Disclaimer: I'm not even remotely a biologist, and this is a vague memory of some announcements (albeit by reputable scientific sources) a year ago. This gist

Disclaimer: I'm not even remotely a biologist, and this is a vague memory of some announcements (albeit by reputable scientific sources) a year ago. This gist of what I remember was that viruses mutate some "percentage" of their envelope on average annually. It's somewhat predictable for more than six months into the future, which is how flu vaccines are developed and manufactured ahead of time before the next flu season. This unpredictability accumulates, so after three years the viruses become nearly totally unpredictable, not just to vaccine manufacturers, but also to the human immune system. If you get the flu every year, each year the new flu might only be 30% different than the one before, so you have some lingering immunity to it. If you catch it every year, you'll generally get mild cases and refresh your immunity each time. If you skip three years, the viruses become 66% different, which might be sufficient to evade your immune system and give you a really bad case. There were also a few coincidental compounding issues recently, such as particularly virulent strains of viruses such as RSV going around.