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Since the beginning of this year, it was obvious that the "season" of the Australian flu in 2019 would be different. Normally, the influenza season coincides with the winter months of July and August, sometimes until September and October.
But this year, things have happened much earlier, with a record number of influenza cases reported in summer and fall.
So what is happening and is it really as serious as the media has reported? Here we look at some of the most recent case data and results to see if this is a "horrible flu season".
The impact of influenza on the community is measured in several ways. The most basic measure is simply to count the number of people presenting to their GP with a flu-like illness.
Sometimes the doctor takes a cotton swab, which is then tested in the laboratory to confirm the presence of the flu virus (it is possible that another virus or other respiratory bacteria is causing the symptoms flu-like).
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Cases of influenza-like illness increased in early March, peaked in early June, and are decreasing. Laboratory confirmed cases (the results of which are shown in the graph above) show a similar trend. We did not include July in this chart because it has not been completed yet, but we still see a high number of cases in July.
Compared to previous years, 2019 announces as a big year with more than 120,000 cases of influenza confirmed in the laboratory until the end of June. But this is not as serious as in 2017, when more than 250,000 cases had been reported to the National Notifiable Disease Surveillance System (NNDSS) by the end of the year. This season having occurred much later than 2019, only 24,000 cases were reported in 2017, up to July 7.
The good news is that since the 2019 season started earlier, it should also end sooner than usual. When the main influenza season begins, it usually ends about 12 to 16 weeks later, when the number of susceptible people falls below the level required to maintain effective circulation.
FluCAN (via Ministry of Health Influenza Surveillance Report), CC BY-ND
Another measure of the severity of the influenza season can be measured by the number of hospitalizations, including admissions to ICUs (intensive care units).
Hospital admissions show from 1 April to 30 June this year, 1,309 admissions to Australian sentinel surveillance hospitals (several hospitals in which influenza admissions are monitored each year).
This figure is much higher than previous years at the same time. In 2018, there were 90 entries and in 2017, 311. But in 2017, the season arrived much later and more seriously and finally resulted in 3,969 entries for this year.
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It is also useful to examine the proportion of hospitalized people with influenza infections hospitalized directly in the ICU. In 2019, this represented 6.7% of admissions compared to 2018 (a mild influenza year), with 8.1% of admissions and in 2017 (a very severe year) with 8.9% of admissions.
The USI rate in 2019 is in the lower part of the historical figures, ranging from 8.7% in 2015 to 14.2% in 2013. According to this measure, the 2019 season is of a serious similar to that of previous seasons and so is not exceptional.
Although hospital admissions can be measured relatively easily, it is more complicated to measure deaths from influenza for several reasons. Influenza often paves the way for secondary bacterial infections, such as pneumonia, which can lead to hospitalization and death, especially in the elderly. When this happens, it can be difficult to directly link the death to an earlier influenza infection.
And data on deaths are often very delayed. The readily available death data collected by SNDSN is considered a significant underestimate of the actual number.
By the end of June 2019, 231 influenza-related deaths had been reported to SSNDN. Virtually all of these cases were due to the influenza A strain. They covered all ages, but most deaths were in the elderly (80 years and older).
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This compares to 24 and 21 deaths over the same period in 2018 and 2017 respectively. But these numbers have risen to 55 deaths and 598 deaths by the end of 2018 and 2017 respectively.
It is clear that 2019 is more severe than 2018, based on the measures detailed above, but at this point, it seems that it will be less severe than 2017. However, we will have to wait several weeks to be sure.
When we look at what has happened in every Australian state so far this year, we're seeing some interesting differences in how the season unfolds. The number of cases began to increase significantly in most states in April, while South Australia had already peaked in April and this number was maintained in May. This means that most states still have several weeks of influenza circulation to cope with.
People of all ages are susceptible to flu, which is reflected in the wide range of ages of infection. Young children (especially those under 10 years of age) and the elderly (especially those over 80) are more vulnerable and are often more severely affected by influenza infections, as are pregnant women.
It is interesting to note that different types of influenza affect different age groups, with influenza B and influenza A (H1N1) being more common in young people and influenza A (H3N2) more common among older people.
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At this point, we can conclude that the 2019 influenza season is very different from our usual seasons and will be one of the most severe seasons of the last 20 years.
So, although 2019 does not seem to be the worst season we've ever seen – it's likely to stay in 2017, it could well be a tight second place. But we will have to wait another month or two before we can be sure.
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