

(Chair: Jacqueline Katz)• This session included a description of the genetic and

antigenic heterogeneity of H5N1 viruses and antibody responses in poultry workers.

• Immune responses to several avian influenza vaccine candidates (inactivated & LAIV) were presented from clinical studies performed in the US as well as in Europe.

• Dr Fred Hayden (WHO) described the South East Asia Influenza Clinical Research Network that will facilitate international collaborative epidemiology and immunologic studies of pandemic influenza.

7

Session III: Assays to evaluate vaccine immunogenicity (Chair: Maria Zambon)

• This session included a discussion of the limitations of current assays to detect antibody responses to HA and NA, and described new assays to evaluate cell mediated immunity and M2-specific antibody responses. Novel assays that use pseudotyped viruses and H5N1 genomic phage display libraries were also described.

• The traditional HI tests based on chicken or turkey RBC are not optimal for anti-H5N1 HI. Horse RBC seems to have more sialic acid 2-3 linked glycans, the preferred receptors for H5N1 strains. Horse HI needs validation

8

Session IV: Correlates of Protection against Avian Influenza : potential insights from animal studies

(Chair: Kanta Subbarao)• In this session animal models for pandemic influenza

were described. Results of wild-type virus challenge in mice and ferrets to determine the immunogenicity and efficacy of new vaccines were presented.

• These animal models provided important information about vaccine immunogenicity and correlates of protection, including heterologous protection.

• Vaccine effects included reduced viral loads in URT and lungs, lower morbidity and less lung pathology.

• Lethality is often not an optimal endpoint for vaccine effect and/or dose finding.

9

Workshop Outcome: Identified gaps and general recommendations-1

• It may be premature to extrapolate what we know from seasonal influenza vaccination to pandemic influenza vaccines. Particularly the use of a given antibody end-point to predict pandemic vaccine efficacy.

• Use of HI may not be appropriate for all types of pandemic influenza vaccines. Additional immunogenicity measurements need to be defined and the protective levels associated with the newly defined endpoints determined.

• Novel assays should be developed to measure mucosal immunity, cell-mediated responses, and antibody responses to NA and other targeted antigens.

10

Workshop Outcome: Identified gaps and general recommendations-2

• Animal models (mice and ferrets) can provide important insight regarding “correlates of protection” against emerging avian strains

• In order to facilitate the standardization of assays to evaluate and compare vaccine responsiveness there is an immediate need for standard reference reagents, low pathogenicity viral stocks, working cell banks, and shared SOPs.

• Programmatic approach to pandemic vaccine trials, with use of standardized assays should facilitate comparison of vaccine candidates and expedite pivotal studies, licensure, and pre-pandemic preparedness