Vaccines being delivered to New Zealand ahead of this year's flu season have been updated to include a new strain, after problems with their effectiveness overseas.
Pharmac is funding a new "quadrivalent" seasonal flu vaccine, to be available by April, that's designed to protect against four strains of influenza, including two "A" and two "B" strains.
There were concerns vaccines had proven ineffective against a certain strain of Type A flu - AH3N2 – which had been prominent in Australia and in the Northern Hemisphere, but was seen to a lesser extent here last season.
Immunisation Advisory Centre director Associate Professor Nikki Turner said the vaccines arriving here next month had been bolstered with an AH3N2 strain.
Their four-strain design was also expected to give better protection than those traditionally used, which only had one B strain in them.
However, vaccine effectiveness was still difficult to accurately predict season-to-season and person-to-person.
Last year was considered a low-activity season, with 50 GP visits for influenza-like illness for every 100,000 people per week.
That compared to the 2013 and 2015, seasons which were moderate seasons with about 80 influenza-like illness cases per 100,000 people per week – and the 2009 influenza pandemic, which saw a peak of more than 250 cases per 100,000 people per week.
While it was known that vaccines would be more effective when they closely matched the strains likely to be circulating, other factors could come into play.
"Older people, infants, and people with a range of underlying chronic conditions do not create such a vigorous immune response, whereas children and healthy adults create a more vigorous response.
"However, those who are most at risk of severe flu and its complications are those who are less likely to mount a strong immune response, so we cannot guarantee that even when they are vaccinated they are well protected."
A further problem was that, because there were many different strains of flu, an individual's history of flu and of vaccination could affect their response to the current vaccine - either to boost immunity or, in some examples, to blunt the immune response.
"At times, therefore, a vaccine may not be effective to a strain or it may not be as effective as expected."
Turner said there were a range of other options to improve protection for people at high risk of flu, including vaccines not yet available in New Zealand.
Other countries were trialling "herd immunity" approaches, where all children were vaccinated to reduce the spread of flu across entire communities.
"Children are very effective spreaders of viruses so even with relatively low rates of vaccination this appears to be quite an effective strategy," Turner said.
"I have been misunderstood recently, particularly in the anti-immunisation social media, as implying this is mandatory vaccination, but it is not.
"Herd immunity does not require everyone to be vaccinated, the vaccinated will protect those around them who are unvaccinated.
"The UK, in particular, is trialling this strategy at the moment with a different sort of vaccine, a live attenuated vaccine delivered by a squirt up the nose."
Even with relatively low rates of immunisation coverage, it appeared to be effectively reducing the spread of flu in the community, Turner said.
"The advantage of this vaccine is that it does not require needles. This type of vaccine is not yet available in New Zealand, or anywhere in the Southern Hemisphere."
Turner added that, alongside vaccination, people could help reduce spread of the flu by practicising good hygiene, staying home when sick, and keeping distance from others.
"The holy grail of flu vaccines is to create a vaccine that will cover all strains and so won't need to be changed every season – the world is working hard on this one but we are still quite a few years away.
"So in the meantime, we are stuck with imperfect vaccines that are still important to use against such a nasty and common virus."