That H1N1 virus mutates a lot--not like the SARS-COV, which has mostly mutated just along the spike protein. This is why the vaccines are proving more effective against the variants--the vaccines all focus on the spike protein.   Also in 1918-21, 1/3 (!!!) of the human race caught it (herd, massive variant possibilities) and that H1N1 flu's direct descendants still rise up every once in a while on grand scale, 2009 being the most recent bigger moment, and are in circulation every year.