Public Health England (PHE) is urging pregnant women and those over 65 to get the flu jab, after a fall in take-up compared to this time last year.
PHE said it was best to get the jab ahead of Christmas, before flu typically starts to circulate.
Social gatherings can increase the risk of it spreading.
Early indications suggest the vaccine will be more effective this year because it is well-matched to the predominant strain circulating.
This is thought to be Influenza A (H1N1), commonly called swine flu.
Uptake of the vaccine among pregnant women and those over 65, who are at a higher risk of flu complications, is lower than last year.
Festive spread risk
Paul Cosford, medical director at PHE, said only around 40% of pregnant women and 65% of over-65s had been vaccinated so far.
"It is even more important than ever that all those eligible take up the offer of the flu vaccine, especially before Christmas when many people will be gathering together with the added risk of spreading infection that this brings," he said.
PHE said there is strong evidence that pregnant women have a much higher risk of serious illness as a result of flu compared with the general population, with the risks highest in the last three months of pregnancy.
Serious complications of flu include pneumonia, septic shock, meningitis and encephalitis (inflammation of the brain).
An enhanced flu jab for people over 65 was introduced this year which contains extra ingredients to give better protection.
In November, GPs reported a shortage of the new vaccine for the over-65s after a staggered delivery of supplies from the manufacturer to GPs and pharmacies meant some older patients had to wait to be vaccinated.
The final batches of the vaccine have now delivered.
PHE estimates this enhanced or adjuvanted vaccine will save hundred of lives this winter and ease pressures on the health service.
A 'quadrivalent' vaccine, which offers protection against four strains of flu, is being given to those under 65 with underlying health conditions.
Another change this year has seen the nasal spray vaccine being offered to an extra school year - Year 5.
Children are seen as super-spreaders of the disease, as they are likely to infect more vulnerable older family members.