The government has ordered enough doses for the whole UK population
The UK swine flu vaccination programme is being extended to children aged between six months and up to five years.
Isn't the vaccination programme already under way?
Yes. GPs started offering it to the initial priority group in October.
This includes people with health problems and pregnant women as well as NHS workers.
In total, there are nearly 14 million people in this group, and doctors expect to finish vaccinating them by Christmas.
However, there is likely to be some overlap with the under fives vaccination due to start in December - subject to negotiations with doctors.
It also seems likely that children will need two jabs - they are given half-doses of the vaccine - although this is to be reviewed as there is evidence to suggest there is a good immune response after one.
Why are young children now going to be vaccinated?
Data suggests healthy children below the age of five are the most likely of all the age groups to need hospital treatment for swine flu.
More than 80% of those who have needed hospital treatment have not had previous health problems such as asthma or heart disease.
For children aged between five and 15 the proportion drops to 60%, while for adults it is much lower.
Government health officials have been particularly concerned about the increasing number of serious complications.
At the beginning of the pandemic one out of every 10 people treated in hospital required intensive care, but that proportion is now one in five.
What is the evidence it is safe?
There are two vaccines against swine flu - Pandemrix made by GlaxoSmithKline and Celvapan made by Baxter.
The European Medicines Agency, the official regulator that licenses treatments, approved both vaccines after thousands of people, including children, took part in trials.
The regulator concluded the jabs were safe and effective.
The most common side-effects reported during the trial process were minor ones - headaches, joint pain, muscle pain, fever and fatigue.
The trial and approval process has been quick - the first trials started during the summer.
But this fast-tracking is not unusual for flu, as it happens for the seasonal version every year.
This can be done because the basic ingredients are the same.
People who are suspicious of flu vaccines often talk about the risk of Guillain-Barré syndrome (GBS), a rare disorder in which a person's own immune system damages the nerve cells, causing muscle weakness and sometimes paralysis.
Most of the evidence suggests vaccines do not increase the risk, however there is research to indicate that getting flu increases it seven-fold.
Another allegation sometimes made is that being given the vaccine could actually give you flu.
Doctors are adamant this is not the case - the two vaccines being used in the UK do not contain the live virus.
As for deaths, the World Health Organization has been monitoring this and while a handful of people have died after being given the vaccine, officials have ruled out any link to the jab.
Should I take my child to have the jab?
The government has said vaccination is the best defence against the virus.
However, officials recognise people are jumpy about immunisations and have said it is a personal decision.
A recent poll for the BBC showed nearly half of people had doubts about vaccination.
Safety and the mild nature of the virus - the fact that not that many people have become really ill - were the main concerns.
What about the rest of the population?
As well as young children, those caring for elderly and disabled people will also be offered the jab as part of the second wave of vaccination.
They have been included in the programme as health officials are worried about the impact of the vulnerable people they are caring for if they fall ill.
No decision has been made yet on the rest of the population.
The five to 15-year-old age group also has high rates of healthy people being admitted to hospital.
But it will be next year before the health service can start seeing anyone else.