In recent days, a 76-year-old patient was registered at the Policlinico S. Martino in Genoa not only respiratory but also neurological symptoms because he was infected by A/H3N2 variant of the seasonal flu, commonly called “Australian” and which will be predominant this year. The symptoms affecting the central nervous system, in any case, they seem to be infrequent in the case of the Australian flu and range from mild headaches to serious complications such as encephalitis. This year’s variant appears rather aggressive, but this is currently only a deduction deriving from the analysis of epidemiological data recorded in the southern hemisphere, where winter has now passed and influenza viruses have already had effects on the population. So, for now you shouldn’t worry too much because for Europe and for Italy it is only one risk prediction. In fact, we must consider that the progress of the virus is also conditioned by local factors such as climate, average age of the population and diffusion of the flu vaccine, which remains the best way to prevent the infection or at least alleviate its symptoms.
What is Australian flu (A/H3N2)
This year the seasonal flu is caused by viruses with a predominance of type A of the strain coded as H3N2 and is generically defined as influenza Australianbecause it has already hit the Australian population quite hard. In fact, that’s not the only strain we’ll have. Every year in fact multiple viral strains contribute to seasonal flu syndromes, but one of these is predominate compared to others in spreading by contagion among the population. Last year the predominant variant was the one coded as A/H1N1, this year it should be the one A/H3N2 variant.
The A/H3N2 strain has been known since 1968 as Hong Kong virus as it caused a particularly severe influenza pandemic in China and other Asian countries at the time. Now this virus no longer has a pandemic nature but endemichas therefore become one of the viruses that circulate seasonally in the world. Just since 1968, most seasonal flu epidemics have been caused by H3N2.
Australian influenza causes respiratory symptoms and in some cases encephalitis
In the case of the H3N2 variant, the main manifestations are respiratory as for all influenza viruses, with symptoms affecting the upper (nose and throat) or lower (bronchi and lungs) airways but can also lead to neurological complications, from symptoms such as headache and dizziness to serious conditions like encephalitis; scientific literature therefore suggests a potential role of this variant also in the development of symptoms at the level of the Central Nervous System. There are documented cases of patients of various age groups who were hospitalized for neurological complications and patients with this type of manifestations presented a risk of hospitalization 10 times higher than other types of symptoms. It seems that the virus can pass into the CNS via the olfactory route and the nasal cavity.
In the southern hemisphere and especially in Australia, this year’s flu has had a certain impact on the population also in terms of deaths. In particular, from a statistic published on theAustralian Bureau of Statistics it appears that the deaths from influenza between January and August 2024 were 810double the value compared to the same period in 2023 with 416 deaths, to 2022 with 402 cases, but still significantly lower than the 1005 deaths in 2019.
Which strains of influenza viruses will cause the 2024-2025 seasonal flu in Italy
Influenza viruses are classified into four types (A, B, C and D) and the A strains are coded by convention with the letters H and N corresponding to the initials of two proteins that characterize the surface of these pathogens, hemagglutinin (HA) and neuraminidase (NA), this is why we talk about H1N1 and H3N2. Strains of influenza viruses have the characteristic of undergoing continuous mutations over time, generating different forms of influenza every year.
As far as Italy is concerned, confirmation of which strains will be in circulation between now and spring 2025 will be provided by the RespiVirNet system coordinated by the Istituto Superiore di Sanità which every year from the 42nd week (around mid-October) begins epidemiological surveillance of flu-like syndromes and from the 46th week (around the first ten days of November) the virological surveillance of viruses in circulation. Weekly surveillance reports are published throughout the winter and until the first days of May; only then will the picture be complete.
