Upvote:3
Most of the information provided below can be found on Brazilian Health Ministry website, in the section for Traveler's Health. I recommend to read it, as it is a credible source of information for traveler's coming to Brazil.
Brazil does not require the International Certificate of Vaccination or Prophylaxis for entry into the country.
About Dengue (and other diseases transmitted by mosquitoes) now that the winter is coming (sorry GoT fans) it becomes less of a threat. In any way it is a good measure to stay in places that have protective screens on doors and windows, especially away from the capital and major cities and also apply repellent to exposed areas of your skin.
You specifically asked for Rio and Salvador, those are major cities so Dengue is less of a concern than rural areas. Are you planning trips off those cities?
Upvote:5
With regards to before you go - it depends where you are from, where in the country you are going and what you are concerned about.
The UK Foreign Office provides advice here. http://www.nathnac.org/ds/c_pages/country_page_BR.htm
It draws attention to: