In NYC, pretty much everyone takes the subway from time to time, 2 meter distancing is impossible, and the underground recycled air is not good. Los Angeles is a much less dense city where most people never take public transit.

Having lived in New York (but only having visited LA), the difference in city design that is immediately salient to me is the presence/absence of the Subway.

According to MIT health economist Jeffrey Harris, the subways seeded the massive coronavirus epidemic in New York City:

New York City’s multitentacled subway system was a major disseminator – if not the principal transmission vehicle – of coronavirus infection during the initial takeoff of the massive epidemic that became evident throughout the city during March 2020. The near shutoff of subway ridership in Manhattan – down by over 90 percent at the end of March – correlates strongly with the substantial increase in the doubling time of new cases in this borough. Maps of subway station turnstile entries, superimposed upon zip code-level maps of reported coronavirus incidence, are strongly consistent with subway-facilitated disease propagation. Local train lines appear to have a higher propensity to transmit infection than express lines. Reciprocal seeding of infection appears to be the best explanation for the emergence of a single hotspot in Midtown West in Manhattan. Bus hubs may have served as secondary transmission routes out to the periphery of the city.

Entries and exits from buildings create pinch points where people are likely to be collected together in small spaces. Access controls and door designs have different throughput rates so conceivably you could design architectural solutions around the goal of minimising contact.

I'm minded to think of how revolving doors could be adopted to facilitate a reduction in infection communication through a restriction in total diameter to necessitate single-occupant compartments and the introduction of air filters to constantly sweep the compartments. These systems seem already to be in place to deal with larger particles.

Though fixating on this small element of architectural design might seem a little less significant than transport and so on it does have the benefit of being fairly mutable. Here's a good article that covers a huge amount of ground on revolving doors that might prove useful in furthering this discussion.