Among her many achievements, philanthropist Melinda Gates was the first woman to have given away more than $40 billion.

As co-chair of the Bill and Melinda Gates Foundation — alongside her husband, Microsoft co-founder Bill — Gates has been part of a movement that has supported work in more than 100 countries that face challenges in education, poverty, hunger and health.

Consequently, heading such an organization means needing to hit milestones and potentially even exceed targets.

In an interview with CNBC, Gates said she is particularly proud of what the private foundation had achieved in improving world health.

"The work that we've done in vaccines, to really get vaccines out to hundreds of millions of children," Gates told CNBC Meets when asked about what her proudest moment of the foundation had been so far.

"There are 3 million children alive today because of those vaccines and when we started in this work, there was you know a 20 or 25-year lag between when a vaccine would come out in the United States and when it would get to the developing world. Even when it got there, it wasn't all the right strains that they needed."