Flu occurs every winter; however, this year the trend is unusually early and concerning. One of the seasonal flu viruses suddenly mutated during the summer, enabling it to evade parts of our immune defenses and triggering a flu season more than a month ahead of schedule. This mutated virus is more severe, according to the trends of the past. With the growing concern, the NHS has declared a “flu jab SOS” warning, noting that these factors may interact to cause a harsh winter. Although scientific uncertainty remains, influenza experts would not be surprised if this becomes the worst season in years with respect to the influenza season. Nicola Lewis, the director of the World Influenza Centre of the Francis Crick Institute, described the current viral activity as unusual and concerning, but not yet alarming.
Flu viruses evolve continuously, and research centers monitor these changes because vaccines must be updated annually. The virus evolves through processes known as antigenic shift and drift. An H3N2 strain circulating in June was found to have seven mutations. Prof Derek Smith of the University of Cambridge said that this led to the surge in cases of the modified virus. It was particularly unusual when it happened, because such mutations are not common in the summer of the northern hemisphere. Prof Smith anticipates that the strain could spread globally and potentially become a pandemic.
In September, the situation escalated dramatically because children returned to school and temperatures fell. The precise effects of the mutations remain under research, but early evidence suggests they help the virus evade immunity from previous infection and immunization. This has contributed to easier transmission, and this is why the UK, along with countries like Japan, is experiencing an early flu season. The virus is now spreading effectively without relying on typical winter conditions. Early assessments also indicate that the R number of the new strain is above the anticipated value. The annual flu usually has an R of approximately 1.2, but the initial estimate made this year stands at 1.4. Practically 100 infected individuals would normally transmit the flu to approximately 120 individuals; they may pass it to around 140.
According to the University of Oxford, Prof Christophe Fraser warns that it may be a very severe season that may be even worse than what has been experienced in the last ten years. In a typical year, roughly one in five people catch the flu, and that proportion can rise under certain conditions, yet uncertainty remains. In Australia, the most recent season was the worst case of flu recorded in history, though it was not of the same mutated strain of H3N2. Children are primary drivers of transmission, but experts are paying close attention to the spread of the infection to older individuals who are more susceptible. Historically, H3N2 strains have been particularly dangerous for the elderly. Among the high-risk groups, flu can be fatal and causes symptoms such as fever, body aches, and fatigue. An average of 8,000 individuals died of flu last year in the UK, and almost 16,000 died in the 2022-2023 season.
Health authorities strongly recommend vaccination. According to the NHS, 2.4 million vaccination slots are available, and this year’s flu jab is considered especially important. Experts report that, although the vaccine was developed before the June mutation, it still offers partial protection and can help prevent severe illness and hospitalisation. It covers multiple flu strains, any of which could cause an epidemic. Doctors are also being reminded that early antiviral treatment can reduce the risk of complications. Some schools in Japan have been closed temporarily to slow the outbreaks, but these measures are not comparable to COVID-19 lockdowns. While the ultimate severity of the season remains uncertain, Prof Lewis cautions that the threat is unlikely to disappear quickly.
References: BBC. New flu virus mutation could see ‘worst season in a decade’. Published November 9, 2025. Accessed November 18, 2025. New flu virus mutation could see ‘worst season in a decade’Â
