As many as four in five frontline medical staff working in England's hospitals are increasing the risk of being struck down with winter flu after failing to get immunised, health bosses have warned.

In an open letter to staff across the country, senior figures within NHS England, Public Health England (PHE) and the Department of Health have urged the take-up of a winter vaccination against a backdrop of rising numbers of patients with flu and respiratory illnesses.

Chief Medical Officer Dame Sally Davies, NHS National Medical Director Sir Bruce Keogh, Chief Nursing Officer Professor Jane Cummings and PHE Director for Health Protection Professor Paul Cosford said "substantial levels of seasonal influenza" have placed "further strain" on care facilities, with admissions to hospital and intensive care increasing.

Provisional data showed 59.3% of frontline healthcare workers were vaccinated by the end of November, up from 55.6% the previous year.

But while that figure was up to 80% in some hospital trusts, it dropped to 20% in others - putting potentially vulnerable staff at risk of sickness at a time when A&E waiting time performance levels are at a record-equalling low.

The letter warns: "Although flu can produce severe symptoms which prevent the individual from working as normal, the range of illness is very broad with perhaps 30% of infections being asymptomatic and a similar proportion with only mild respiratory symptoms.

"Such individuals, with mild or no symptoms, can still pass on the virus to vulnerable people they come into contact with. This is why vaccination of healthcare workers is a critical part of our flu prevention strategy and helps to ensure the well-being of our most vulnerable patients.

"Flu-related staff sickness absence can significantly affect the ability to deliver services safely and effectively."

What are the symptoms of the flu?

People with flu normally feel very unwell for two or three days, and will continue to experience symptoms for around another five days. After that, you may feel tired and run down for a further two or three weeks.

The most common symptoms of flu are:

• a sudden fever (above 38°C or 100.4°F)
• headache
• chills
• runny or stuffy nose
• tiredness
• aching muscles
• dry cough
• sore throat

Symptoms of a cold?

Symptoms of a cold can include:

• runny or blocked nose
• sneezing
• sore throat
• cough

People suffering from a cold may also have a mild fever, which can make a cold easy to confuse with flu.

Flu symptoms usually develop very quickly, whereas the symptoms of a cold usually develop over one or two days.

How do they both develop?

A cold develops gradually over one or two days and you're most contagious during the early stages when you have a runny nose and sore throat.

You should begin to feel better after a few days but some colds can last up to two weeks.

Flu usually comes on much more quickly than a cold, and symptoms appear one to three days after infection.

You should begin to feel better within a week or so, but you may feel tired for much longer.

Who is at risk?

You may be more at risk of serious complications, such as pneumoniaand bronchitis if you:

• are over 65
• have serious heart or chest complaints, including asthma
• have serious kidney disease or liver disease
• have diabetes
• have lowered immunity
• had a stroke or transient ischaemic attack (TIA)

How are they treated?

You can treat the symptoms of flu by taking:

• paracetamol to lower your fever
• ibuprofen for muscle aches
• cough syrup if you have a cough
• a decongestant if you have a blocked nose

Resting and taking care of yourself are usually enough to cure a cold. You should:

• drink plenty of fluids
• rest your body
• eat healthily

You can treat the symptoms of a cold to help you feel better, but this will not make you recover sooner.

You could:

• take cough syrup or throat lozenges
• take painkillers such as ibuprofen, paracetamol or aspirin
• gargle salt water, which may help ease a sore throat and nasal congestion
• take decongestants to help with a blocked nose. These can either be taken orally or as a spray in your nose