The one where you actually need the certificate to get in.
Yellow fever is a viral illness spread by mosquitoes in parts of sub-Saharan Africa and tropical South America. A single dose of the vaccine gives lifelong protection under WHO rules, and comes with an International Certificate of Vaccination or Prophylaxis (ICVP) — the little yellow card border officials actually check.
For a small group of countries you cannot enter without proof of the jab. For a larger group, you only need proof if you're arriving from (or transiting through) a country with yellow fever transmission risk. For many others, it's recommended on health grounds even though no certificate is required.
If the country requires a certificate for all travellers, border staff can and do refuse entry without it. There's no 'I forgot' workaround — some airports will vaccinate on arrival at your expense and hold you back 10 days for the certificate to become valid.
A layover of 12+ hours in a yellow fever transmission country usually triggers the requirement at your final destination. Check both departure and transit countries against the WHO list before you book.
Infants under 9 months, pregnant travellers, and people with weakened immune systems may not be able to have the vaccine. A doctor can issue a medical exemption on the same yellow certificate.
Don't leave it to the week before you fly. If your trip is in less than 10 days and the country requires a certificate, you'll need to change your plans.
Border staff can refuse entry without proof. Get vaccinated at least 10 days before travel.
Required for all travellers, from 9 months old.
Required for all travellers, from 1 year old.
Required for all travellers, from 1 year old.
Required for all travellers, from 9 months old.
Required for all travellers, from 9 months old.
Required for all travellers arriving from any country.
Check your departure and any transit stops of 12+ hours against the WHO risk list.
Certificate required within 6 days of arriving from a risk country.
No certificate required at the border, but public health bodies recommend the jab for these destinations or regions.
Recommended on health grounds for the northeast (Iguaçu Falls region).
Under WHO's 2016 amendment, a single dose gives lifelong protection and the certificate is valid for life. Some countries still ask for a booster after 10 years — always check the destination's current rules.
The UK is not a yellow fever transmission country, so transiting through London on the way to (say) Kenya doesn't trigger the certificate rule. But a layover in Kenya on the way to India would.
You can — but the certificate only becomes valid 10 days after vaccination. If your flight is inside that window and your destination requires proof, you can't travel.
No — those are separate vaccines you might also need for the same trip. Yellow fever is the only one that comes with an international entry certificate. The others are recommended on health grounds only.