Pregnant women should ensure they have the whooping cough jab to protect their babies, Public Health England is urging.
Cases of whooping cough, which can be deadly, were higher in the first six months of 2015 than in the same period last year.
Yet figures show only half of women are being immunised.
Newborns are highly susceptible to the infection until they start vaccination at two months old.
The vaccine for pregnant women was introduced three years ago in the midst of an outbreak of whooping cough.
The idea is that the mother produces protective antibodies in response to the vaccine, which are then passed on to the child until they themselves are vaccinated.
Public Health England (PHE) said vaccination reduced the risk of a newborn developing whooping cough by 91%.
It said 56.4% of pregnant women were vaccinated in 2014-15. In that time there were seven baby deaths from whooping cough.