Healthy children aged 12 to 15 should be offered a single dose of the Pfizer vaccine, the Chief Medical Officers of all four UK nations said this week. The jab was already recommended for youngsters with serious underlying conditions, or those living with vulnerable adults.

So what’s behind the decision? How do the risks of Covid compare to the possible side effects from the jab? And what happens if parents and children disagree?

Let’s look at these questions and more.

How do the risks of the vaccine compare to the risk of Covid for kids?

For the “vast majority” of children aged 12 to 15, catching coronavirus leads to very mild symptoms or none at all, according to the independent Joint Committee on Vaccinations and Immunisations (JCVI), which advises the government on who should get the jab.

Nevertheless, the JCVI said earlier this month that for kids in the age group, “the benefits from vaccination are marginally greater than the potential known harms”.

The benefits they looked at were avoiding serious complications from Covid-19. The Committee estimates that if we give a million healthy youngsters a first dose of the jab, we’ll prevent 2.4 admissions to intensive care, 87 hospitalisations and 31 cases of a paediatric inflammatory syndrome linked to coronavirus.

The “potential known harms” from vaccination are mainly in the form of myocarditis – a rare inflammation of the heart muscle that has been linked to the Pfizer jab. (We looked at this in more detail earlier this month).

The JCVI says that for every million kids aged 12 to 15 to get a first dose, we might see between 3 and 17 cases of myocarditis.

The risk of the condition goes up after a second dose to between 12 and 34 cases per million. That’s partly why the Chief Medical Officers’ advice is to only administer a single jab to kids in this age group.

Why is this happening if the JCVI chose not to recommend it?

If you’ve been following the news, you’ll have seen that while they said the benefits outweighed the risk, the JCVI concluded earlier this month that it was not worth offering all kids in this age group a vaccine.

That’s because “substantial uncertainty remains” over the exact rate of side effects from the jab – so it wouldn’t take much to tip the balance away from vaccination, according to the criteria they were examining.

But the JCVI’s decision was based solely on the benefits and risks in terms of avoiding Covid complications versus possible side effects from the jab. As the Committee said, there are many more factors in play, so they suggested the Chief Medical Officers might look at the broader health implications of vaccinating young teens.

And the Officers concluded a major benefit of vaccination for these kids is that it is “likely” to reduce transmission and therefore cut down on school absence.

In the Officers’ view, this is not simply an educational benefit, but a health one too. They wrote in August that they were “confident that multiple sources of evidence show that a lack of schooling increases inequalities, reduces the life chances of children and can exacerbate physical and mental health issues.” They said that “school improves health, learning, socialisation and opportunities throughout the life course including employment” and that home-schooling during the pandemic has not been an adequate substitute, despite efforts from teachers and parents.

With the effect on schooling in the mix, the Chief Medical Officers say there’s an even stronger case in favour of vaccinating 12 to 15-year-olds.

Is this about protecting adults?

Speaking on behalf of his counterparts in Scotland, Wales and Northern Ireland on Monday, England’s Chief Medical Officer, Chris Whitty, was keen to stress that their advice is about the balance of risks and benefits for 12 to 15-year-olds themselves.

Adults and wider society may well benefit from lower infection rates among youngsters (and therefore fewer opportunities for the epidemic to spread and new variants to emerge), but these effects were not part of the Officers’ calculations.

Does the vaccine reduce the chance of Long Covid?

The JCVI’s analysis didn’t consider Long Covid. Dr Liz Whittaker, senior clinical lecturer in paediatric infectious diseases and immunology at Imperial College London, explains why: “although 2-13 per cent of this age group report persistent symptoms 12 weeks after infection, we do not know what causes this, or whether it would be prevented by vaccination.”

On the question of whether jabbing kids might avert Long Covid, Dr Whittaker says “there is an early signal in adults to suggest that vaccination decreases the risk, this is likely related to the decreased risk of infection following vaccination. However, to date, we do not have good enough medical evidence.”

The published advice to ministers from the Chief Medical Officers of the UK doesn’t explicitly mention Long Covid either – though research on the condition, which remains little-understood, was included in a list of papers that formed part of their analysis.

What are other countries doing?

Children over 12 without underlying conditions have been offered jabs in countries across Europe, including France, Spain, and the Netherlands, since the summer. The Chief Medical Officers say they have used data from the United States, Canada and Israel – which have already vaccinated millions of children – as part of their advice this week.

Who decides if a child is jabbed?

This question boils down to a legal concept called “Gillick competence”, named after a legal case from the 1980s. If there’s disagreement between a child and their parent over a medical intervention like taking a vaccine, a registered healthcare professional will take a view on whether the child is capable of understanding the implications of the treatment.

In the first instance, schools and doctors will seek consent from parents to vaccinate a child under 16. We understand that if a child wants the jab and their parent disagrees, schools – who will be administering the majority of vaccines – will bring them together to discuss the pros and cons. If no agreement is reached and the child is understood to be Gillick competent, the vaccination will go ahead.

What if parents disagree between each other?

Consent is only needed from one person with parental responsibility as long as there is no dispute about vaccination with any other parent responsible for the child.

If one parent agrees and the other doesn’t, the immunisation won’t go ahead unless there is a specific court approval that says it’s in the best interests of the child.

But again, if the youngster wants to be vaccinated, is found to be Gillick competent and voluntarily consents, the immunisation will proceed as a parent cannot override the decision of a Gillick competent child.

What will it feel like for children to get vaccinated?

Responding to a question from a FactCheck reader, the Department of Health confirmed to us that the jab will be administered with a needle. It should feel like a short, sharp scratch.

Dr Whittaker says the jab is “well tolerated” but that many teenagers experience reactions: “sore arm, flu-like symptoms for a couple of days”.

Will younger kids be offered the jab?

Responding to this question on Monday, England’s Chief Medical Officer Chris Whitty said that while “there are some nations that are doing this”, there were no current plans to re-examine the question. “It hasn’t even got to the point that it is being considered by MHRA [the UK medicines regulator], so we are a long way from considering this so let’s not rush that one at all,” he said.