According to work published in 1973, 1983, and 2004, after vaccination against seasonal flu, antibody titres peak after typically two to four weeks. They decrease by about 50% over the next six months (the decrease is less for older adults), then remain stable for two to three years; protection without revaccination persists for at least three years for children and young adults.
It was previously thought that vaccination provided lifelong protection against specific strains. This is not totally untrue; a 2010 study found a significantly enhanced immune response against the 2009 pandemic H1N1 in study participants who had received vaccination against the swine flu in 1976. And a study published in Nature found that 90 years after the 1918 pandemic, survivors had antibody-producing cells that produced antibodies with “remarkable power to block 1918 flu virus infection in mice, proving that, even nine decades after infection with this virus, survivors retain protection from it”. (This immunity was a consequence of infection, not vaccination.)
