I was reading with interest some of the discussion surrounding the WHO's recommendation regarding the malaria vaccine. We know that malaria is a big problem, yet we also know that this vaccine has relatively limited efficacy and is quite complex to administer. The key question is how much effort and resources should be invested in rolling out this vaccine compared to alternative approaches to the problem (e.g. malaria treatment, or bed nets, or mosquito eradication, or transgenic mosquitos, or whatever).

This took me down the rabbit hole of public health researchers debating the best way to roll out vaccines in a resource limited environment - which vaccines to prioritize, how to acquire/source them, how to distribute them, etc, etc. There's several competing schools of thought that have their own backers (financial and political) - all of them have broadly similar goals of improving the health of some of the world's most vulnerable populations, but there are fierce debates about how to best do that.

This got me thinking about the much broader question: what is the most effective use of the marginal aid dollar today? If I were a government who wanted to spend another $1 billion on aid writ large, where would I spend it? Or, for that matter, how should a philanthropist spend her marginal million dollars, or a joe shmoe spend his marginal $1000? What is the most effective way to deploy additional resources?

Obviously there are various definitions of 'effective', but let's assume that our goals are to improve the well being of as many people as possible as much as possible, with as little waste/fraud/corruption as possible. I'm not interested in answers that are essentially 'policy' answers (e.g. put money into lobbying Congress for X), but rather given the current state of the world, where can a dollar of aid do the most good?

I'm kinda assuming it will be something associated with food assistance, vaccination, or education, but maybe there's a different perspective I'm not thinking about (e.g. water scarcity issues, or funding microcredit, or refugee resettlement, or climate associated efforts like building/protecting carbon sinks). Where do you get the biggest bang for your buck?