New evidence shows that the risk of infections increases with time since the second dose of the Covid booster jab, so ministers are urging millions of Britons to get their jab by 11 December.
The new warning comes after cases broke records in parts of Europe on Wednesday, with the continent once again the centre of a Pandemic that has prompted new restrictions.
16 million people in the UK have had a booster vaccine. Everyone aged 40 and over and the clinically extremely vulnerable can get a booster six months after their second jab. If you haven't gotten your first, second or booster dose yet, please come forward for it as soon as possible, said the vaccines minister.
People who have had their booster vaccine by 11 December will have high protection against Covid. Those eligible for a booster have been urged to take up the offer as soon as possible to protect themselves and their families, and help to reduce the pressure on the health service.
Two weeks after a booster, people have a high level of protection from getting a case of Covid.
New research shows that the immunity against infections decreases in six months after two doses of the Pfizer/BioNTech vaccine. The major study shows the importance of boosters.
The second dose of Pfizer/BioNTech increased the chances of a breakthrough infection by more than tenfold, according to the findings of a study of 80,000 people in Israel.
Booster programmes are required to prevent the spread of infections because the vaccine is highly effective at preventing hospitalisations for at least six months but the protection against infections falls more steeply.
The message was that boosters are needed after about five months.
The study was based on the electronic health records of 80,000 people who had a test after their second vaccine. There was no evidence of previous Covid-19 infections in the sample.
The study found that the rate of positive results went up after people had had their second dose. 1.3% of people tested positive after a second dose, but this increased to 2.4% after 90 days, 4.6% after 120 days, and 10.3% after 150 days or more.
The authors said it appears clear that immunity fades after three months of being double-vaccinated but returns to a high level after a booster dose.
They said that large-scale Covid-19 vaccination campaigns can achieve control over the spread of the virus, but even in countries with high vaccination rates, breakthrough infections can occur. Adults who received two vaccine injections had a higher risk of Covid infections compared to individuals who had not received a vaccine in the last 90 days.
They said it was too early to say if immunity waned after a third booster dose.
The jab is thought to offer long-term T-cell immunity for older people, which could be better than other vaccines.