Effective October 1, 2011, proof of yellow fever vaccination is required for all travelers 1 year of age and above if traveling from or transiting through a country with risk of YFV transmission regardless of the amount of time spent at the airport.
This requirement also applies to the following countries with low potential for exposure to YFV: São Tomé and Principe, Somalia, Tanzania, Zambia, and Eritrea (as of January 1, 2012).
Travelers not meeting this requirement can be refused entry to South Africa or be quarantined for up to 6 days. Unvaccinated travelers with a valid medical waiver will be allowed entry.
Source: CDC Yellow Book