Blood donors are being urged to keep regular appointments up to Christmas, after national blood stocks hit a low last week.
Flu and Christmas will mean fewer blood donors
The National Blood Service says while there is no crisis, a spate of flu and people being busy before the festive period will mean fewer donors.
It said the number of people with flu-like illness last week was more than double that of last Winter.
The NHS in England and North Wales needs up to 9,000 units of blood a day.
The service said blood stocks traditionally drop by 10% at this time of year as people prepare for the festive season.
The surge in the number of people with flu means the figure is likely to be boosted because bad health forces them to stay away.
"Blood stocks are lower than we'd like them to be," a spokesman said.
"But they are improving and it's certainly not at crisis level.
"We always take action to make sure we don't hit that level at this time of year as people often forget to give blood in the run-up to Christmas," he said.
The service says only 6% of those who could give blood actively donate.
The blood collected is used for a wide range of treatments from accidents and emergencies, to routine surgery and cancer treatment.
Although north east and north west Wales fall within the NHS area reporting a blood shortage, the rest of Wales has sufficient replies.
A spokesman for the Welsh Blood Service, which covers mid, west and south Wales, said they had enough supplies to cover the holiday period.