Young people with asthma or diabetes who do not get a flu jab this winter could be putting themselves at risk, the Department of Health has said.
Last year, 58% of people in at-risk groups under the age of 65 did not receive an inoculation, leaving more than two million vulnerable to the effects of the virus.
Influenza contributes to more than 20,000 deaths every winter, with thousands more people suffering due to complications from the virus.
Free flu jabs are available for the over-65s, patients with serious heart or respiratory conditions, liver or kidney disease, diabetes, multiple sclerosis, conditions of the nervous system, and those who have had a stroke.
Chief Medical Officer Liam Donaldson there is a "common misperception" that only older people suffer badly from the flu.
He said: "Many children and adults under the age of 65 are putting themselves at just as much risk to the effects of the virus.
"If you suffer from illnesses such as asthma, diabetes or serious heart or kidney disease, or you are 65 years or older, you are particularly at risk from flu.
"For those most at risk, flu can lead to more serious illnesses, including bronchitis or pneumonia, and could result in hospitalisation or, in the most serious cases, death.
"People and children at risk from the effects of flu need to get a flu jab every year."