When does the flu season start?

In the northern hemisphere, flu season traditionally spans from mid-November to mid-February, though its arrival can vary, starting as early as October and extending into May. Health experts declare the onset of the flu season when 10% of suspected cases confirm positive for the virus. As of early November, England reported a positivity rate of 11%, a significant rise from 3% the previous year, marking an unusually early start to the season, by four to five weeks.

What is causing this year’s outbreak?

Flu seasons are characterized by the presence of multiple influenza strains, primarily caused by influenza A and B viruses. Among these, common subtypes of influenza A include H1N1 and H3N2. Currently, in the UK, a specific form of H3N2 dominates the season. This virus traces its lineage to a strain responsible for Australia’s severe flu outbreak earlier this year and has since evolved through seven new mutations. These mutations resulted in a drifted strain, known as subclade K, which spreads more rapidly, although it hasn’t shown to cause more severe illness.

What is happening in continental Europe?

Across Europe, the situation varies, with some countries experiencing more severe outbreaks than others. On the continent, the flu season began around three to four weeks ahead of the usual schedule. However, the drifted H3N2 strain has only recently become the primary cause of infections in several areas.

In Germany, the Robert Koch Institute reported that the flu season began two to three weeks early. Both H1N1 and H3N2 strains are present, but there’s been a noticeable rise in H3N2 cases over the past three weeks.

In contrast, France saw a slightly delayed onset, with flu cases emerging just a week ahead of the typical timeline. Dr. Vincent Enouf, the deputy director of France’s national respiratory virus center at the Pasteur Institute in Paris, noted that both H1N1 and the H3N2 subclade K are equally prevalent in the country.

France’s public health agency, Santé Publique, recently reported a significant increase in flu activity across metropolitan regions, sparing only Corsica, which has lower case numbers. The number of individuals seeking medical care for flu at hospital emergency departments, as well as the number of hospital admissions, has risen considerably over the past week.

Elsewhere in Europe, in Spain cases are surging, with infection rates already higher than last year’s winter peak and hospitalisations doubling in a week. Romania and Hungary are also experiencing a surge in cases. In Ireland, nearly 3,000 cases were reported in the first week of December, up 49% on the week before, with hospital admissions up 58%.

How effective is the vaccine?

The mutations acquired by the drifted H3N2 flu virus mean that it is not recognised as well by the immune system and is not well matched to the H3N2 virus used in this year’s seasonal flu vaccine. As expected, data from the UK’s Health Security Agency show that the vaccines are less effective at blocking infections caused by the drifted H3N2, but they still provide useful protection against severe disease. The data from the start of England’s flu season show that protection against hospital attendance and admission was within the normal range for flu vaccines, at 70-75% for children and 30-40% for adults.

Despite the vaccine being less effective than hoped, health officials strongly recommend people have the shot to reduce their risk of severe illness. Nearly eight million French people have already had the flu vaccine, Enouf said, 21% more than at this time last year.

Data published by Sante publique this week show that among patients being treated for flu in intensive care units, 58% were aged 65 and older and 90% had at least one other medical condition. Of those whose vaccination status was known, 98% were not vaccinated. In Ireland, 73% of people admitted to intensive care for flu had not had this year’s flu vaccine.