  A fractional-order compartmental model was recently used to describe real
data of the first wave of the COVID-19 pandemic in Portugal [Chaos Solitons
Fractals 144 (2021), Art. 110652]. Here, we modify that model in order to
correct time dimensions and use it to investigate the third wave of COVID-19
that occurred in Portugal from December 2020 to February 2021, and that has
surpassed all previous waves, both in number and consequences. A new fractional
optimal control problem is then formulated and solved, with vaccination and
preventive measures as controls. A cost-effectiveness analysis is carried out,
and the obtained results are discussed.
