How bad will the upcoming flu season be?
The southern hemisphere’s flu season is ending now. What can it tell us about what to expect in the northern hemisphere this winter?
- 14 October 2026
- 5 min read
- by Linda Geddes
At a glance
- Based on observations of the southern hemisphere’s 2026 flu season, which has just ended, the upcoming season in the northern hemisphere is expected to be moderate, though predictions are difficult.
- Scientists are watching the emergence of small changes in H1N1 influenza viruses that could potentially influence how the coming season unfolds.
- Regardless of such changes, vaccination is expected to play a key role in reducing the toll of influenza in the coming months.
As the nights draw in and coughs and sniffles start to spread, perennial questions are emerging about how bad this year’s flu season might be, and how well vaccines are likely to protect us.
After a southern hemisphere flu season broadly in line with previous years, the US Centers for Disease Control and Prevention (CDC) is anticipating that its upcoming season there will be “moderate”, based on historic trends and expert opinion.
However, it cautions that severity is difficult to predict before the season begins, and vaccination remains a key defence against severe disease.
One development that scientists are closely monitoring is the emergence of small changes in H1N1 influenza viruses that could potentially influence how the coming season unfolds.
What can we learn from the southern hemisphere’s flu season?
Although influenza viruses circulate year-round, infections tend to peak during the winter months in temperate regions.
Monitoring influenza activity in southern hemisphere countries such as Australia, New Zealand and South Africa, where the flu season is just ending, can therefore offer early clues about which flu viruses are circulating and what the northern hemisphere might face. Its flu season usually begins in October and can last until May.
Even so, differences in population immunity, circulating flu strains and vaccination coverage mean the same patterns don’t necessarily repeat.
Overall, the World Health Organization (WHO) reported fewer influenza detections globally between February and August 2026 than during the same period last year.
The main viruses circulating were familiar forms of seasonal flu. These fall into two main groups: influenza A and the influenza B Victoria lineage. The former includes H1N1, a descendent of the virus that caused the 2009 swine flu pandemic, and H3N2, which has circulated in humans since 1968.
Of these viruses, H3N2 was most common across the southern hemisphere, with a recently emerged subgroup known as subclade K accounting for the majority of H3N2 viruses analysed since February 2026.
When Subclade K was first detected at the tail end of the southern hemisphere’s 2025 flu season, its rapid accumulation of mutations initially sparked concerns that it might evade existing immunity and vaccines.
But despite the northern hemisphere’s 2025–2026 flu season starting unusually early in parts of Europe, its overall severity varied between countries, and wasn’t uniformly more severe than previous winters.
Nor does this year’s southern hemisphere flu season appear to have been unusually severe overall. “Within the variation of flu seasons, it wasn’t really anything to write home about,” said Dr Ruth Harvey, Deputy Director of the Worldwide Influenza Centre at the Francis Crick Institute in London.
What could this mean for the northern hemisphere?
While the southern hemisphere’s experience offers some reassurance, it doesn’t guarantee that northern hemisphere countries will face a similar flu season.
Speaking at a WHO meeting on influenza vaccine composition in late September, Prof Nicola Lewis, Director of the Worldwide Influenza Centre at the Francis Crick Institute, said: “In recent weeks, we’ve seen the continued circulation of influenza, perhaps to higher levels than we saw at the same time last year in 2025, and we’ve also continued to see both influenza A and influenza B.”
One development that scientists are watching is a recent increase in detections of H1N1 influenza viruses.
Last year influenza vaccines prevented an estimated 180,000 hospitalisations and thousands of deaths in the US alone.
Within a subgroup group of these viruses, known as D.3.1.1, researchers have identified small changes in the haemagglutinin protein found on the viruses’ surface.
Laboratory tests have suggested that antibodies may recognise some of these variants less well than the H1N1 virus targeted by this winter’s flu vaccine, although it is currently unclear whether this will translate into reduced protection.
Researchers at the Francis Crick Institute are currently running further tests using a technique that measures how well antibodies from large numbers of people with different infection and vaccination histories recognise emerging flu viruses.
This approach helped reveal that people had greater pre-existing immunity against last winter’s H3N2 subclade K variant that earlier laboratory studies had suggested, and that vaccination boosted these responses.
Have you read?
Should I get the seasonal flu vaccine?
Vaccination remains a key means of preventing serious flu-related illness, hospitalisation and death, particularly among people who are most vulnerable to complications.
Last year influenza vaccines prevented an estimated 180,000 hospitalisations and thousands of deaths in the US alone.
This year’s Northern Hemisphere flu vaccines target H1N1, H3N2 and influenza B viruses, and were updated earlier this year to better match recently circulating strains.
They are expected to play an important role in reducing the toll of influenza in the coming months, so it is worth taking up the offer of a seasonal flu vaccine if you’re eligible for one.
