If it turns out that having antibodies does not provide immunity for developing Covid19, what does that mean for the development of a vaccine?

For context, this is a question I answered on Quora

You’re probably referencing a relatively recent Belgian serological study conducted with blood samples collected from vaxxed cancer patients (n = 2,958) that concluded that anti-spike fragment 1 antibody titers are not a correlate of protection against omicron infection. The prospective observational cohort study investigated the relationship between anti-S1 SARS-CoV-2 IgG antibody levels and protection against breakthrough SARS-CoV-2 infections in cancer patients prior to their first COVID vaxx and 6 months after their second booster (n = 760). They found that while higher anti-S1 IgG titers were correlated with a lower probability of breakthrough SARS-CoV-2 infection in the first year of the pandemic there was no clear correlate of protection during omicron as known antibody levels reached statistical significance of the prior year.