Uptake of the whooping cough vaccine in pregnant mothers has been "extraordinarily good" in England, the Department of Health says.
The jab was introduced UK-wide in the autumn as an emergency measure to protect newborn babies during the worst outbreak of the disease in 20 years.
There have been nearly 10,000 UK cases this year and 13 babies have died.
A month after the scheme was announced, 44% of eligible women in England had had the jab.
The director of immunisation in England, Prof David Salisbury, told the BBC: "It really is an extraordinarily good result.
"October's the first month we've got figures and to go straight in at 40% is fantastic."
He said the figures were better than for flu vaccine uptake in pregnant women, but he still wanted more women to take up the offer.
Whooping cough in adulthood can be grim, but is rarely life-threatening. The vaccine was introduced to protect newborn babies as they are the most vulnerable to the infection as they will not begin their childhood vaccines until two months of age.
The infection can stop a baby breathing or lead to pneumonia, brain damage, weight loss and death.
Pregnant women, between 28 and 38 weeks, are offered the jab and in October, about 18,000 women were vaccinated.
It prompts the mother's immune system to create more antibodies to attack the whooping cough bacterium. The antibodies should pass from the mother to the child in the womb and offer protection when a baby is born.
'Not bad at all'
It is still too early to fully assess the success of the scheme. Early data shows that in August, there were 72 infections in children under the age of one. That fell to 67 when the vaccination programme started in mid-September and fell again to 46 in October.
Prof Salisbury said the knowledge that whooping cough could be fatal had been a "strong motivating force" for expecting mothers.
Dr David Elliman, from the Royal College of Paediatrics and Child Health, said the figures were "not bad at all" considering how quickly the vaccination programme was introduced.
He added: "Obviously you want to aim much higher, but I would expect it to go up.
"There has been quite a lot of publicity about deaths in babies, which would have tipped it [in favour of having the jab] for many pregnant mothers."
There are surges in whooping cough cases every three to four years. However, the current outbreak has affected nearly 10 times as many people as the previous outbreak in 2008.
Equivalent vaccine uptake figures for Scotland, Wales and Northern Ireland are not yet available.