Health officials are raising alarms after a seasonal flu strain unexpectedly mutated during the summer months. This new variant appears to partially bypass existing immunity, triggering an unusually early start to the flu season - more than a month ahead of schedule - and is linked to more severe outbreaks. In response, the NHS has issued an urgent call for increased flu vaccinations, warning that the combination of the early start and the virus's characteristics could make this winter particularly harsh. While there remains considerable uncertainty, experts have indicated they would not be surprised if this turns out to be the most serious flu season in ten years.

"We haven't seen a virus like this for a while, these dynamics are unusual," said Prof Nicola Lewis, the director of the World Influenza Centre at the Francis Crick Institute. "It does concern me, absolutely," she added. "I'm not panicking, but I am worried."

Influenza viruses are constantly evolving, which is why the seasonal flu vaccine needs to be updated every year. Scientists track these changes carefully to anticipate which strains might dominate. The virus evolves through a pattern called "shift and drift." Most of the time, it drifts - making small, incremental changes - but occasionally it undergoes a dramatic shift, with major mutations emerging all at once.

This scenario unfolded in June, when a strain of H3N2 flu acquired seven significant mutations. The changes caused cases of this new variant to surge quickly, according to Professor Derek Smith, director of the Centre for Pathogen Evolution at the University of Cambridge.

Researchers are still investigating the exact impact of the mutations, but they likely help the virus partially escape the immunity built up from past infections and vaccinations. This allows the virus to infect more people and spread more easily, which is why the flu season has arrived earlier than usual in the UK and other countries, including Japan.

Experts warn that H3 is a particularly concerning strain, known to have a greater impact on the population. Last year, flu took nearly 8,000 lives, and during the 2022-23 season, the toll was close to 16,000, according to BBC reports.

Flu symptoms can vary widely: some people may carry the virus without feeling ill, while others experience sudden fever, body aches, and extreme fatigue. The infection can be especially dangerous for older adults and vulnerable individuals.

Because of this, getting the seasonal flu vaccine is crucial. The NHS in England has issued an urgent "flu jab SOS", announcing that 2.4 million vaccine appointments are available in the coming week.