Which statement correctly distinguishes direct horizontal transmission from indirect horizontal transmission?

What is the definition of the basic reproduction number (R0) in infectious disease epidemiology?

A novel virus has R0 = 5.0. A vaccine with 80% efficacy is available. What minimum proportion of the population must be vaccinated to prevent sustained transmission? Show your calculation.

A disease with a long incubation period and moderate R0 begins circulating in a partially immune community. Synthesize an optimal combined strategy that addresses both early detection and protection of susceptible individuals.