Why low levels of immunity could lead to a severe flu season

Experts across the globe are warning that this year’s flu season could be more severe than usual (1). And this is despite historically low levels of flu activity during the Northern Hemisphere’s 2020/21 flu season – levels not seen any winter since towards the end of the 19th century (2)!

So, why might the low levels of flu in circulation last winter lead to a severe flu season in 2021/22?

Not only did we see extremely low levels of flu virus in circulation last winter; we also saw a mild flu season in winter 2019/20 (3). Together these will have led to lower natural immunity levels to the flu than we might typically expect to see (3)(4).

Why? Because every flu season, anywhere between 5% and 30% of people catch the flu (5). Getting infected boosts their immune response and, with that, their immunity to the flu viruses (5).

So, a flu season with low flu activity followed by a flu season with very low flu activity means we haven’t had that flu immunity boost in 5% to 30% of the population (5). In other words, population immunity to flu is likely lower than usual (5).

What will happen is very hard to predict (4). But as lockdowns and border controls ease and international travel resumes, the low level of immunity to the flu might very well lead to a surge in flu cases (4).

Added to that, COVID measures mean a large number of infants and young children have never been exposed to the flu (4). So, we can expect higher than average flu infection levels in these younger age groups this flu season (4).

Experts believe we may also be more exposed to zoonotic flu infections – human flu infections from animal flu viruses such as the bird flu and swine flu viruses (6).

The flu will return. Don’t let the extremely low levels of flu circulating last year make you complacent. Get your flu vaccine.



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