Nearly a third of over-65s in Northamptonshire are not protected against the flu.
A report by NHS England says nearly 40,000 over-65s in the county did not claim their free winter flu jab by the end of 2016.
NHS England is urging the people of the county to make sure older friends and relatives claim their jab and stay protected against flu this winter.
Dr Tim Davies, screening and immunisation lead for NHS England for Northamptonshire, said: "It might feel like the end of winter isn’t too far away, but it’s certainly not too late for people over the age of 65 to protect themselves against flu by claiming their free jab."
Everyone over the age of 65 is entitled to a free flu vaccination from their GP or a participating local pharmacy.
But out of 115,378 registered patients aged over 65 by NHS Nene, only 78,873 have claimed their jab.
Meanwhile, out of 10,760 patients under NHS Corby, only 7,290 have had their jab.
Dr Davies said: “Older people are at an increased risk of serious illness or even death if they contract flu, and yet they can be protected through a quick and easy jab that costs them nothing.
“We’re pleased that a great many over-65s have already received their free flu vaccinations for this winter, but more can be done to ensure that the most at-risk groups in our community are protected.
“People might think they are unlikely to catch flu but it’s just not worth the risk, and the complications caused by flu can be very serious for even the healthiest people. What’s more, by getting vaccinated yourself you are also helping to protect those around you.
“For the flu vaccination to be effective, you need to be vaccinated every year because the virus can change, and the previous year’s vaccine may not offer the right protection against the virus in circulation this year.
“I would urge anyone with an elderly friend, relative or neighbour to check that they’ve had their jab this winter – and encourage them to contact their GP or local pharmacy if they haven’t.”
As well as over-65s, people with long-term health problems, pregnant women and carers who receive carers allowance are also entitled to a free flu jab. Children aged two, three and four are also eligible for a free flu vaccination as a painless nasal spray, while primary school children in years 1, 2, and 3 receive nasal spray vaccinations at school.