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.
In the first six months of this year there were 1,744 cases - up from 1,412 in the same period last year - although levels remain below those seen in the outbreak.
Dr Mary Ramsay, PHE's head of immunisation, said: "It's important that pregnant women visit their GP surgery or midwife to get vaccinated, ideally between weeks 28 and 32 of their pregnancy.
"Being vaccinated against whooping cough while you're pregnant is a highly effective way to protect your baby in the first few weeks of their life."
Prof John Watson, England's deputy chief medical officer, said whooping cough was "an extremely distressing illness".
He added: "Deaths in infants with whooping cough have reduced significantly since the introduction of the vaccine for pregnant women in 2012 so I encourage all pregnant women to take up the pertussis vaccine when offered."
What is whooping cough?
It is also known as pertussis and is caused by a species of bacteria, Bordetella pertussis
It mostly affects infants, who are at highest risk of complications and even death
The earliest signs are similar to a common cold, then develop into a cough and can even result in pneumonia
Babies may turn blue while coughing due to a lack of oxygen
The cough tends to come in short bursts followed by desperate gasps for air (the whooping noise)
Adults can be infected - but the infection often goes unrecognised
Prof Alan Cameron, from the Royal College of Obstetricians and Gynecologists, said: "We understand some women may have concerns about receiving vaccinations during their pregnancy, but we can provide reassurance that the whooping cough vaccine is safe for use during pregnancy, with no known adverse side effects for mother or baby."