Fewer babies in Gloucestershire were vaccinated against whooping cough last year than a decade ago, new figures show.
The World Health Organisation says 95% of children should be vaccinated against preventable diseases such as whooping cough.
It comes as cases of the respiratory disease have exploded this year, with the UK Health and Security Authority confirming the number of reported cases in 2024 is more than three times as many as last year.
A leading health expert has warned more babies will die if vaccination rates across the country do not rise.
UKHSA figures show 94.4% of babies in Gloucestershire had received their six-in-one vaccine by their first birthday, which provides immunisation against a range of diseases including whooping cough.
This was down slightly from 94.8% the year before, and 95.8% a decade earlier.
It means Gloucestershire did not reach the 95% vaccination target set by the UKHSA.
Professor Sir Andrew Pollard, consultant paediatrician and chairman of the Joint Committee on Vaccination and Immunisation, told BBC Radio 4’s Today programme under-vaccination is putting "the most vulnerable – those who are too young to have been vaccinated – at greatest risk".
He said the "only thing we can actually do" about rising cases is to ensure higher vaccination rates.
He warned: "The troubling thing is that if we continue to have high rates of spread and low rates of vaccination, there will be more babies severely affected and sadly there will be more deaths."
The UKHSA confirmed five babies in England died after being diagnosed with whooping cough in the three months to March.
Meanwhile, in the year to April 21, GPs nationally reported 9,575 suspected cases of whooping cough to the UKHSA.
This included 44 in Gloucestershire.
Not all these cases will be confirmed as whooping cough. The UKHSA, which does not release local data, said there were 2,793 confirmed cases in England in the three months to March.
That compares to just 858 cases for the whole of 2023, while in March alone, some 1,319 cases were reported, according to the provisional data.