The Yellow Fever vaccination takes at least 10 days to take effect and many countries will require proof of effective vaccination on condition of entry.
On the 11th July 2016, the World Health Organisation (WHO) concluded that the Yellow Fever vaccination is now only required once in a lifetime (ref: http://www.who.int/ith/annex7-ihr.pdf): ‘Accordingly, as of 11 July 2016, for both existing or new certificates, re-vaccination or a booster dose of yellow fever vaccine cannot be required of international travellers as a condition of entry into a State Party, regardless of the date that their international certificate of vaccination was initially issued. This lifetime validity of these certificates applies automatically to certificates issued after 11 July 2016, as well as certificates already issued.’
It should be noted however that consultants over the age of 60 should consult their doctor. On the 4th April 2017, WHO (http://www.who.int/csr/don/04-april-2017-yellow-fever-brazil/en/) states that ‘Travellers with contraindications for yellow fever vaccine (children below 9 months, pregnant or breastfeeding women, people with severe hypersensitivity to egg antigens, and severe immunodeficiency) or, over 60 years of age should consult their health professional for advice’.
The Geo People has verified the above advice with International SOS, (https://internationalsos.com), a large medical and travel security services firm that provides oil & gas industry consultants with OGUK medicals.