  Herein, we provide estimations for the effective reproduction number $R_e$
for the greater metropolitan area of Athens, Greece during the first wave of
the pandemic (February 26-May 15, 2020). For our calculations, we implemented,
in a comparative approach, the two most widely used methods for the estimation
of $R_e$, that by Wallinga and Teunis and by Cori et al. Data were retrieved
from the national database of SARS-CoV-2 infections in Greece. Our analysis
revealed that the expected value of Re dropped below 1 around March 15, shortly
after the suspension of the operation of educational institutions of all levels
nationwide on March 10, and the closing of all retail activities (cafes, bars,
museums, shopping centres, sports facilities and restaurants) on March 13. On
May 4, the date on which the gradual relaxation of the strict lockdown
commenced, the expected value of $R_e$ was slightly below 1, however with
relatively high levels of uncertainty due to the limited number of notified
cases during this period. Finally, we discuss the limitations and pitfalls of
the methods utilized for the estimation of the $R_e$, highlighting that the
results of such analyses should be considered only as indicative by policy
makers.

Populations and Evolution;Physics and Society;Applications