Direction and level are different
Imagine one community with many cases and an estimated Rt below 1, and another with few cases and Rt above 1. The first may currently face greater healthcare demand even though its transmission is shrinking. The second may face increasing demand if growth continues.
Neither number alone describes the whole situation.
Rt is estimated, not read directly from a counter
Models use surveillance data and assumptions about transmission timing. Reporting delays, changing ascertainment, and changes in the relationship between infections and the observed outcome can affect the estimate.
An interval spanning 1 calls for more cautious language than a point estimate alone suggests.
R0 answers another question
The basic reproduction number, R0, concerns transmission in a fully susceptible population under specified conditions. It is not a fixed personal risk score or a universal constant independent of context.
Read several indicators together
Use burden measures, trend estimates, severity, and system capacity for the decision at hand. A statement that transmission is declining should not be shortened to “risk is low” without considering the current level. Likewise, growth does not specify exactly how large the next wave will become.
