Basically,
Roman sestertii are
scarce from 193-222, that is from the reign of Septimius until the end of
Severus Alexander's first year as emperor. From 223 on they become common again, until the
denomination was more or less discontinued during the sole reign of
Gallienus.
This thirty-year period of general scarcity, however, includes a twelve-year period of great
rarity:
sestertii of
Septimius Severus and
his family from 198 to 209 are all very
rare, on a
par with the
rarity of large bronze medallions in other periods. That is why
sestertii of
Plautilla (202-5) or those of
Geta as
Caesar (198-209) are very hard to obtain. Naturally this fact makes
sestertii of Septimius himself as a class appear that much rarer, since we are really dealing only with
his coins struck between 193 and 197 and then again in 210 and 211, those of 198-209 all being of great
rarity.
As far as I am aware, nobody really knows why sesterius production was
reduced over the years 193-222, and virtually suspended from 198-209.