Risk Wise

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mined, if the protection induced by a vaccine is clade or

subclade-specific. Ideally, an H5N1 vaccine would protect

not only against the virus strain used for vaccine manu-

facture but also against viruses, which have undergone

antigenic drift. However, traditional inactivated split and

sub-unit H1N1 and H3N2 influenza vaccines induce rela-

tively strain-specific serum antibody and are ineffective

against antigenically drifted viruses. In contrast, studies

with the Vero cell culture derived whole virus H5N1

vaccine have demonstrated that it is capable of inducing

broadly cross-reactive neutralizing antibodies against a

range of H5N1 clades and subclades.

Immunization

studies in guinea pigs have demonstrated that two immu-

nizations with a clade 1 A/Vietnam/1203/2004 strain

vaccine resulted in induction of high titre neutralizing

antibody responses to a clade 0 (A/Hongkong/156/1997),

a variety of clade 1 (A/Vietnam/1203/2004, A/Vietnam/

1194/2004, A/Thailand/83/2004), clade 2.1 (A/Indonesia

/05/2005), clade 2.2 (A/turkey/Turkey/1/2005, A/chicken

/Egypt/03/2006) and clade 2.3 (A/Anhui/1/2005) strains.

Although whole virus vaccines are reported to be

more immunogenic in immunologically naïve individ-

uals than split or sub-unit vaccines, they are considered

to be associated with enhanced reactogenicity, partic-

this derivation of new reassortants requires several weeks, resulting

in significant delay in the delivery of a new pandemic vaccine. In

addition, the vaccine may provide an optimal antigenic fit with the

WT circulating virus only with respect to the HA and NA genes and

not with respect to the rest of the genes including the nucleoprotein

and the matrix genes which are derived from the A/PR/8/34 virus.

A novel strategy was developed to avoid the delay and potential

antigenic mismatch associated with vaccine production using egg-

adapted, reverse genetics-derived reassortant virus. This involves use

of wild-type virus to produce vaccine antigen in Vero cell culture,

one of the most advanced cell culture systems for production of

influenza viruses. For vaccine production, the virus harvest is inac-

tivated using a highly stringent procedure involving two separate

steps, formalin and UV treatment. Formalin alone was sufficient to

achieve total inactivation with a large safety margin, as confirmed

by safety (passage) assays of the bulk vaccine in two highly suscep-

tible cell systems, that is Vero and chicken embryo cells. Double

inactivation was chosen to enhance the safety margin. The inacti-

vated virus is then purified by continuous sucrose gradient

centrifugation followed by ultra/dialfiltration steps prior to formu-

lation. Re-sequencing of the HA gene of both strains at the

production level confirmed that virus grown in Vero cells did not

result in the selection of antigenic variants.

This process can result in the first batches of vaccine being avail-

able approximately 11 weeks after receipt of the pandemic vaccine

strain. This contrasts with a lag time of 20-28 weeks, which is

required for production of vaccine based on RG-derived attenuated

virus in embryonated eggs.

Safety and immunogenicity of Vero cell derived whole virus

H5N1 vaccines

In addition to vaccine supply, the other critical issues in the event of

a pandemic are vaccine efficacy and safety. The H5N1 virus has

diverged into three distinct lineages or clades (0, 1, 2) and multiple

subclades within clade 2. Therefore production of pandemic vaccine

can possibly only be initiated once the exact candidate strain is deter-

Cell culture (Vero) facility

Used for manufacture of H5N1 vaccine in Bohumil, Czech Republic

Source: Baxter Bioscience

Vero cell culture comparison

Source: Baxter Bioscience

Comparison of Vero cell culture with embryonated eggs with

respect to process and timelines