Parents are being told that kids are at risk of catching potentially lethal measles with more than one in 10 children still unvaccinated - the lowest level in a decade.
The UK Health Security Agency (UKHSA) is urging that children have their measles, mumps and rubella (MMR) vaccines as well as other routine jabs before starting school.
Measles is a highly infectious disease which can lead to serious complications such as pneumonia and encephalitis (inflammation of the brain).
On top of this, measles infection damages and suppresses the whole immune system, meaning children can be left much more susceptible to catching other infections.
Then measles can lead to a condition called SSPE (subacute sclerosing panencephalitis), which can cause progressive destruction of the central nervous system, loss of motor control, epilepsy and death.
All children are invited for their first MMR vaccine on the NHS aged one, with the second dose given when they are three years and four months.
The most recent vaccine figures, covering July to September last year, show that just 88.6% of children have had their first MMR dose by the age of two while just 85.5% have had both doses aged five.
Worryingly, it means that more than one in 10 children aged five are not up to date with their two doses of MMR vaccine.
The World Health Organisation (WHO) says 95% of children need to be vaccinated to keep measles away.
In a new campaign, the UKHSA and the NHS are now calling on parents and guardians to get their children vaccinated against these preventable diseases.
Covid has led to a big drop in the number of kids being vaccinated against MMR and other diseases at the right time, said the UKHSA.
As international travel resumes, the UKHSA said it is more likely that measles will be brought in from countries that have higher levels of the disease and so it is important that MMR vaccination rates increase in the UK.
Since the introduction of the measles vaccine in 1968, it is estimated that 20 million measles cases and 4,500 deaths have been prevented in the UK.
Dr Vanessa Saliba, consultant epidemiologist at the UKHSA, said: "The MMR vaccine offers the best protection from measles, mumps and rubella which is why we're calling on parents and carers to make sure their children are up to date with their two doses.
"Even a small drop in vaccine coverage can have a big impact on population immunity levels and lead to outbreaks.
"I would urge parents to check if their children are up to date with their MMR vaccines and if not to get them booked in as soon as they are able. It's never too late to catch-up."
Dr Nikki Kanani, GP and medical director for primary care at NHS England, said: "It is incredibly important that all parents and guardians ensure their child is up to date with their routine vaccinations, including MMR, as these vaccines give children crucial protection against serious and potentially deadly illnesses and stop outbreaks in the community.
"If your child has missed a vaccination, please contact your GP practice to book an appointment as soon as you can to make sure they have maximum protection against disease."
Parents who are unsure if their child is up to date with all their routine vaccinations, should first check their child's Red Book (personal child health record).
GP practices are also able to carry out checks and book vaccine appointments.
As part of the campaign, research commissioned by the Department of Health and Social Care and the UKHSA of 2,000 parents and guardians of children aged five and under found 48% were unaware that measles can lead to serious complications such as pneumonia and brain inflammation.
Only four in 10 (38%) were aware measles can be fatal and 56% were unaware that two doses of the MMR vaccine gives 99% protection against measles and rubella.
