On 13 June, donors to the GAVI Alliance will gather in London to affirm their commitment to fund immunizations in the developing world. At the meeting, participants will address the estimated $3.7 billion financing gap needed over the next four years to scale up childhood vaccination efforts to meet the demand forecasts for those countries that receive assistance from the Geneva-based organization. But attendees of the pledging conference will also be discussing something not on the formal agenda: the announcement last month that Seth Berkley, who founded and heads the International AIDS Vaccine Initiative (IAVI), will take over the helm of the alliance in August. Berkley will lead a unique chapter in GAVI's development as the organization narrows in on the looming deadline set by Millennium Development Goal 4, which aims to reduce child mortality by two thirds by 2015. Yet, in a sense, these efforts will be a continuation of the work Berkley has fostered at IAVI since he formally launched the New York-based nonprofit in 1996. Berkley, an epidemiologist who previously held jobs with the Rockefeller Foundation, the Carter Center and the US Centers for Disease Control and Prevention, has witnessed ups and downs in the vaccination field, from the disappointing STEP trial in 2007 to the more recent good news from the 2009 Thai study, which reported as much as 31% protection against HIV. Roxanne Khamsi spoke with Berkley about what he has learned in his quest for a preventative shot against AIDS.