Referee 1's review:

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N.B. These are the comments made by the referee when reviewing an earlier version of this paper. Prior to publication the manuscript has been revised in light of these comments and to address other editorial requirements.
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Review of the first revised manuscript:
The authors present an interesting idea to use intradermal instead of subcutaneous injection and thus use a fraction of the usually required vaccine dose. This may be very much in the interest of donors including the WHO but it is probably not in the interest of the vaccine producers. This work suggests a policy change and probably deserves consideration even though yellow fever is currently not very topical as there have been few outbreaks recently with big outbreaks occurring between 1985 and 1995. But considering the devastating effect of yellow fever outbreaks and the continued recommendation for travellers to use YF vaccine there should remain interest. The study has to be independently financed by the Infectious Diseases of the Leiden University Medical Center which is assuring.

Backgrounds
It may be helpful if the authors could elaborate two points of background information:
• Could the authors provide us with more detailed information regarding the current limited supply of vaccine? The authors inform us that in case of simultaneous outbreaks in megacities the current stockpile of yellow fever vaccine will not be sufficient to protect the large populations from the disease. More helpful may be a balanced assessment between the number of doses available as part of a stockpile (6 million?) and potential demand. Clearly the use of a reduced dose would also be more economic.
• What is the minimal conventional sc vaccine dose for YF vaccine. The authors refer to 0.5mL as conventional dose, while the WHO position seems to be that the minimal conventional dose is lower than 0.5mL. The authors should report the recommended ranges or clarify that the WHO unambiguously defines the dose at 0.5mL; i.e. there is no range?

Methods
Why was only a subgroup of primovaccinees tested by PCR?

Conclusions
The conclusions are appropriate - but could be more cautious (in the abstract) clarifying that the available evidence is not sufficient for a policy change. Yet the findings strongly suggest that 1/5th of the dose is sufficient for immunisation therefore more research is needed.

The discussion is lengthy and may benefit from cutting not essential paragraphs.

Review of the second revised manuscript:
My concerns have been adequately addressed and the manuscript is now ready for publication.