What does a basic understanding of science have to do with deciphering this advice?
I don't think understanding the
science is a prerequisite for being able to decipher the information and recommendations from the federal government. What is useful is to be able to distinguish the evidence from the guidance.
For residents of the United States (I haven't seen reliable information from other countries), the
evidence is that "fully vaccinated" individuals who wear effective face masks, and observe physical-distancing and hand-sanitizing precautions, can probably travel on public conveyances with relatively low risk of infection and very low risk of hospitalization or dying, even if they are exposed to people who are infected.
The
guidance is that nobody should travel unless absolutely necessary (e.g., hoping to see a dying family member for the last time) because (1) some proportion of even fully-vaccinated individuals will get infected (probably between five and ten percent, according to the current data, which are likely to change as more evidence is gathered), and (2) the Centers for Disease Control and Prevention only have fragmentary evidence about the effectiveness of the vaccines that have been administered against the mutations of the SARS-CoV-2 virus that are currently circulating.
If you don't want to deal with complexity, or are risk-averse, it isn't very difficult to figure out what to do: stay put for now.