This is not an issue. "Large lobby groups" also include organizations such as the EFF, ACLU, unions, and foundations such as the Gates Foundation. And let's not forget the biggest lobby group of all: the American citizenry. If you ever wrote your elected representative to vote a certain way, you are lobbying.
>"...because that would deny its pharma companies a lucrative & desperate market."
Correct me if I'm wrong, but this doesn't make sense. HIV/AIDS is also a lucrative desperate market with no vaccine currently available. Heck, why is the US okay with malaria being on there even?
Also, wouldn't the Latin-Am governments purchase supplies and any future vaccines from the pharma companies? In a national emergency, the government throws more money at the issue. So, when there is a Chagas vaccine, the Latin-Am government can throw more money at the pharma company who is producing the vaccine. How is the pharma industry benefiting from the US striking out Chagas?