Do you need a yellow fever vaccine to visit the border?

Quick answer: No, a yellow fever vaccine is not an entry requirement for border visits. However, it is recommended for Foz do Iguaçu and the Falls area, as these regions are on health ministry lists. For day trips to Ciudad del Este, Paraguay does not require the certificate, but it may be requested if staying in the country or continuing your journey from a risk area.

In Brazil (Foz): it is not an entry requirement, but Foz is on the Ministry of Health's list of areas with a recommendation for the yellow fever vaccine. Get it at least 10 days before your trip.

Paraguay: for a day shopping trip to Ciudad del Este, the certificate is normally not required (border traffic rules). To stay in the country or continue your journey, the International Certificate of Vaccination may be requested of those coming from a risk area.

Argentina: no requirement for tourism at the Falls; a recommendation similar to Brazil's.

Summary: get vaccinated as a precaution (the region is heavily visited) and carry the certificate if you're going to stay in Paraguay. Confirm the current rules at a health center before traveling.

Source: Ministry of Health · border guidance