Vaccinating teenagers is beneficial, even if their vulnerability to Covid-19 is low

The UK government has announced that all 16 to 17-year-olds will be offered a first dose of the Pfizer Covid-19 vaccine in the coming weeks. Many countries are already vaccinating children over 12, so this decision is not out of step with what is going on elsewhere. Despite this, the reaction in Britain is likely to be mixed.

If it was just a question of: "Does this vaccine work in this age group?", then the answer is easy. The Pfizer vaccine's phase 3 trial showed that it was safe and effective in people over 16.

But the question of whether it's right to give the vaccine to 16- to 17-year-olds is more complicated. The benefits of vaccinating someone against Covid-19 generally fall as the people being vaccinated get younger, and all vaccines carry some risk, even if almost always very, very small. And you also need to take into account the wider impact that vaccinating young people will have on the rest of British society and the world.

It's been well documented that for most young people, Covid-19 is a mild or asymptomatic infection. It can't be argued that vaccination will significantly reduce severity of disease, hospitalisations or deaths among young people. But there are other benefits.

In a very small number of children, Covid-19 leads to a more serious condition called paediatric multisystem inflammatory syndrome (PIMS), where inflammation appears throughout the body. PIMS tends to affect younger children, but cases have also been recorded in teens and young adults. By lowering the risk of infection, vaccination may prevent it.

There's also the risk of long Covid, a much more common issue. Research suggests just under 2% of children who catch the coronavirus have symptoms that last for more than eight weeks. Long Covid is poorly understood and for some is a prolonged, debilitating illness. We currently have no treatment options for it and don't know how long it lasts. It also isn't limited to people who develop severe illness when infected – so using vaccines to prevent mild disease may reduce the number of long Covid cases.

We also know that the immune response following vaccination is significantly higher than following natural infection. Therefore, although a proportion of 16 to 17-year-olds will have already been exposed to Covid-19 and have some immunity, taking a vaccine will still offer the best protection.

These benefits need to be balanced against the risk of side-effects. For the Pfizer vaccine, these are thought to be similar across all age groups. Mild side-effects – including pain at the injection site and chills – are common, resolve quickly and are well tolerated. There are, however, concerns about more serious side-effects such as a severe allergic reaction (anaphylaxis) and inflammation of the heart (myocarditis or pericarditis).

But these are very rare – so much so that the UK government hasn't been able to reliably estimate how frequently they occur. But because of this, and because the risk of severe illness stemming from Covid-19 in young people is so low, the Joint Committee on Vaccination and Immunisation hasn't stated outright that the benefits of vaccinating 16 to 17-year-olds clearly outweigh the risks – only that getting the balance between these benefits and risks is important.

But the wider impact of vaccinating young people strengthens the case. Young people have already suffered significant disruption to their academic and social development, and suppressing the spread of the virus among them should mean that fewer have to isolate and miss school. It will also lessen any surge in cases when schools reopen after the summer holidays.

Lowering infections among children should also mean fewer parents and carers have to take time off work. Under the current guidance, if exposed to a positive case at home they still have to isolate, even if vaccinated.

Many young people will also have a close relative or contact who is clinically extremely vulnerable. It's possible that such people will have responded less well to their Covid-19 vaccination and so will be choosing to continue to shield. Vaccinating their family and friends helps reduce the chance of them catching the virus.

This idea of vaccinating young people to prevent infection in others isn't new. Giving the pneumococcal (pneumonia) vaccine to infants to prevent severe infection in early life has had a hugely beneficial effect in preventing community-acquired pneumonia in older adults. Vaccinating a further 1.4 million people in the UK population for Covid-19 is likely to further reduce transmission of the virus to those more vulnerable to severe disease.

We also know that random mutations occur when the coronavirus infects people and reproduces, and that the more of these mutations that occur, the more likely it is that viral variants will arise that can escape the effects of vaccines. Anything that reduces case numbers reduces this risk.

Despite all of the above, the ethics of offering vaccines to those unlikely to become very unwell will be debated, as rates of vaccination in many parts of the world are low. Many highly vulnerable people remain unprotected.

Recognising this, the World Health Organization has called for wealthier nations not to give fully vaccinated people booster doses until there has been a more equitable distribution of vaccines to the global population. A similar argument could be made against vaccinating younger people.

The challenge of balancing individuals' and societies' best interests in the context of a pandemic is challenging. We remain in an evolving situation and there is still so much we don't know about the behaviour and threat of the Covid-19 virus. It's likely that only history will be able to judge whether the UK vaccination strategy was the right one. But in the meantime, we may see vaccines being offered universally to younger age groups in the UK in due course.