On 1 September, millions of children and pregnant women across England became eligible for their flu vaccination as the NHS began the first phase of this year’s winter immunisation programme. On its own, that is a routine seasonal story. What made it worth a closer look was what sat behind the jab itself: last winter’s flu season arrived early and was driven by a fast moving new variant, and this year’s vaccine was reformulated specifically in response.

What Opened on 1 September

From 1 September 2026, all children aged two and three on 31 August, every pupil from Reception through to Year 11, and children aged six months to 18 years in clinical risk groups became eligible for a free flu vaccine. Most children received it as a nasal spray rather than an injection, a live but weakened form of the virus that cannot cause flu itself. Pregnant women became eligible on the same date, at any stage of pregnancy, and could book through their GP practice or midwife, receiving the vaccine as an injection.

A second wave of eligibility opens on 1 October, bringing in everyone aged 65 and over, adults aged 18 to 64 in clinical risk groups, care home residents, unpaid carers receiving carer’s allowance, close contacts of people who are immunocompromised, people experiencing homelessness and frontline social care workers without access to an occupational health scheme. The National Booking Service, which lets people book appointments directly, opened on 17 August this year, earlier than in previous seasons.

Dr Amanda Doyle, NHS England’s National Director for Primary Care and Community Services, described flu as something that “can really knock you for six,” a reminder that for most healthy adults it is unpleasant rather than dangerous, but for young children and pregnant women it carries a meaningfully higher risk of hospital admission. Health and Social Care Secretary Yvette Cooper framed the programme in more system-level terms, calling vaccination “crucial for relieving NHS pressures over winter.” Last winter, close to 19 million people in England had a flu jab, and the NHS estimates that vaccination avoided around 104,000 hospital admissions.

A Season That Arrived Early

The reason this year’s programme got more attention than usual traced back to a variant that emerged in the summer of 2025. Classified as J.2.4.1 and known informally as subclade K, this version of the H3N2 influenza virus carried a cluster of mutations in the haemagglutinin protein, the molecule the immune system primarily uses to recognise the virus. In England, subclade K went from under one per cent of sampled flu cases in July 2025 to around 94 per cent by mid-December, effectively replacing the older H3N2 lineages within a matter of months.

Two things followed. First, the 2025/26 flu season started unusually early, with growth peaking around week 40, in late September and early October, compared with a typical peak between weeks 47 and 51. Second, because subclade K had only been identified after that season’s vaccine strains were already selected, there were early concerns that the vaccine would offer poor protection against it.

The reality was more reassuring than the initial alarm suggested. A modelling study of the 2025/26 season in England, published in Nature’s Communications Health, found that while the season began early, its overall growth rate was comparable to previous years rather than unusually explosive, and total case numbers ended up lower than the 2022/23 season used as a benchmark. Vaccine effectiveness data from the UK put protection against subclade K at 32 to 39 per cent in adults, which researchers described as within the normal range for the H3N2 component of the vaccine in a typical year, and notably higher in children, at 72 to 75 per cent.

Why the Vaccine Was Rebuilt

Flu vaccines are reformulated most years because the virus mutates quickly enough that last year’s strain selection can become outdated. The World Health Organization reviews global surveillance data twice a year and issues recommendations for which strains manufacturers should target in the coming season. In its recommendations for the 2026/27 northern hemisphere season, published in February 2026, the WHO specifically cited the emergence of subclade K as the reason for updating the H3N2 component, recommending an A/Darwin/1454/2025-like virus for egg-based vaccines and an A/Darwin/1415/2025-like virus for cell culture, recombinant and nucleic acid based vaccines.

This was the version of the vaccine offered to children and pregnant women in England from 1 September. It was not a new type of vaccine or a novel technology, but an update to the existing seasonal formula, chosen specifically because the strain circulating most widely no longer closely resembled the one the previous vaccine had been built around.

Why Children and Pregnant Women Come First

Prioritising these two groups is not simply about their own risk, although that risk is real. Flu-related hospital admissions among children are higher than in most other age groups outside the very elderly, and pregnancy brings changes to the immune system, heart and lungs that make severe flu illness more likely. Babies cannot be vaccinated themselves until they are six months old, so the only way to give a newborn any protection against flu in its first weeks of life is for the mother to be vaccinated during pregnancy, allowing antibodies to cross the placenta before birth. As Dr Alison Wright, President of the Royal College of Obstetricians and Gynaecologists, put it as flu cases climbed last December, “the flu vaccine during pregnancy is the most effective way to protect women and their babies from getting seriously unwell with flu.”

Children also matter because of how effective the vaccine is in that age group. The 72 to 75 per cent effectiveness recorded against subclade K in children is substantially higher than the 32 to 39 per cent seen in adults, meaning the same dose delivers considerably more individual protection when given to a child. Combined with how frequently school-aged children mix with each other and with their households, vaccinating this group early in the season is one of the more efficient uses of a limited winter vaccination window.

What Happens Next

Opening eligibility is the easy part. This year’s programme sets uptake targets of 46 per cent for two and three year olds, 56 per cent for primary school-aged children and 47 per cent for secondary school-aged children, figures that reflect how much of the eligible population typically does not end up vaccinated even once the offer is available. Whether this year’s early booking window and the reformulated vaccine translate into meaningfully higher uptake, rather than simply better protection for those who were always going to come forward, will not be clear until the season is well underway. The vaccine itself was updated to meet the virus as it currently stands. Getting it into arms, or noses, remains the harder half of the job.

Sources: NHS England; UK Health Security Agency; World Health Organization; Communications Health (Nature); STAT News; Royal College of Obstetricians and Gynaecologists.