Two countries, two different approaches to protecting children from chickenpox. While the UK prepares to introduce a combined vaccine covering measles, mumps, rubella and chickenpox (MMRV) in a single jab, the US is moving in the opposite direction – restricting parents’ ability to choose that same combination for their youngest children.

Just as the US has just celebrated 30 years of chickenpox vaccination, advisers to its health secretary, Robert F. Kennedy Jr, voted against the use of the MMRV vaccine for children under four years old. Meanwhile, from January 2026, the UK will offer children their first dose of the combined MMRV vaccine at 12 months and a second dose at 18 months old.

This divergence reflects more than just different medical opinions; it highlights how the political climate can shape health policy. In June 2025, RFK Jr dismissed all members of the US Advisory Committee on Immunization Practices and replaced them with advisers who have made a series of recommendations restricting vaccination in the US.

Currently, children in the US receive two doses of measles, mumps, rubella and chickenpox (varicella) vaccines – the first at 12-15 months, and the second at four to six years old. For the first dose, separate chickenpox and MMR vaccines are advised unless parents prefer the combined shot. The new restrictions would eliminate that parental choice.

The US panel’s concern is about febrile seizures – fits caused by high temperatures that can occur in young children after vaccination. Chickenpox (also known as varicella) vaccine can be delivered as a standalone vaccine, or combined with measles, mumps and rubella in a single shot. The combined MMRV vaccine slightly increases the risk of a febrile seizure in the seven to ten days after vaccination, compared with giving separate vaccines.

But this risk needs context. For every 2,300 children who receive a first dose of MMRV, there might be one extra febrile seizure, compared with separate vaccines. There’s no extra risk for the second dose.

Febrile seizures happen in around 2% of children before age five – regardless of vaccines. During a febrile seizure, a child may become stiff, twitch or shake, and be unresponsive. These are frightening to watch but usually harmless, typically resolving within a few minutes without treatment.

This small risk must be weighed against chickenpox’s considerable harms. Though commonly seen as a mild childhood illness, chickenpox causes significant disruption.

A Bristol study found that even mild cases reduce quality of life, disrupting sleep and causing tiredness and pain. More seriously, chickenpox can lead to severe complications including bacterial skin infections, pneumonia, brain inflammation, sepsis, stroke and death.

In England, there are around 4,500 hospital admissions a year due to chickenpox – and this may be an underestimate, as it relies on chickenpox being identified and recorded as the underlying cause.

People with weakened immune systems face particularly high risks.

Chickenpox in pregnancy affects around three in 1,000 pregnancies. In the first trimester, there is a rare but serious risk of a condition called foetal varicella syndrome, which affects the development of the foetus. Pregnant women are at increased risk of severe pneumonia, and foetuses and newborns are also at risk of severe chickenpox infection. While rare, it can be fatal.

Shingles

The virus also creates long-term problems. After infection, it lies dormant and can reactivate as painful shingles later in life.

Countries with chickenpox vaccination programmes have seen dramatic improvements. The US programme has prevented 91 million cases of chickenpox, 238,000 hospitalisations and almost 2,000 deaths.

Hospital admissions have fallen for all age groups in countries such as the US, Australia, Canada and Germany. Beyond health benefits, vaccination saves time off school and work – every dollar spent on chickenpox vaccination in the US saves US$1.70 (£1.26), creating a net saving of US$23.4 billion over 25 years.

You might wonder why the UK hasn’t vaccinated sooner. There was concern that natural chickenpox virus circulating in the population helped boost adult immunity against shingles. Thirty years of US data suggest this belief was unfounded.

An adult with a shingles rash.
Shingles is a painful condition caused by the reactivation of the chickenpox virus in later life. Suriyawut Suriya/Shutterstock.com

Vaccinating children has not been seen to increase shingles in older adults. We also now have effective shingles vaccines for older people.

So why is the UK choosing the combined MMRV approach that the US is restricting? The combined vaccine works just as well as separate shots and offers practical advantages. Babies receive fewer injections.

Surveys in the UK suggest around 85% of parents would accept a chickenpox vaccine and may prefer a combined MMRV vaccine to multiple jabs. Fewer needles and appointments can encourage vaccine uptake. While UK vaccine coverage is high, delays can stack up with each additional appointment.

Vaccine uptake matters enormously. Vaccines protect not just the child who receives them, but everyone in the community by reducing disease circulation – known as “herd immunity”. This is key to eliminating diseases such as measles, but only works with high vaccination rates. The US changes restrict parental choice about using the MMRV vaccine, and risk undermining the public trust that successful vaccination programmes depend on.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Helen McDonald, University of Bath

Read more:

Helen McDonald has previously received research funding from the NIHR (National Institute for Health Research) Health Protection Research Unit in Immunisation. She is a member of the varicella/zoster subcommittee for the Joint Committee on Vaccination and Immunisation. She is writing in a personal capacity, and views are her own.