What is the youngest age at which a baby can be vaccinated against flu?

In the UK as well as most other countries, flu vaccines are only licensed for use in babies aged over 6 months. However, a recent study in the United States of 1375 infants aged just 6-12 weeks confirmed that flu vaccine was safe and effective in this age group also, and therefore might be considered under special circumstance. Vaccinating mothers in pregnancy also passes immunity to the newborn baby.