Discussion and conclusion
The increase of neutralizing antibodies following the administration of
the bivalent booster we documented in our study (i.e., 7.0–fold
increase) was consistent with the conclusions of other studies4,7. As for the humoral response following the first
two doses of BNT162b2 and the homologous booster 8,9,
a substantial waning of the humoral response was observed 6 months after
the administration of the BNT162b2 bivalent booster. This decrease was
especially important considering the XBB.1.5 subvariant. Moreover, the
drop of neutralizing antibodies over time coincides with the decrease of
bivalent booster effectiveness against infection in the recent report of
Lin et al. 10.
A significant decay in the cellular response was also observed over
time. Nevertheless, the proportion of samples still able to generate
IFNγ in response to an in vitro SARS-CoV-2 stimulation remained
high. This observation is consistent with the maintained and superior
effectiveness against severe disease in the report of Lin et al.10. The fact that the release of IFNγ could represent
a good surrogate of the risk of severe infection still remains to be
evaluated. Importantly, these data can also help national competent
authorities in their recommendation regarding the administration of an
additional booster.