The First Wave and the Second Wave of the Flue?

Some people who had been infected by the first wave did have some immunity on that second wave, which is pretty interesting. We don't have any samples from the first and maybe not even the third wave. And so all the studying we've done has been from that second, most deadly wave. So they may not have been the exact same strain, but it is weird people would have an immunity to that second wave if they'd been through the first.