Cases of potentially fatal whooping cough have shot up by 400 per cent in Herts.
The vast majority of new cases involve very small babies less than two months old, who are most at risk of death from the illness.
So far this year there have been 140 cases of whooping cough in the county, compared to 28 last year and just seven in 2010.
Across England and Wales nine babies under three months old have died from the illness.
In response health chiefs in Herts are offering whooping cough vaccinations to pregnant women, who will then pass on their immunity to their newborns.
Jim McManus, director of public health at NHS Hertfordshire, said: “Over the last year we have seen a large rise in the number of whooping cough cases, the most serious of which are in children too young to be protected by routine vaccinations.
“If you are pregnant, getting vaccinated is the best way to protect your baby.
“Whooping cough is highly contagious and can be a serious illness. The main symptoms are severe coughing fits, which in babies and children are accompanied by the characteristic whoop sound as the child gasps for breath after coughing.
“Newborn babies are particularly vulnerable so it is vital they are protected from the day they are born, which is why we are offering the vaccine to all pregnant women.”
The vaccine can be given from when women are 28 weeks pregnant right up until they are about to deliver their baby.
Even if women were vaccinated as youngsters, they will be encouraged during routine ante-natal appointments to be vaccinated again to boost their immunity, as this helps protect their babies in those early months before they can be immunised.
Health bosses say the cause of the increase in cases is not yet understood but investigations are taking place.