Advances in the development of universal influenza vaccines

Advances in the development of universal influenza
vaccines
Sarah C. Gilbert
Jenner Institute, University of Oxford, Oxford, UK.
Correspondence: Sarah C. Gilbert, The Jenner Institute, University of Oxford, Old Road Campus Research Building (ORCRB), Roosevelt Drive,
Oxford, UK. E-mail: [email protected]
Accepted 16 August 2012. Published Online 24 September 2012.
Despite the widespread availability and use of influenza vaccines,
influenza still poses a considerable threat to public health.
Vaccines against seasonal influenza do not offer protection against
pandemic viruses, and vaccine efficacy against seasonal viruses is
reduced in seasons when the vaccine composition is not a good
match for the predominant circulating viruses. Vaccine efficacy is
also reduced in older adults, who are one of the main target
groups for vaccination. The continual threat of pandemic
influenza, with the known potential for rapid spread around the
world and high mortality rates, has prompted researchers to
develop a number of novel approaches to providing immunity to
this virus, focusing on target antigens which are highly conserved
between different influenza A virus subtypes. Several of these have
now been taken into clinical development, and this review
discusses the progress that has been made, as well as considering
the requirements for licensing these new vaccines and how they
might be used in the future.
Keywords Clinical, influenza, vaccine.
Please cite this paper as: Sarah C. Gilbert. (2012) Advances in the development of universal influenza vaccines. Influenza and Other Respiratory Viruses
DOI: 10.1111/irv.12013.
Introduction
The word ‘universal’ has two meanings in the context of
influenza vaccines; vaccines that protect against all influenza viruses, and the vaccination of the entire population
against influenza. This review will cover primarily the former, but will also discuss how these new vaccines could be
used in vaccination policies of the future.
Currently licensed seasonal influenza vaccines, whether
inactivated or live attenuated, split or whole virion, adjuvanted or not, induce antibodies against the highly polymorphic head of the viral haemagglutinin (HA). As the
proportion of the human population with effective antibodies to the HA of the circulating virus increases following infection and recovery, or vaccination, variants of the
virus capable of escaping this immunity by virtue of
mutated HA sequences that either change the protein
sequence or shield it by glycosylation are selected, resulting
in continual antigenic drift of the circulating viruses. The
immunodominant antibody responses induced by vaccination are in most cases highly specific for the HA molecules
that were included in the vaccine, and when there is a significant mismatch between the vaccine and circulating
virus, the vaccine efficacy is markedly reduced.
1
Periodic
Influenza A pandemics occur when a virus of a new subtype infects humans and is transmissible resulting in rapid
spread of the new virus to multiple geographic locations.
The absence of antibodies specific for the pandemic HA
results in an increased number of susceptible individuals,
and high numbers of human infections occur until after a
few years the majority of the population has been exposed
and immune selection pressure again results in antigenic
drift of the pandemic virus, which then becomes the current seasonal influenza virus. Current vaccine formulations
require the precise HA sequence of the circulating virus to
be known to produce the vaccine, resulting in a lag of several months before large numbers of doses of a new vaccine
can be produced once a new pandemic virus has been
identified. The realization from 2004 onwards that H5N1
viruses were repeatedly causing infections in humans,
despite the fact that human-to-human transmission had
only been observed in rare cases involving extremely high
exposure, highlighted our susceptibility to influenza A pandemics, and resulted in new approaches to influenza vaccine development being undertaken. Three main strategies
employing conserved regions of the influenza virus as antigens have emerged as potential solutions, and these will be
reviewed below.
DOI:10.1111/irv.12013
www.influenzajournal.com
Review Article
ª 2012 Blackwell Publishing Ltd 1Animal models for testing candidate
influenza vaccines
Although this review will focus on data from clinical trials,
all of the vaccines discussed will have been tested in preclinical animal models prior to initiating clinical studies.
The species used are most commonly mice, ferrets and
macaques, as reviewed by Bodewes et al.
2
In mice, the pathogenesis of the infection does not resemble that of humans,
and viruses for challenge experiments have to be adapted to
infect mice. There are limited T cell reagents available for
use in ferrets, which also appear to be highly susceptible to
influenza A virus infection, perhaps more so than humans,
and supply of sufficient quantities of animals can be a limiting factor in planning experiments. The immune system of
a macaque is very similar to that of a human, and experimental data obtained in this model is a better predictor of
the outcome in humans, but the cost of conducting experiments in an ethically approved manner, particularly when
high-level containment is required for virus challenge, prevents the model from being used extensively. Pigs have also
been used to test influenza vaccines
3
and have the advantage
that there is no shortage of supply, reagents are available for
T cell analysis, and they can be infected with viruses of
many different subtypes as both a-2,3- and a-2,6-galactose
sialic acid linkages are present on cells lining the pig trachea,
4
which provides an opportunity to study heterosubtypic protection induced by vaccination. A recent
comparison of pandemic H1N1 vaccines in pigs
5
produced
data that were in close agreement with a similar study in
humans,
6
providing further support for the greater use of
this model in future. However all of these models have the
disadvantage that they cannot mimic the complex immune
memory to influenza A virus found in humans after a lifetime of repeated exposures, and the results of experimental
studies must be interpreted with this in mind.
Anti-M2e antibodies
The M2 protein forms a proton-selective ion channel which
plays an important role in virus morphogenesis and assembly. It consists of only 96 amino acid residues, of which 23
are present on the surface of the virion, and only five of
these exhibit any significant degree of polymorphism.
7
Although present in low abundance on the surface of the
virion, M2 is also found on the surface of virus-infected
cells, with approximately twice as many M2 molecules than
HA molecules reported. Anti-M2e antibodies are not found
following influenza infection, but can be induced by vaccination. These antibodies are not virus neutralizing, and
most likely act via antigen-dependent cell cytotoxicity or
complement-dependent cytotoxicity.
8
Various strategies
have been employed to increase the immunogenicity of
M2e vaccines in animal models and a number of studies
have reported partial protection against lethal challenge
and decreased viral shedding following induction of antiM2e antibodies in mice.
9–11
In pigs, contrasting results
have reported either exacerbation of disease following
influenza challenge,
12
or partial protection with reduction
in macroscopic lung lesions, but no reduction in virus
shedding.
13
Clinical trials have been undertaken by Sanofi Pasteur
using the vaccine ACAM-FLU-A, which was found to be
well tolerated, with no serious side effects. Antibody
responses to M2e were induced in the majority of subjects.
VaxInnate has also completed a Phase I trial of M2e fused
to flagellin, again demonstrating immunogenicity with IgG
specific for M2e detected in 96% of subjects after the second dose and acceptable vaccine safety.
14
Other M2e based
vaccines are also in development, but as yet no clinical effi-
cacy studies have been reported.
15
Thus, it has been demonstrated that although antibodies specific for M2e are not
part of the response to influenza A virus infection in
humans, it is possible to induce them by vaccination. It is
still not known whether these antibodies will recognize all
influenza A subtypes, if they can contribute to protection
against disease following infection, what the mechanism of
that protection might be and what antibody titre would be
required to achieve a useful level of protection.
T cell responses to conserved antigens
In contrast to HA, the internal antigens of influenza viruses
are very highly conserved across all subtypes and strains of
influenza A. Nucleoprotein (NP) and matrix protein 1
(M1) are abundantly expressed in virus-infected cells,
16
and
effector T cells capable of recognizing peptides derived
from these antigens that are presented by major histocompatibility complex (MHC) molecules on the surface of
virus-infected cells can kill the virus-infected cells, preventing further spread of the infection within the host. Several
large epidemiological studies have provided evidence that
recent infection with seasonal influenza reduces the risk of
disease caused by pandemic virus. Analysis of the Cleveland
family study found that infection with H1N1 prior to 1957
reduced the risk of infection with H2N2 at the start of the
1957 pandemic.
17
A new analysis of susceptibility to infection in the 1918 pandemic concludes that recent infection
with a seasonal influenza virus provided heterosubtypic
immunity to the pandemic virus,
18
and in the 2009 pandemic, recent seasonal influenza virus infection was associated with protection from infection with pandemic virus
whereas recent vaccination against seasonal influenza
increased susceptibility to pandemic virus.
19
Gilbert
2 ª 2012 Blackwell Publishing LtdThis heterosubtypic protection is mediated through
T cells (either CD4
+
or CD8
+
, or both) specific for antigens
that are conserved between seasonal and pandemic viruses,
rather than by antibodies to external antigens which differ
between viral subtypes. A study in which volunteers were
inoculated with live influenza virus demonstrated that
those with cytotoxic T cell responses detected by lysis
assays cleared influenza virus effectively and exhibited
reduced virus shedding, even in the absence of antibodies
specific for the HA of the influenza challenge virus.
20
Antibodies are undoubtedly the primary protective mechanism
when the antibody specificity is a perfect match for the HA
of the infecting virus, but vaccine failures occur approximately 1 out of 20 years, when the vaccine does not match
the circulating viruses, demonstrating that it is not suffi-
cient to have antibodies recognizing a particular influenza
subtype (H1 or H3 for example), but that the antibodies
must be specific for the precise variant of the subtype.
21
When these are not present and influenza virus is encountered, a strong T cell response can act rapidly to prevent
spread of the infection, resulting in some cases in a completely asymptomatic infection with no viral shedding.
A more recent study of influenza challenge in healthy
subjects who were all seronegative (defined as having an HI
titre <10) for the challenge virus (either H3N2 or H1N1)
at the time of infection found that pre-existing CD4
+
T cells rather than CD8
+
T cells specific for two internal
antigens, NP and matrix protein 1 (M1) correlated with
disease protection.
22
T cell responses were measured in
interferon-c ELISpot assays using pools of 18-mer peptides
spanning the antigens of interest, in which either CD4
+
or
CD8
+
cells were depleted prior to setting up the assay. The
strength of the CD4
+
T cell response to NP and M1
showed a significant negative correlation with both the
total symptom scores and the duration of illness for the
nine volunteers infected with H1N1, and additionally with
virus shedding for the 14 volunteers infected with H3N2,
whereas the correlation was weaker when total T cell
responses to NP and M1 were considered. However, the
numbers in the study were small, and for the H3N2 group
CD4
+
and CD8
+
T cell responses against these two antigens
were very similar in magnitude (56% CD4
+
versus 44%
CD8
+
). Whereas CD8
+
T cells are believed to act directly
on virus-infected cells, in other virus infections the role of
CD4
+
T cells is thought to be in priming or maintaining
the CD8
+
T cell response
23
or in recruitment of CD8+ T
cells to the site of infection.
24
However, CD4
+
T cells may
also act directly as antiviral cytotoxic cells, and Wilkinson
et al. demonstrated that CD4
+
T cells taken from the volunteers in the human challenge study were cytotoxic and
employed the perforin-granzyme pathway. If cytotoxic
CD4
+
T cells are to have a protective effect following
human influenza virus infection, it will require the expression of MHC class II on the respiratory epithelium, and
substantial expression was demonstrated on explanted
human lung tissue and cultured primary human bronchial
epithelial cells,
22
supporting the hypothesis that cytotoxic
CD4
+
T cells recognizing conserved influenza antigens may
act directly to contain the spread of influenza A virus in
the human respiratory tract. Earlier studies that measured
cytotoxic T cell responses to influenza A virus in lysis
assays
20,25,26
would therefore have detected both CD4
+
and
CD8
+
responses.
The high degree of conservation of internal antigens such
as NP and M1 across all influenza A virus subtypes allows
T cells that were primed by infection with one viral subtype
to recognize and kill cells infected with virus of a different
subtype, resulting in the heterosubtypic immunity that is
not conferred by antibodies to the polymorphic regions of
HA. Lee et al.
27
demonstrated that blood donors in the UK
had memory T cells that were capable of recognizing NP
and M1, and to a lesser extent other internal antigens from
H5N1 influenza virus. However the half-life of the T cell
response has been calculated to be only 2–3 years.
26
The
median T cell response to influenza in the population correlates with the number of influenza infections at that time,
and decreases in influenza seasons when the number of
influenza cases is low.
This short-lived period of effective T cell-mediated protection against influenza disease can affect the progression
of an influenza pandemic in ways which have only recently
begun to be understood. At the start of a new pandemic
with a novel influenza subtype, some individuals have
immunity which is capable of preventing symptoms of
influenza infection from developing following infection
with the new virus, although they may experience subclinical infections. New analysis of the progress of the 1918
pandemic
18
highlights the fact that many of those living in
urban environments were apparently unaffected, despite
having had a higher likelihood of exposure than those in
isolated communities. The most likely mechanism for this
is that they had recently been exposed to the former seasonal influenza virus and had sufficient T cell immunity to
prevent disease occurring after exposure to the pandemic
virus. The fact that influenza pandemics occur in waves,
rather than infecting the whole population at the first
exposure may be explained by the short-lived nature of the
heterosubtypic immunity, with waning immunity in some
of those who escaped illness in the first wave resulting in
susceptibility to the second wave despite little antigenic
drift occurring. Furthermore, adults had higher rates of
pre-existing immunity than the young, and this immunity
was better maintained. This may be a consequence of
repeated exposures to influenza virus throughout life
gradually modifying the T cell memory to this acute viral
infection, with each subsequent encounter.
Development of universal influenza vaccines
ª 2012 Blackwell Publishing Ltd 3The aim of boosting heterosubtypic T cell responses to
conserved influenza antigens by vaccination underlies the
development of a number of novel universal influenza vaccines. The first of these to enter clinical development was
Modified Vaccinia virus Ankara (MVA)-NP + M1, using
the replication-deficient poxvirus vector MVA to express
the NP and M1 of influenza A. MVA as a vaccine vector
has been tested in many clinical trials of novel vaccines
against malaria, tuberculosis and HIV.
28
It has been found
to have an excellent safety profile in all sections of the population from children
29
to the elderly (Richard D. Antrobus, Patrick J. Lillie, Tamara K. Berthoud, Alexandra J.
Spencer, James E. McLaren, Kristin Ladell, Teresa Lambe,
Anita Milicic, David A. Price, Adrian V. S. Hill and Sarah
C. Gilbert; unpublished data.), and is highly effective at
boosting T cell responses however they were first acquired.
In a first Phase I study demonstrating safety and immunogenicity in healthy young adults,
30
the T cell response to NP
and M1 was found to be predominantly CD8
+
prior to vaccination and the CD4:CD8 ratio was not altered by vaccination. A subsequent Phase IIa influenza challenge study then
provided the first demonstration of efficacy of a vaccine
designed to boost T cell responses to influenza, with a signi-
ficant reduction in duration of viral shedding in the vaccinated group and also a reduction in the numbers of subjects
experiencing symptoms of influenza virus infection.
31
The
NP and M1 sequences in MVA-NP + M1 are derived from
an H3N2 virus, and the challenge was performed with a virus
of the same subtype. A further study has examined the safety
and immunogenicity of the vaccine in older adults, demonstrating remarkable immunogenicity even in those aged over
70 years (Antrobus, submitted). Indeed, the large increases
in the number of T cells recognizing NP and M1 following a
single vaccination with MVA-NP + M1 are a notable feature
of these clinical studies, with a >10-fold increase (mean of all
subjects) in T cell response to the influenza antigens at the
highest dose tested.
30
Other T cell boosting vaccines are also in development,
with BiondVax, SEEK, Immune Targeting Systems and Bionor Pharma all employing peptide or protein-based vaccinations to increase T cell responses to influenza. Vical has
tested a trivalent DNA vaccine formulation, in which the
three plasmids express H5 HA, NP and M2. T cell (CD4
+
and CD8
+
combined) responses to NP were assessed by
interferon-c ELISpot assay, and a threefold increase following vaccination was recorded in between 20% and 60% of
subjects.
32
BiondVax is developing Multimeric-001, a protein consisting of conserved regions of the virus (including
five linear epitopes from HA, three from NP and one from
M1 of influenza A and B) which is produced in Escherichia
coli and administered with Montanide ISA 51VG adjuvant.
Clinical trials have been completed in younger and older
adults with good safety. IgG titres against the vaccine were
increased by up to 50-fold, and cellular responses were
assessed by proliferation of peripheral blood mononuclear
cells (PBMCs) from donors with up to 90% of subjects
demonstrating a twofold increase in proliferation following
vaccination.
33
SEEK have produced a synthetic multiepitope vaccine FLU-V which is administered with Montanide ISA 51 adjuvant,
34
and has been tested in Phase I and
Phase IIa clinical trials. The vaccine consists of an equimolar mixture of four peptides encoding regions from NP,
M1 and M2. Immune response was assessed by measuring
interferon-c in the supernatant of PBMCs from vaccinees
following incubation with the four peptides, with all vaccinees in the high-dose group demonstrating a twofold
increase over the pre-vaccination response.
35
Immune
Targeting Systems have produced a synthetic nanoparticle
vaccine FP01 consisting of six peptides each conjugated to
a fluorocarbon molecule which is now in clinical testing.
Bionor Pharma also has a peptide-based influenza vaccine
based on conserved regions of influenza antigens in development.
15
There is still much work to be done in defining the phenotype of protective T cells and determining the duration of
immunity induced by vaccination, as naturally acquired
T cell-mediated immunity to influenza is short lived. However, the known protective effect of T cell responses acquired
by influenza virus infection and the potential to protect
against all influenza A viruses with a single vaccine makes
this an extremely important area of vaccine development.
Heterosubtypic anti-HA antibodies
Although the humoral response to influenza HA is generally highly subtype specific, in recent years human antibodies that recognize a large number of subtypes have been
identified. In 2009, Ekiert et al.
36
reported the isolation of
antibody CR6261, which recognizes a highly conserved
region in the stem region of HA, and can neutralize influenza virus by preventing membrane fusion. This is contrast
to the majority of anti-HA antibodies which bind to hypervariable regions around the receptor binding site and prevent binding of the virus to host cells. CR6261 is able to
bind to most group 1 HAs, including H1, H2 and H5. This
was followed by the isolation of CR8020 which is capable
of neutralizing most group 2 HAs including H3 and H7.
37
Corti et al.
38
reported the isolation of an antibody capable
of binding all group 1 and group 2 HAs. These antibodies,
used singly or as a cocktail of monoclonal antibodies, could
be used to provide passive immunity in cases of severe
influenza, providing a new therapeutic opportunity.
Although isolated from human blood samples, these
broadly neutralizing anti-stem antibodies appear to constitute a very minor component of the human immune
response to influenza. However following the 2009
Gilbert
4 ª 2012 Blackwell Publishing Ltdinfluenza pandemic, it was demonstrated that the anti-HA
response was dominated by broadly neutralizing antibodies,
raising the possibility that with the right immunogen
design, this type of antibody could be induced to protective
levels by vaccination.
39
There was evidence of extensive
affinity maturation suggesting that these antibodies were
produced after multiple exposures to antigen, and that it
may be necessary to employ a complex multi-stage vaccination protocol to achieve broadly neutralizing antibodies.
The structure of HA and binding sites of broadly crossneutralizing antibodies has been reviewed by Nabel and
Fauci.
40
In pre-clinical studies, vaccination with plasmid
DNA encoding HA followed by boosting with homologous
inactivated influenza vaccine resulted in broadly neutralizing antibodies, including stem-specific antibodies that were
protective against infection in mice and ferrets.
41
Broadly
neutralizing antibodies were also induced in non-human
primates using the same regime.
41
Steel et al.
42
, have
designed a novel vaccine based on the stem of HA without
the globular head, which can be produced as protein in
HEK293 cells. Mice vaccinated with this construct produced broadly neutralizing antibodies and were protected
against lethal influenza virus challenge. A recombinant protein consisting chiefly of the HA2 portion of HA produced
in E. coli and refolded is highly immunogenic in mice.
Antibodies induced by vaccination were protective against
homologous challenge, and exhibited cross-strain protection within the H3 subtype, but were not protective against
H1 challenge.
43
The approach of DNA priming and inactivated influenza
vaccine boosting using H5 monovalent inactivated vaccine
(MIV) has now been tested in clinical trials.
44
The regime
resulted in increased humoral responses to H5 HA compared with two doses of MIV alone. Anti-stem antibodies
were induced, which in some cases were capable of neutralizing a distinct H5 virus and an H9 virus. This provides
evidence that broadly neutralizing anti-stem antibodies can
be induced in humans by vaccination, and that the induction of increased helper T cell responses following the
DNA vaccination may underlie the increased breadth of
humoral responses. However, it is by no means certain that
a neutralizing antibody response of a sufficiently broad
specificity and titre can be induced in all humans by vaccination. It may be more realistic to aim to induce broadly
neutralizing antibodies rather than universally neutralizing
antibodies. A human monoclonal antibody recognizing a
conserved epitope on the globular head of the majority of
H1N1 viruses has been identified.
45
The use of an adjuvant
with trivalent inactivated vaccine (TIV) or viral-vectored
delivery of HA
41,46,47
also results in greater cross-reactivity
than immunization with inactivated virus or recombinant
protein alone, and these approaches have the potential to
improve protective immunity against drifted variants of the
same subtype at least, with the possibility for some crosssubtype neutralization.
What do we expect from a universal
vaccine?
Having reviewed the different approaches that are being
followed with the aim of developing a universal influenza
vaccine, it is useful to consider what we expect a universal
influenza vaccine to achieve. Will it be a ‘one shot for life’
vaccine given in infancy? Will it be a vaccine to be stockpiled in case of a pandemic rather than used to prevent
seasonal influenza infections? Or a vaccination given to the
whole population every year with efficacy against seasonal
influenza at least as high as the currently licensed vaccines,
but the same level of efficacy against drifted seasonal variants and pandemic viruses. If the latter, and the vaccine
was used worldwide, the resulting immunity could prevent
any new pandemic from occurring as the number of ‘susceptibles’ in the population would be very low. This could
achieve containment of disease caused by Influenza A,
although the continued presence of large reservoirs of the
virus in avian species will require the rate of vaccination to
be maintained continually.
48
Although we tend to categorize people as ‘susceptible’ or
‘immune’ to influenza, in reality there are more possible
outcomes of exposure to influenza virus than either no illness or severe illness⁄ death. The possible outcomes and the
immune mechanisms that are thought to be responsible for
them are shown in Table 1. It is also necessary to consider
how influenza vaccines are tested for efficacy. In Phase IIa,
or controlled challenge studies, healthy individuals aged
18–45 years with low haemagglutinin inhibition (HI) titres
to the challenge virus receive intranasal inoculation of the
challenge virus while housed in a quarantine unit. Twice
daily symptom questionnaires and daily nasal washes for
virus quantification maximize the chances of detecting ‘laboratory-confirmed influenza’, which in this population is
generally a very mild illness. Due to the unpredictable and
sometimes low rate of infection of unvaccinated subjects, a
control group of the same size as the vaccinated group
must be included and it may be necessary to repeat the
study in multiple cohorts of volunteers to achieve a statistically significant estimate of vaccine efficacy. It is essential
to have the control group challenged with the vaccinated
group rather than using data from a historical set of control subjects, as the reasons for the rate of infection in the
control group are still not well understood and may be
affected by the strain and prevalence of the seasonal viruses
circulating in the months prior to the challenge.
Phase IIb or field efficacy studies require several 100 or
1000 people to be recruited at the beginning of the influenza season, with half of them receiving the vaccine under
Development of universal influenza vaccines
ª 2012 Blackwell Publishing Ltd 5test, and the other half receiving placebo, or TIV as a comparator, as has been done for some studies of live attenuated influenza vaccine.
49
Follow-up consists of weekly
monitoring telephone calls or web-based questionnaires to
capture information on influenza-like symptoms, plus use
of nasal swabs to sample virus when symptoms are present.
This requires participants to remember to report all possible influenza symptoms for several months, take swabs correctly when indicated and provide them for virus detection.
Thus, Phase IIa studies will capture all instances of mild
disease, but provide no information about more severe illness and can only be used in a the age range least likely to
suffer severe disease. Phase IIb studies will miss some cases
of mild disease but can include a much wider age range,
and if sufficiently large may be able to indicate vaccine
efficacy against severe disease.
Phase IIa studies can only determine efficacy at a given
time point following vaccination, which is usually only a
few weeks. Phase IIb studies collect information for a whole
influenza season, and may be extended to a second season.
Virus isolation allows an assessment of efficacy against both
strains that are antigenically similar to the vaccine and
drifted variants to be assessed.
49
Either of these approaches may be used to test the effi-
cacy of novel influenza vaccines, but only against seasonal
influenza. Efficacy testing against virus subtypes other than
H1N1 and H3N2 can only be conducted in animal models,
or using functional in vitro tests for neutralizing antibodies
and cytotoxic T cells to predict vaccine efficacy against
pandemic viruses. As novel ‘universal’ influenza vaccines
can only be fully tested for efficacy against mild or possibly
severe seasonal influenza in humans, if the efficacy is suffi-
cient to recommend their use, they could then be used in
place of the current seasonal vaccines.
Vaccines for all
It should not be forgotten that the number of cases of
severe influenza disease and death in different age groups is
affected more by naturally acquired immunity than either
exposure to the virus, or vaccination, with the majority of
deaths from seasonal influenza occurring in the very young
or the elderly. Any form of immunosuppression, including
pregnancy and obesity, increases the probability of severe
illness,
50
and in the elderly, currently licensed vaccines are
considered to have low efficacy, although robust evidence
is lacking.
51
Repeated use of TIV in influenza-naı¨ve individuals prevents the acquisition of heterosubtypic T cell
immunity.
52
In animal models, the heterosubtypic immunity acquired following virus infection is partially protective
against infections with influenza viruses of other subtypes,
and acquisition of heterosubtypic immunity is prevented
by use of TIV or whole inactivated virus vaccines.
53–55
The
ideal influenza vaccine for infants or young children would
Table 1. Possible outcomes of human interactions with influenza A
Outcome
Virus shedding and
likelihood of onwards
transmission
Immune mechanism responsible
for protection
Scored as lab-confirmed flu?
(symptoms and virus shedding)
1 No exposure None Can only be achieved by non-pharmaceutical
interventions such as masks, mobility restriction
No
2 No infection None High-titre neutralizing antibodies (NAb) to the
circulating virus
No
3 Asymptomatic infection None or very low Lower NAb titre, or protective T cell response,
possibly anti-M2e antibodies
No
4 Mild illness: ‘a cold’ or
‘man flu’
Moderate to high Insufficient pre-existing immunity to prevent
disease, but rapid increase in NAb and T cells
to prevent spread of infection resulting from
expansion of immune memory
Yes in quarantined challenge
study, possibly in field study
5 Severe illness: ‘the flu’ High Insufficient pre-existing immunity to prevent
disease, lack of appropriate immune memory
to rapidly control spread of infection
Yes
6 Serious illness requiring
hospitalization
High Insufficient pre-existing immunity to prevent disease,
lack of appropriate immune memory to rapidly control
spread of infection, immunodeficiency from any cause,
secondary bacterial infection
Yes
7 Death High Insufficient pre-existing immunity to prevent disease,
lack of appropriate immune memory to rapidly control
spread of infection, immunodeficiency from any cause,
secondary bacterial infection
Yes
Gilbert
6 ª 2012 Blackwell Publishing Ltdtherefore be designed to prime broad immunity, either
cytotoxic T cell or neutralizing antibody mediated (but
preferably both), paving the way for further development
of the immune memory at each subsequent encounter with
influenza virus rather than providing sterilizing immunity.
For older children and adults, this broad immunity would
be boosted by periodic vaccination with a different vaccine
which may contain both ‘universal’ and ‘seasonal’ components. For example, use of MVA-NP + M1 co-administered
with TIV results in broadly cross-reactive T cell responses
to NP and M1 as well as high-titre antibodies specific to
the HA components of TIV (Figure 1, and Caitlin E.
Mullarkey, Arjan van Laarhoven, Amy Boyd, Eric Lefevre,
Teresa Lambe, Sarah C. Gilbert; unpublished data). Use of
such a vaccination regime would accelerate the onset of
highly effective, broad immunity. For the elderly, boosting
immune memory has a greater chance of success than
priming new immune responses, and this approach could
increase the upper age limit at which vaccination ceases to
become effective in the face of immunosenescence. At the
extremes of age the chance of exposure to the virus will
be reduced by effective, broad immunity in the rest of the
population, and vaccination to induce neutralizing antibodies in late pregnancy could improve protection of
infants prior to their first vaccination.
56
Influenza A B C
All of the above refers to vaccination against influenza A,
whereas we currently vaccinate against influenza B as well,
but not influenza C which causes only very mild disease.
The lack of a significant animal reservoir of influenza B
57
means that pandemics do not occur, and that with widespread use of an effective vaccine this virus could in theory
be eradicated. Infections result in disease in children; once
immunity has been acquired influenza B rarely causes disease in healthy adults but then affects the elderly. The same
approaches to inducing universal or broad immunity to
influenza A with a vaccine that has an improved level of
efficacy in the elderly, who form the main reservoir for
influenza B virus, could also be applied to influenza B, and
vaccine formulations could continue to cover both virus
types.
The path ahead
There has been little significant change in our approach to
vaccination against influenza for many years, but there is
now enormous scope for applying novel technologies to
produce vaccines that will provide better protection against
seasonal influenza in all age groups at the same time as very
useful protection against pandemic influenza that is at least
capable of reducing the number of deaths and reducing the
severity of disease in those who do become infected. The
diversity of approaches being pursued and uncertainty over
what each could achieve has resulted in reluctance from
large vaccine manufacturing companies to commit to any
one of them until larger efficacy studies have been completed. These studies will therefore require public funding,
which is unquestionably warranted when the return on
investment in terms of improved public health and security
against pandemic influenza is taken into account. Influenza
A cannot be eradicated, and to gain control over this virus
it may be necessary to vaccinate a high proportion of the
population at intervals throughout life. However, universal
vaccination with universal vaccines would put an end to the
threat of global disaster that pandemic influenza can cause,
and is a goal well worth pursuing