A spike in emergency admissions for influenza has signalled that the flu season is gathering pace, with nearly 2000 confirmed cases last week in NSW.
This year's predominant strain - influenza A (H3N2) - is particularly menacing to the very young and the elderly, with 22 aged care centre outbreaks last week alone.
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What goes into the flu vaccine?
This year's National Immunisation Program flu jab protects against the four strains of flu predicted to circulate in Australia over the winter months.
Authorities warned people with flu-like symptoms to stay away from aged care centres, where it is difficult to control because the vaccination is not as effective.
There have been 79 outbreaks in residential aged care facilities this year, affecting 942 residents and staff, and causing 45 deaths.
![Type A influenza is covered by the 2016 seasonal vaccine.](/web/20160819191845im_/http://www.smh.com.au/content/dam/images/g/o/6/w/b/s/image.related.articleLeadwide.620x349.gqtiu5.png/1471321558861.png)
NSW Health's Vicky Sheppeard said this was double the number of outbreaks compared to last year, when the bulk of cases were caused by type B influenza.
"Last year when we had type B and other years when we've had H1N1 the elderly haven't been as susceptible," Dr Sheppeard said.
"It's interesting when swine flu came out in 2009 we hardly had any outbreaks in elderly people and it's thought that because it was quite similar to the 1918 strain that they had developed immunity early in life and maintained that."
The latest influenza report shows 2341 people cases of influenza were confirmed across NSW last week, causing 141 presentations to the emergency department.
This was an increase on the previous week, but within the usual range for this time of year.
Although emergency departments start to see an increase in flu admissions at the beginning of winter, it usually snowballs as more people become infected and peaks towards the end of winter and beginning of spring.
"It's very similar to 2015 and we're saying on that basis we're nearing the peak of the season, so this is really the time when there's most infections around and people are most likely to catch flu," Dr Sheppeard said.
Influenza A is covered by the 2016 seasonal influenza vaccine.
Pregnant women are especially advised to get the vaccine, which will also increase the immunity of their babies in their first six months of life.
The latest influenza surveillance report shows 2016 has been a quieter year for pneumonia than the last five years and there have been fewer admissions to the critical care wards for people affected by flu-like illness.
Northern Sydney bore the brunt of the spike last week, with 319 notifications, but western Sydney has had the worst month, with 18.46 people infected per 100,000 of the population, followed by northern Sydney and the Nepean Blue Mountains.
Western NSW and the Central Coast have had the fewest cases.
Western Sydney manager of communicable diseases Shopna Bag said the region was broadly comparable to other local health districts in terms of weekly notifications.
But western Sydney could be recording more infections because more people were getting tested, following public awareness campaigns around vaccination and infection.
"We have seen flu notifications increasing in the last couple of years," Dr Bag said.
"We do know that there are practices that can vary between regions as well as between years. "We're asking people to go and get tested and if you do have the flu, stay home."