When entering Tanzania proof of yellow fever vaccination may be required if some one is entering from a country with a risk of yellow fever such as Kenya. People arriving from areas with out yellow fever should not need proof of it. However, a study done last year showed unpredictable checks at borders and even people who were arriving from Europe or Dubai were sometimes checked. Checks were more likely at land borders than at airports and more likely at Arusha than Dar es Salaam and Zanzibar. Consequences of not having a valid certificate or exemption note included fines, clearance after a short discussion, inability to obtain a visa and even vaccination at the border. It seems everyone arriving in Tanzania should either be vaccinated or carry an exemption note. This should be unnecessary for those not entering from endemic countries however after reading the study it seems necessary.
A big change is that from July 11 2016 when you have a yellow fever vaccination the certificate will now say that the vaccine is valid for the lifetime of the person vaccinated instead of 10 years as it was previously. This follows a decision from WHO because the protection from the vaccine is longer than 10 years for most people. The American CDC however warns that if someone is going to a high yellow fever risk region, were pregnant when they were first vaccinated or HIV positive a second dose may be indicated. It also remains to be seen if the border officials of all countries accept the new duration of protection. If travelling with an expired certificate is should not be altered in any way but should be considered adequate.
There has been a yellow fever outbreak in Angola since December 2015 with over 3,000 cases and over 300 deaths. Because of it nearly 11 million doses of yellow fever vaccine have been given there in emergency vaccine campaigns. The numbers of new cases have declined but it continues to spread. An outbreak is now occurring in neighbouring Democratic Republic of Congo and travellers with yellow fever have gone from Angola to Kenya and China as well. A different yellow fever outbreak is also occurring in Uganda . Because of this yellow fever certificates will probably be checked at borders more strictly than usual so be sure you have your certificate if travelling in such countries in the near future and certainly be vaccinated before you go.