In this phase 2a study, we have demonstrated the safety of MVA-NP+M1 at a dose of
1.5 × 108 PFU given as a single intramuscular injection.
The majority of adverse events were mild in severity, with no serious systemic adverse
events and no rigors experienced by any of the 15 subjects who were vaccinated, indicating a
satisfactory safety profile at this dose. This dose is now being tested in an additional
phase 1 study of subjects aged >50 years.
In the phase 1 study, the T-cell response was measured by ex vivo IFN-γ ELISpot assay
at the peak of response 7 days after vaccination, and at 21 days. Median responses were 2793
and 2088 SFUs per million PBMCs at 7 and 21 days in the high-dose
(2.5 × 108 PFU) group when fresh PBMCs were used in the
assay. In the phase 2a study reported here, employing an intermediate dose of MVA-NP+M1
and using fresh PBMCs, the median response of the vaccinees 21 days after vaccination was
980, falling to 627 on the day prior to influenza challenge. Although it is not unexpected
that the response measured by this assay is reduced when the vaccine dose is reduced, the
small numbers of volunteers in both studies do not allow an accurate determination of the
magnitude of this reduction.
Following influenza virus challenge, only 5 of 11 control subjects developed
laboratory-confirmed influenza, defined as symptoms of influenza disease plus virus
shedding. This figure is lower than expected for challenge studies of this type,
although it has previously been shown that approximately one-third of individuals undergoing
influenza challenge are protected despite not having detectable antibodies against the
challenge virus [3
] and it is a known feature of
this challenge model that not all control subjects will develop influenza. In this study
only 2 vaccinated volunteers developed laboratory-confirmed influenza, the total number of
symptoms recorded was lower in the vaccinated group at all time points following challenge,
the number of grade 2 and 3 symptoms recorded was lower, and virus shedding was
significantly reduced, supporting a protective effect of the vaccine against both disease
severity and virus shedding.
It was notable that there was no consistent rise in HI titer following influenza challenge,
even among volunteers who developed laboratory-confirmed influenza.
Having demonstrated a significant increase in the number of T cells producing IFN-γ in
response to NP and M1 following vaccination, and with fewer vaccinated volunteers developing
influenza than control subjects, we attempted to confirm the association of vaccine-induced
T-cell responses with this protective outcome. In a large study of 2172 children in the
Philippines and Thailand, it was found that the majority of infants and young children with
>100 SFUs per million PBMCs in an IFN-γ ELISpot assay utilizing whole influenza
virus as antigen were protected against clinical influenza [9
]. In our own small-scale study of adults, who would have had
multiple prior exposures to influenza prior to vaccination resulting in memory populations
of influenza-specific T and B cells, we were not able to define a correlate of protection
based on responses detected in PBMCs using the IFN-γ ELISpot assay prior to challenge.
Following influenza challenge, only minor fluctuations in the IFN-γ ELISpot were
detected for a period of 4 days, increasing by the seventh day in subjects who developed
influenza disease, whereas virus shedding was detected on the second and third day. This
suggests that changes in responses measured in circulating PBMCs are occurring only after
respiratory tract symptoms, and cannot be used to predict protection or susceptibility.
However an anamnestic mucosal T-cell response predictive of protection cannot be excluded.
For future studies, a systems biology approach should be taken to understanding
multifactorial mechanisms of protection that may be missed when only a small number of
measures of immune system status are used.
This study provides evidence that intranasal challenge with influenza virus appears safe in
individuals with elevated T-cell responses after MVA-NP+N1 immunization. The absence of
any lower respiratory symptoms or signs, together with normal oxygen saturations and
spirometry after influenza challenge, makes immunopathology highly unlikely . This supports
previous work in several nonhuman species (particularly mice and ferrets [10
]) and pigs [11
], indicating the apparent safety of intranasal influenza virus
challenge after immunization with T-cell–inducing vaccines.
This first efficacy study of a vaccine designed to boost T-cell responses to conserved
influenza antigens has demonstrated the safety of this vaccination approach. Vaccinees were
exposed to influenza virus at a time when anti-influenza T-cell responses had been increased
by vaccination with no ill effects and no evidence of lower respiratory tract infection or
inflammation. It also elucidated the efficacy of the vaccine in boosting the T-cell response
to the vaccine antigens and in reducing laboratory-confirmed influenza in the vaccinees
compared with control subjects. This reduction equates to 60% vaccine efficacy, which
is a similar level to that shown for inactivated influenza vaccines when the circulating
virus and the strain used in the vaccine are well matched [12
], although further studies using a larger sample size will be
required to reach a more precise and robust estimate of vaccine efficacy.
The majority of studies on T-cell–mediated protection against influenza have been
conducted in the mouse model. A small number of studies in other species have indicated that
T-cell responses to conserved influenza antigens can protect against disease and virus
], but this is the first clinical efficacy study of a vaccine
designed to protect in this way. The results of this first clinical study are encouraging
and provide initial evidence that this approach will be successful. Further studies are
indicated to characterize safety and efficacy in larger numbers of individuals and to assess
vaccine immunogenicity in both older and younger age groups.