You bring up an excellent question here. In many cases, the -ING following a preposition can be understood to be the object of that preposition, and thus a gerund (noun). For example: The meeting focused on selling new products. Here, it doesn't make sense for "selling" to be modifying "new products," so "selling" must be the noun. In the previous example, "... focused on vaccinating humans..." is similar. "Vaccinating" isn't modifying humans, or it would mean "humans who are vaccinating." Instead, the vaccination is being done to the humans; it makes sense that "vaccinating" is the noun here.

In some cases, though, there is ambiguity with an -ING after a preposition. OG #130 states, "... lead to lowering interest rates..." Here, it seems that "lowering" is modifying "interest rates" - they're interest rates that are lowering. If that's the case, then "rates" is the noun, not "lowering." We'd need to add "lowering of interest rates" to make it clear that "lowering" is the noun. It's not so much that we added the article "a"; it's that we put "interest rates" into a prepositional phrase of its own, making "lowering" stand alone as a noun.

Distinguishing between these verbals can be tricky. Rather than memorizing rules (if it follows a preposition it's this, if it follows something else it's that...), try to think about FUNCTION. What role is the word playing? Is it acting as an idea of its own? If so, it's a noun. Is it giving you information about another thing/idea? If so, then it's a modifying participle.