A better understanding of the durability and breadth of serum neutralizing antibody responses against multiple SARS-CoV-2 variants elicited by Covid-19 vaccines is crucial in addressing the current pandemic.  In this study, we quantified the decay of serum neutralization antibodies after the second and third dose of the original Covid-19 mRNA vaccine.  Using an authentic virus neutralization assay, we found that decay half-lives of WA1- and Delta-neutralizing antibodies were both ~60 days post second and third vaccine dose.  Unexpectedly, the durability of serum antibodies that neutralize three different Omicron subvariants (BA.1.1, BA.5, BA.2.12.1) was substantially better, with half-lives of ~6 months or longer.  A booster dose of the original Covid-19 vaccine was also found to broaden antibody responses against not only multiple SARS-CoV-2 strains but also SARS-CoV and four other sarbecoviruses.  These findings suggest that repeated vaccinations with the Covid-19 vaccine may confer a degree of protection against future spillover of sarbecoviruses from animal reservoirs.