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.

Sources
(1) https://www.pgeu.eu/wp-content/uploads/2019/04/PGEU-Statement-on-Flu-Vaccination-2021-2022.pdf
(2) https://togetheragainstflu.com/news/interview-john-mccauley/
(3) https://acmedsci.ac.uk/file-download/474780
(4) https://www.vaccineseurope.eu/news/press-releases/implementation-of-influenza-vaccination-campaign-2021-22
(5) https://togetheragainstflu.com/news/interview-richard-webby/