The state's governor says it is the worst flu season in years after nearly 20,000

A sign in a New York pharmacy warns of the outbreak

A public health emergency has been declared in New York after nearly 20,000 flu cases have been reported this season.

Officials are tackling the worst influenza outbreak in several years and an immunisation programme has been expanded to reduce the spread of the illness.

Pharmacists are not usually authorised to vaccinate children under 18 but New York Governor Andrew Cuomo has suspended this order to increase the number of people being protected.

The 20,000 reported cases are more than four times the 4,400 positive laboratory tests reported for the whole of the previous season.

"We are experiencing the worst flu season since at least 2009, and influenza activity in New York state is widespread, with cases reported in all 57 counties and all five boroughs of New York City," Mr Cuomo said in a statement.

"Therefore, I have directed my administration, the State Health Department and others to marshall all needed resources to address this public health emergency."

The flu season in the US got under way a month early, in December, driven by a strain that tends to make people sicker. That led to worries that it might be a bad season, following one of the mildest flu seasons in recent memory.

The latest numbers do show that the flu surpassed an "epidemic" threshold last week. That is based on deaths from pneumonia and influenza in 122 US cities.

The government doesn't keep a running tally of adult deaths from the flu, but estimates that it kills about 24,000 people in an average year. Nationally, 20 children have died from the flu this season.

Flu usually peaks in mid-winter. Symptoms can include fever, cough, runny nose, head and body aches and fatigue. Some people also suffer vomiting and diarrhoea, and some develop pneumonia or other severe complications.