All are true about epidemiological features of cholera except ?

The El Tor biotypes have greater endemic tendency than its classical counterpa in that it causes a higher infection-to-case ratio (i.e. more inapparent infections and mild cases). About other options Cholera epidemic has an abrupt onset. It stas as common source epidemic and often treat an acute health problem. Then it continues as a propagated epidemics as cases become the source for the other persons. Cholera epidemic in a community is self-limiting. This is attributed to the acquisition of temporary immunity, as well as due to the occurrence of a large number of subclinical cases. Poor environmental sanitation is the most impoant predisposing factor for epidemic