There are three important findings from the clinical trials reported here. First, we replicate our previous findings that vaccination with MVA85A in subjects previously vaccinated with BCG is safe and well tolerated. Second, we also replicate our previous findings that BCG induced immune responses can be significantly boosted with MVA85A, as measured by the peak and plateau vaccine induced effector immune responses after vaccination. Third, we show that the boosting potential of MVA85A does not seem to be dependant on interval between BCG vaccination and boosting with MVA85A. We investigated the relationship between boosting interval and peak vaccine induced immune response, 1 week after vaccination. No correlation was found between boosting interval and peak response (Spearman's correlation r
0.49, data not shown).
Importantly, these higher antigen specific responses are maintained at a significantly higher level than in the BCG alone arm for at least 24 weeks after vaccination. The same result was found in the previous clinical trial in which a longer boosting interval was investigated. There were no significant differences in plateau responses 24 weeks after vaccination, when the short and long boosting intervals were compared. This persistence of ex-vivo responses cannot be attributed to persistence of the MVA85A vaccine, as MVA85A does not replicate in mammalian cells and does not persist. It is more likely that the MVA85A boost has expanded the memory T cell population, which is either persisting without antigenic exposure or is being constantly re-stimulated or ‘boosted’ by exposure to environmental mycobacteria. We and others have previously demonstrated the existence of anti-mycobacterial immunity induced by environmental mycobacteria in BCG naïve adolescents and adults in the UK 
. In contrast, the BCG induced immune responses at 24 weeks after vaccination are not significantly different from baseline, despite the fact that BCG will almost certainly persist for longer than MVA85A.
The results of the BCG alone trial are very comparable with our previous data on the immune response induced after BCG vaccination, using the BCG Glaxo strain 
. Other groups have also found no significant differences between the two strains of BCG used in the UK over the last 5–10 years 
. Subjects in the short and long boosting interval groups differ in which strain of BCG they were vaccinated with. The short boosting interval subjects presented here were all vaccinated with the SSI strain of BCG, whereas the long boosting interval group previously published were vaccinated with Glaxo BCG. However the comparability of immunogenicity of these two strains found both by us and others suggests that this is not an important factor when comparing the boosting potential of MVA85A between these two groups.
A limitation of the work presented here is that the short boosting interval BCG prime–MVA85A boost trial and the long boosting interval trial previously published were not performed as a single study. In comparing results between trials, the power to detect any differences may be small, particularly given the small sample sizes used in these Phase I studies. However we believe the promising immunogenicity in the short boosting interval trial presented here justifies the further evaluation of this vaccine in efficacy trials as discussed below.
In the absence of a pre-defined immunological correlate of protection, the key question when developing a new TB vaccine is whether such significantly enhanced immune responses seen after the MVA85A boost in both the short and long interval boosting studies are accompanied by an improvement in protective efficacy. This question can only be addressed in large scale efficacy trials. Such trials will need to be conducted in a high incidence population, to obtain efficacy data within a realistic time frame. Even so, these trials are likely to require approximately 10000 subjects and will also require follow-up periods of up to 2 years. A key question when considering the deployment of a new TB vaccine designed to boost BCG is when to administer the boost. One option is to boost in infancy at about 4–6 months of age, and ideally this boost would coincide with an existing EPI schedule vaccine visit, providing no interference occurred between new and existing vaccines. Another potentially useful time to boost BCG is in early adolescence, just before the rise in incidence of TB disease that occurs in adolescence and young adults.
There are two possible efficacy trials which correspond to potential deployment in either infancy or adolescence. Both scenarios have advantages and disadvantages. Boosting in infancy is attractive as there is a well established infrastructure within the EPI for such an additional vaccine. If such a boost were scheduled to coincide with an existing EPI schedule visit, then vaccine take-up would likely be higher. However the major disadvantage with conducting an efficacy trial in infancy (but not necessarily with deployment in this age group once efficacy had been established) is that disease end points can be difficult to define 
. In contrast, boosting in adolescence is an attractive option as disease endpoints are clearly defined and easy to diagnose in this age group. If effective, boosting in adolescence would have a more immediate impact on the mortality and morbidity of this disease than boosting in infancy. A considerable disadvantage of boosting in adolescence is that there is currently no infrastructure for routine vaccination in this age group, particularly in the developing world. However with the recent licensing of a vaccine against human papilloma virus, scheduled to be administered from 9–15 years of age, such an infrastructure is likely to become established in the future 
. Ultimately a prophylactic vaccine against HIV would also be likely to be administered in early adolescence.
The aim of these comparative Phase I studies was to investigate the effects of boosting BCG soon after vaccination (thus modelling the infant boosting scenario) and boosting many years after BCG vaccination (thus modelling the adolescent scenario).The immunogenicity data presented here suggest that, at least using this immunological readout, both options of boosting in infancy or of boosting in adolescence may be effective. The data presented here supports the further evaluation of this promising candidate vaccine, which is currently in Phase II clinical trials in South Africa.