The 1918 influenza is called the H1N1 strain and it is still around today. The flu differs from COVID-19 in one big way. Influenza virus mutates very quickly, which is why we need a new vaccine yearly that covers for different strains. Luckily with Influenza, you can get some protection against similar strains because the mutations are usually small changes. SARS-CoV-2, on the other hand does mutate, but not as rapidly. We don't yet know how our immune systems are going to respond to different strains of SARS-CoV-2. There is some evidence that previous infection with omicron can protect against other variants, but this is still under investigation.