Over 45 million Brits have already received two doses of a Covid-19 vaccine. Now the government is offering a third “booster” shot to at-risk groups.

So who is eligible, how can you get the jab, and will this be enough to stop the government bringing in tougher restrictions?

How does it work?

Booster shots are being offered to people over 50, those with underlying health conditions and health and social care workers.

If you’re in one of those groups, the NHS will invite you for a top-up jab six months after you had your second dose.

Some 3.8 million people in England have had their booster shot since the scheme began on 16 September.

If you’ve not yet had your first or second dose, the government advises “you should have them as soon as possible”. You’ll be offered a third dose in due course.

Those getting boosters will be offered either the Pfizer or the Moderna vaccines, which are mRNA jabs. Common side effects from these include tenderness in the arm, feeling tired, headaches and “mild flu like symptoms”.

In extremely rare cases, mRNA vaccines have been linked to heart inflammation. Though as FactCheck has reported before, the best available evidence shows that Covid-19 itself is a much greater risk factor for this condition, plus a host of others including stroke, kidney injury and blood clots.

Why is this happening?

“There is evidence from real-world data in England of waning of vaccine effectiveness against symptomatic disease from approximately 10 weeks after second dose.” That’s what the scientists on the government’s SAGE committee wrote in early September.

They stressed that “protection against hospitalisation and death remains high up to at least 20 weeks in healthy adults”, but said that “indications of waning with respect to hospitalisation and death appear to be predominantly in higher risk groups”.

That’s why older adults and those with underlying conditions are being offered the booster.

The government says: “Protection against severe disease from the first 2 doses seems to decline very slowly. So don’t worry if your booster vaccine is given a few weeks after the 6 months time-point. The booster dose should help to extend your protection into the next year.”

Speaking on Radio 4 yesterday, Professor Neil Ferguson of Imperial College London said there was “good data coming through from Israel that shows that if you’ve had the third booster dose of the vaccine then you get very high levels of immunity, better even than you had after the second dose”.

For this reason, he said, “I do think it’s critical we accelerate the booster programme”.

Will this prevent tougher restrictions?

Booster shots for the vulnerable are a key plank of the government’s “Plan A” for the winter. But, it says, “if the data suggests the NHS is likely to come under unsustainable pressure, the Government has prepared a Plan B for England”.

That Plan B involves mandatory face coverings in some public places, as well as “vaccine-only COVID-status certification in certain settings”.

The government says it “hopes not to have to implement” this contingency strategy, but acknowledges that “it is possible that Plan A is not sufficient to prevent unsustainable pressure on the NHS and that further measures are required.”

Lawrence Young, professor of molecular oncology at the University of Warwick, told the Science Media Centre today that “we must do everything to encourage those eligible to get their booster jabs and to vaccinate healthy 12 to 15 year olds” but that “we can’t rely on vaccination alone to protect us all over the winter and to prevent the NHS being overwhelmed.”

Professor Young says that “With the rising number of hospitalisations, Plan B measures […] are looking increasingly likely.”

His comments echo those of Professor Azeem Majeed of Imperial College London, who told Channel 4 News this week that the government should push on with vaccination but that “it’s very important to get case numbers down using other measures as well, such as face masks and being careful with large indoor gatherings”.

The business secretary Kwasi Kwarteng told Times Radio this morning that “there is the risk of greater infections but the critical thing here is looking at the hospitalisation and sadly death rates, and those, compared to where we were even just in January, are much, much lower.”

Asked on Sky News whether the government would consider an unofficial “Plan C” involving a return to lockdown, Mr Kwarteng said he “would rule that out”.