“Dont we get hit with a new mutated flu strain every single year ? Do we need a vaccine every year for the new Wuhan strain ?”
No. Usually it’s one or more of the old strains. They have some process for deciding which flu strains to include in the vaccine. Sometimes they get it right, sometimes, not so much.
When a new strain appears it is usually in the form of a pandemic because nobody’s immune system has seen it before. Then it becomes a member of the “old strains club” and gets rotated into the vaccine cycle when they develop one.
Vaccine development is normally about an 18 month process, so it is typically two flu seasons before it gets included. The second season the strain won’t generally be as bad because there will be some herd immunity acquired from the prior year.
Viruses also mutate. DNA based viruses (like some that cause some kinds of warts, IIRC) not nearly as quickly as RNA (like CCP-19, the cold and influenza) viruses. But how the virus mutates will affect how immunity is carried forward. If it mutates in a way that is invisible to your immune system, you stay immunized. If it mutates in a way that makes it look different, you lose at least some immunity.
Right, so the guess was every two years a new mutation. My point was. Perhaps every year. The RNA is never exactly copied right ? Always a couple bits off.