A new study has shown that the Covid booster jab provides five times as much protection as the second.
Experts believe that the booster shot is more effective than the original two-dose regimen.
Researchers looked at vaccine reactions before and after boosters in healthy middle-aged adults.
All of them had received their second jabs around nine months ago.
The antibody levels of the patients had dropped almost ten-fold by the time they received their booster shots.
However, antibody levels had increased 25-fold in six to ten days following top-up and were five times higher after just two doses.
Yesterday was the opening of an online booking site for people aged 40 to 49, who can book a slot six-months after their second dose.
Teens aged 16-17 can also now book in for their second vaccine jab, to be even more protected against Covid.
Everyone able to get their extra doses have been urged to come forward, as it means they will be fully protected by Christmas Day.
Health and Social Care Secretary Sajid Javid said: “Getting your Covid-19 booster vaccine is the best way to keep yourself and your loved ones safe this winter and will help reduce the pressure on the NHS.
“While the government is continuing to monitor a wide range of data to ensure the country remains protected, we have very sadly seen a surge in cases in parts of Europe.
“The most important thing we can do to stop a similar rise in this country is get the jab – so please get your vaccines as soon as you can so we can keep the virus at bay.”
New data last week from the first real-world study on the effectiveness of booster vaccines by the UK Health Security Agency showed how important the booster jab is.
Research has shown that top-up jabs can increase protection against symptoms of Covid in older adults over 50.
Two weeks later, it was also shown that protection against symptomatic infections in adults over 50 years old was 93.1 percent in those who had Oxford/AstraZeneca primary treatment and 94.0 percent for Pfizer.
This is because Europe failed to give the AstraZeneca covid jab to the elderly, according to a vaccine boss.
Several countries are facing fresh lockdowns because infections are so high and hospitals are coming under strain due to the influx of admissions.
But the UK, while seeing high Covid case numbers, has not recorded drastic hospital admissions or deaths for months – although there are still hundreds of people becoming seriously unwell every day.
Vaccines have largely broken the link between infections and severe illness that so strongly existed before.
Pascal Soriot, the chief executive officer of AstraZeneca, has suggested this is because the UK used the company’s Covid jab, created by Oxford University, to protect the elderly.
He told the BBC Radio 4 Today programme: “It’s really interesting when you look at the UK – there was a big peak of infections, but not so many hospitalisations relative to Europe.
“In the UK, this vaccine was used to vaccinate older people. Whereas in Europe, people initially thought the vaccine didn’t work in elderly people.”
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