Abstract:
Three safe and effective vaccines against SARS-CoV-2 have played a major role in combating
COVID-19 in the United States. However, the effectiveness of these vaccines and vaccination
programs has been challenged by the emergence of new SARS-CoV-2 variants of concern. A new
mathematical model is formulated to assess the impact of waning and boosting of immunity against
the Omicron variant in the United States. To account for gradual waning of vaccine-derived immunity,
we considered three vaccination classes that represent high, moderate and low levels of immunity. We
showed that the disease-free equilibrium of the model is globally-asymptotically, for two special cases,
if the associated reproduction number is less than unity. Simulations of the model showed that vaccinederived
herd immunity can be achieved in the United States via a vaccination-boosting strategy which
entails fully vaccinating at least 59% of the susceptible populace followed by the boosting of about
72% of the fully-vaccinated individuals whose vaccine-derived immunity has waned to moderate or
low level. In the absence of boosting, waning of immunity only causes a marginal increase in the
average number of new cases at the peak of the pandemic, while boosting at baseline could result in
a dramatic reduction in the average number of new daily cases at the peak. Specifically, for the fast
immunity waning scenario (where both vaccine-derived and natural immunity are assumed to wane
within three months), boosting vaccine-derived immunity at baseline reduces the average number of
daily cases at the peak by about 90% (in comparison to the corresponding scenario without boosting
of the vaccine-derived immunity), whereas boosting of natural immunity (at baseline) only reduced
the corresponding peak daily cases (in comparison to the corresponding scenario without boosting of
natural immunity) by approximately 62%. Furthermore, boosting of vaccine-derived immunity is more beneficial (in reducing the burden of the pandemic) than boosting of natural immunity. Finally, boosting vaccine-derived immunity increased the prospects of altering the trajectory of COVID-19 from
persistence to possible elimination.
