UK health officials are putting a temporary limit on who can have a hepatitis B vaccination because stocks are running low.
The move is in response to the wider, global shortage of the jab caused by manufacturing issues.
UK travellers are being told they may not be able to get it before they leave Britain for higher-risk countries.
Babies and high-risk groups will be prioritised under the measures that are expected to continue into 2018.
A spokeswoman for Public Health England said: "All those who need to have a hepatitis B vaccine will be offered it in due course."
But she said some people wanting to be vaccinated now may not be able to have the jab.
Who needs a hepatitis B jab?
All UK infants should be vaccinated because the infection can persist for many years in children and can eventually lead to complications, such as scarring of the liver or liver cancer.
The chance of catching hepatitis B, which is spread by contact with infected blood and other body fluids, is very low in the UK.
But some people - those who inject drugs and share needles, for example - are at higher risk.
In some parts of the world, including East Asia and Sub Saharan Africa, the infection is more widespread.
The risk for travellers to such countries is still low, but people visiting these countries from the UK may still want to consider getting immunised.
Can I buy the vaccine?
The NHS does provide free doses, based on clinical need. Some people may be charged for the vaccine.
Private clinics may also have stock that they can offer patients for a fee.
The current global shortage is affecting stocks in private clinics as well as the NHS. Although manufacturers are getting more stock in, supplies are limited.
How else can I protect myself?
- avoid having unprotected sex
- avoid having tattooing, piercing and acupuncture in an unhygienic environment with unsterilised equipment
- avoid exposure to contaminated needles (not sharing needles)
Travellers may also want to consider taking a sterile medical equipment kit if travelling to areas with poor resources.