This may not be the 'biggest flu season on record', but it is a big one – here are some possible reasons

This year, the number of laboratory-confirmed influenza (flu) virus infections began rising earlier than usual and hit historic highs in some Australian states. If you have been part of any gathering this winter, this is probably not news.

States in the south-east (central and southern Queensland, New South Wales, Victoria, Tasmania and South Australia) are more inflamed by flu than those in the north and west. For example, Queensland has seen more hospital admissions than in the last five years, mostly among an older population, while younger demographics more often test positive without needing hospitalisation.

Meanwhile, flu numbers in New Zealand and elsewhere in the Pacific have not matched the same elevated levels. But is Australia really experiencing the biggest flu season on record in 2017, or are we just testing more and using better tools?

This is hard to answer for certain because the information we need is not usually reported until later and public databases only show the past five years. We can say for sure that 2017 is on track to be a historically big flu year.

Read more: Have you noticed Australia’s flu seasons seem to be getting worse? Here’s why

Really, a big flu season

Flu can be a nasty illness. Sometimes it’s deadly. Other times it can be mild. But even for cases that fall in the middle you may not be able to work for days, or you’ll have to look after ill children home from school, or visit the very sick who have been hospitalised.

Years ago, detection of influenza viruses mostly relied on slow, finicky methods such as testing for virus in artificial cell cultures. But, in Australia today, most laboratories use either sensitive tools to detect viral gene sequences in samples from the patient’s airway, or less sensitive but rapid dipstick methods, where a special strip is placed in a sample to detect viral proteins.

These tools have been in use since 2007 in the larger Australian laboratories, so it’s unlikely we are just seeing more positives in 2017. While newer versions of these tests are being rolled out this year, they are unlikely to detect more cases. Equally, it’s unlikely more people with suspected flu decided to change their behaviour in 2017 and get tested, compared to 2016, or the year before.

As in all years, there are many people in the community with flu who don’t get tested. The proportion of people with flu who are tested likely remains roughly the same year to year.

State-wide flu reports provide reliable, laboratory-confirmed results. By looking at them, we can also be confident that “man flu” and severe common colds aren’t contributing to this specific and large increase in flu. We’re very likely seeing a truly huge flu season.

Why so bad this year?

Flu, caused by infection with an influenza virus, is mostly a disease with an epidemic peak during July and August in non-tropical countries. Flu viruses are broadly grouped into two types: Influenza-A and Influenza-B. Influenza-B viruses have two main sub-types while the Influenza-A viruses are more variable.

The Influenza-As you get each year are usually A/H3N2 (the main player so far this season) or A/H1N1, which lingers on from its 2009 “swine flu” pandemic. Multiple flu viruses circulate each year and serial infections with different strains in the same person in a single season are possible.

H3N2 has played a big role in the past five flu seasons. When it clearly dominates we tend to have bigger flu seasons and see cases affecting the elderly more than the young.

H3N2 is a more changeable beast than the other flu viruses. New variants can even emerge within a season, possibly replacing older variants as the season progresses. This may be happening this winter, driving the bigger-than-normal season, but we won’t know for certain until many more viruses are analysed.

Outside winter, flu viruses still spread among us. This year, in particular, we’re being encouraged to get vaccinated even during the peak of flu season. Vaccines are a safe way to decrease the risk that we or loved ones will get a full-blown case of the flu.

Yet Australian flu vaccination rates are low. Data are scant but vaccination rates have increased in adults and some at-risk groups, but remain lower than for childhood vaccines.

Read more: Disease risk increasing with unvaccinated Australian adults

The flu vaccine

Each season new flu vaccines are designed based on detailed characterisation of the flu viruses circulating in the previous season. But the viruses that end up dominating the next season may change in the meantime.

It is not clear whether that was a factor for this year’s high numbers in Australia this year or precisely what the vaccine uptake has been in 2017. Much of this detail will not be reported until after the epidemic ends. Some testing suggests this year’s vaccine is well matched to the circulating viruses.

The flu vaccine is not the most effective of vaccines, but it is safe and the only preventive option we have for now. Of those vaccinated, 10-60% become immune to flu virus.

Read more: Flu vaccine won’t definitely stop you from getting the flu, but it’s more important than you think

Future flu vaccines promise to account for the ever-changing nature of flu virus, reducing the current need for yearly vaccination. Until they are available, though, it remains really important to book an appointment with your vaccine provider and get a quick, safe vaccination, because we are unarguably in the midst of the biggest flu season Australia has seen in years.

We have both vaccines and drugs to help us prevent and minimise disease and the extra load on hospitals caused by flu. The young, elderly, those with underlying disease and Indigenous Australian people are most at risk of the worst outcomes and this is reflected by government-funded vaccination for these groups.

The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond the academic appointment above.