The UK’s four chief medical officers (CMOs) have recommended that Covid-19 vaccinations be offered to all children aged 12 to 15. The the Joint Committee on Vaccination and Immunisation (JCVI), that guides vaccination decisions, originally advised that there were insufficient medical grounds to recommend vaccination of healthy 12- to 15-year-olds without other medical conditions – in other words that the margin of benefit was too small to make it worthwhile. But the CMOs were asked by the secretary of state for health and social care to also consider possible benefits for education and mental health – issues beyond the JCVI’s remit. Decisions about vaccinating children have multiple moving parts. We must balance the risks and benefits of vaccination for the young person themselves, while at the same time considering the ethical issues involved in securing an overall increase in the vaccination rate of wider society by vaccinating children – a group who are less likely to experience serious Covid symptoms themselves. The only vaccines being considered for children are the Pfizer and Moderna mRNA vaccines, both of which appear highly effective for this age group, for whom the CMOs have recommended just a single dose. There are about 2.6 million 12- to 15-year-olds in England, and 350,000 have already been offered two doses because they have a significant medical condition leaving them at higher risk of complications from Covid-19. Data suggests that one vaccine shot is around 55% effective in preventing infection; based on this, Department of Health modelling estimates that if 60% of the UK’s 2.2 million currently unvaccinated teenagers were given a single dose, this would prevent around 30,000 infections in this group over the next six months. The main concerns about vaccination in children are side effects, both known and unknown. The key worry is rare heart inflammation (myocarditis or pericarditis), which from US data appears to occur in up to 160 cases per million among 12- to 15-year-old boys and 13 per million in girls, mostly after the second shot. Most cases are mild, but small numbers have ongoing heart rhythm problems, although these are rarely serious. Other unknown side-effects may surface, but are likely to be vanishingly rare given that over 11 million teenagers now have been vaccinated worldwide. My colleagues and I analysed data for the whole of England from the first year of the pandemic that showed just nine deaths among 12-to 15-year-olds, at a time when more than 100,000 adults had died of Covid. While the death of any child is one too many, seven of these deaths occurred in those with other serious medical conditions who are already eligible to be vaccinated. About 150 young teenagers were admitted to intensive care with Covid-related problems in the same year, again most with other conditions. Putting these numbers together suggests that vaccinating English 12- to 15-year-olds not already eligible for a shot could be reasonably expected to prevent two deaths and 30-40 intensive care admissions in a year, but at the cost of up to 170 to 180 cases of myocarditis – all of which are likely to be mild. On a society-wide scale these numbers are very small, though of course devastating for any individuals affected. The CMOs’ decision to give healthy young teenagers only one shot will dramatically reduce the risk of myocarditis, but will also reduce the chance of preventing the – very rare – serious illnesses arising from infection. Vaccination may also reduce post-Covid syndromes. The best estimates suggest between 2% and 14% of children and young people have multiple persistent symptoms after infection, although the great majority recover very quickly. The broader benefits of vaccination for children aged 12 to 15 potentially include less disruption of education, a factor that played an important role in the CMOs’ decision. The disruption of the summer term was very largely due to compulsory isolation of large school bubbles, which has now been replaced with daily antigen testing while in school. But vaccination will reduce the number of children being infected and having to stay off school – the benefit being about 100,000 fewer days of school loss over the next six months. The benefits for broader society largely arise from a potential reduction in transmission. Children bringing infections home from school has been a problem throughout the pandemic. It is difficult to put an exact figure on how many cases may be averted, and our hopes that vaccines would prevent transmission have been dented by the Delta variant. The benefits in terms of averting transmission will be reduced if teenagers receive only one shot of vaccine, although high natural immunity (around 40-60% of teenagers currently have antibodies) should help with this. Medically speaking, vaccinating the children and young teenagers in this group carries low personal benefit and also low risk, with these risks very finely balanced. The CMOs have judged that broader benefits, particularly relating to keeping children in school, have swung the decision towards offering vaccination in order to reduce the harms the pandemic is doing to them. I strongly support this decision, and it is right that it is being made now, when we finally have sufficient safety data to support vaccinating young teenagers. Decisions will come in the future about vaccinating even younger children, which may make this decision look like a walk in the park. Any vaccine is only as good as the total number of people who end up being vaccinated. The UK has a great record so far, but the low benefits for an individual child are very likely to make some parents and young people hesitate. Data on parents is reassuring, with 86% in a recent survey reporting they would have their child vaccinated. However colleagues from the OxWell Student Survey have found that hesitancy may be much higher among younger teenagers themselves, particularly those from poorer families. Experience and the data tell us that vaccine uptake is likely to be lowest in poorer families, who have both suffered most from Covid and whose children have lost the most schooling. Offering vaccines to younger children and teenagers was the right decision, but we must engage directly with them to address concerns and design new ways to reach vaccine hesitant families for the overall program to be a success. Russell Viner is a paediatrician and professor at the UCL Great Ormond Street Institute of Child Health