Hygiene advice is being offered to people
Enough doses of swine flu vaccines for everyone in Wales should begin arriving in the next few weeks.
Latest figures show 64 confirmed Welsh cases, but new counting methods mean up to 1m people in Wales could be diagnosed with the illness long term.
Up to six million doses would become available, with two per person, and those most at risk would be first in line to receive a jab.
Experts will carry out tests and work out how to administer the vaccine.
Wales' chief medical officer Dr Tony Jewell said it would be a huge logistical exercise.
Dr Jewell said the vaccine would reduce the impact of a second phase of swine flu.
"It will put us in a good position to modify it. It is an unprecedented situation," he said.
Cover your nose and mouth when coughing or sneezing, using a tissue where possible
Dispose of tissues promptly and carefully
Wash hands frequently with soap and water
Clean hard surfaces such as door handles regularly with a normal cleaning product
Help your children to follow hygiene advice
Source: National Public Health Service for Wales/Welsh Assembly Government
So far 64 cases of swine flu in Wales have been confirmed by laboratory testing.
But the National Public Health Service for Wales has shifted to counting patients who see their GPs reporting flu-like symptoms.
Latest figures across Wales reveal that 426 people have gone to their local doctor in the past week with flu-like symptoms.
Three were admitted to hospital over the last few days, including a Merthyr boy, aged three, from Merthyr Tydfil , a Powys woman, 45, and Monmouthshire man, 41.
The epidemic could last around 18 months, according to experts, and by the end of that period up to 1m people in Wales could have been affected.
Health officials said for every 100,000 people there have been 14.2 cases of flu-like illnesses.
But Wales is behind other parts of the UK for infection rates.
In Scotland the rate is 23.6 cases, while in England it is 51.9 cases.
Seven people in Wales with swine flu had to be hospitalised but five have since been discharged.
17 people in the UK have died - all but one of them had underlying health problems.
People are advised that if they develop flu-like symptoms they should stay at home and check them using the various sources of information supplied - by leaflet, 24-hour telephone information lines or websites.
Experts say that for most people the illness is mild and gets better within five to seven days. Anybody with severe symptoms is urged to contact their GP or NHS Direct services."
NHS Direct can be contacted on 0845 46 47.