You are advised to contact your doctor or clinic around 4-8 weeks before your trip to check whether you need any vaccinations and to get their professional medical advice regarding travel to Kenya.
There is a risk of malaria in all areas of Kenya, except Nairobi and the highlands - check with your doctor about suitable antimalarial tablets. Dengue fever can also be transmitted via mosquito bites. Try to avoid mosquito bites wherever possible - wear loose long-sleeved clothing and trousers, and use a repellent on clothing and exposed skin.
Kenya falls into the yellow fever region in Africa. There is only a low potential for exposure to yellow fever in Nairobi, Mombasa, Lamu, Malindi and Shimba Hills National Park, but in the rest of the country there is a higher risk. It is therefore advisable for all travellers aged 9 months and older to obtain a yellow fever vaccination no less than 10 days prior to travel, but depending on the rest of your travel plans and country of origin, it is not compulsory:
Travellers from the UK who are only travelling directly to and from Kenya are not required to produce a vaccination certificate upon their return to the UK.
If you are arriving into Kenya from a country which has a risk of yellow fever transmission, you will be required to provide a certificate of your vaccination upon entry into Kenya. These countries include Angola, Argentina, Ecuador, Ethiopia, Nigeria, Peru, Rwanda, Uganda and Venezula but it is up to the traveller to check the full list via http://www.who.int/ith/2015-ith-annex1.pdf?ua=1
Certain countries including South Africa and Tanzania will deny entry if you are arriving from Kenya without the vaccination. Please ensure you check the full list via http://www.who.int/ith/2015-ith-annex1.pdf?ua=1 to see which countries impose this rule.
If your doctor advises that it is not safe for you to have the vaccination then you should obtain a medical waiver and travel with this instead.