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Influenza – or flu – is caused by the flu virus. It often occurs quickly, with high fever, chills, muscle aches, fatigue and a dry cough. These symptoms worsen during the first days.
Most people will get better without medical care, but some people are at higher risk of dangerous complications. This includes pregnant women, children, people over 65, people with chronic diseases, and Aboriginal and Torres Strait Islander peoples.
According to press reports, Australia is about to experience a particularly bad flu season. But it is too early to say if that is the case – and it is impossible to predict. Here is what we know so far.
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How was the flu season last year?
The World Health Organization (WHO) classifies the severity of influenza seasons according to the speed with which circulating viruses spread, the severity of the disease – which can be measured by the number of hospitalizations or death – and the impact of the disease, for example: pressure on hospitals.
Based on these measurements, the 2018 influenza season was relatively mild. The season started late, relatively few cases were seen in clinics and general hospitals, they had a low impact on workplaces and hospitals and caused an average level of illness.
This contrasts with 2017, which had an early start to the season, a high activity in the eastern United States, a significant impact on GPs and hospitals, a high absentee rate and at least 1 255 deaths.
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It is too early to evaluate the 2019 season – this is usually done after the end of the influenza season. But general practitioners see more influenza-like illnesses for this time of year compared to previous ones.
Hospital surveillance usually does not start before the end of April, so it is difficult to assess the activity and the severity of it, but the number of presentations in the emergency departments of the Northern Territory and New South Wales has been bred.
Why is it so difficult to predict?
Part of the problem with the influenza season forecast is that we are talking about a season, but four separate influenza viruses cause a clinically important disease in humans.
These viruses are classified into two types of influenza: A and B. Influenza A is subdivided into two subtypes: H1N1pdm09 and H3N2. Influenza B is divided into two lineages: B / Victoria and B / Yamagata.
All four viruses are covered by the four-strain influenza vaccine provided to Australians under 65 years of age.
For adults 65 years and older, a strengthened vaccine containing both influenza A subtypes and a B virus is available. This year, strain B is a B / Yamagata lineage virus because in 2018 we saw a lot more B / Yamagata than B. / Victoria virus.
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As influenza viruses are constantly changing, any immunity acquired during an infection or an earlier vaccine may provide little protection against the viruses that will circulate during the next season.
There is also little cross-talk. An A (H1N1) pdm09 infection, for example, may not protect you from A (H3N2). It is even possible, though unlikely, to be infected with two viruses simultaneously or as a result.
The four influenza viruses rarely circulate with equal frequency during the winter months. As a rule, one of the influenza A viruses will dominate.
In Australia, it is rare that influenza B dominates. It is even more rare that the two lines circulate at the same time, but it happened in 2015.
The burden of each of these viruses also varies. Children may be more susceptible to infections with influenza B than adults, while older adults are relatively less likely to be infected with A (H1N1) pdm09 but are more susceptible to A (H3N2) infections ).
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Although deaths have been associated with the four viruses, the A (H3N2) virus usually causes more deaths than the others, especially among the elderly.
Unfortunately, our disease surveillance systems rarely collect information about influenza viruses that patients present in clinics and hospitals. This is because the tests for influenza can be expensive and knowing the strain – or if it is the flu – would not change the course of treatment.
Instead, we learn which virus has dominated and its likely impact. While this is useful for monitoring global trends, it is not enough to study specific patterns of virus circulation that may allow us to better predict the seasons.
What is happening elsewhere in the world?
Influenza viruses are mutating fairly quickly and circulating fairly efficiently around the world, including through air transport.
That said, the circulation of influenza viruses in Europe does not necessarily help us predict the viruses that will circulate in Australia, because there is no consistent pattern of one hemisphere dominating the other in terms of viral circulation.
Even in the hemispheres, variations occur. During the winter of 2017-2018, influenza B viruses dominated Europe, while A (H3N2) viruses dominated in North America.
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In tropical and subtropical regions, where there is no "winter", there may be more than one influenza season a year, or a season all year with low levels of a virus, usually influenza B.
The reasons why influenza viruses circulate at different times are unclear. They may be related to climatic factors such as temperature and humidity and, in some places, may be dictated by tourism. But we do not really understand the cause-and-effect relationships between these factors.
The good news is that with the increase in computing power, the availability of high quality surveillance, more years of data and a better understanding of virology and the Immunology of the flu, our ability to predict the season is improving.
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