The document discusses the basic reproduction number (R0) of COVID-19. R0 estimates the average number of secondary infections produced by one infected individual in an entirely susceptible population. For COVID-19, estimates of R0 range from 1.5 to 4.5, with most estimates between 2-3. An R0 above 1 means the number of cases will increase, while below 1 means cases will decrease. The document also discusses secondary attack rates, which measure the proportion of exposed individuals who become infected from a specific exposure, such as within a household.