This is an extremely illuminating Twitter thread from a St. Andrews infectious diseases specialist; a few excerpts:

2147 close contacts of 157 #COVID19 cases were followed up: Overall infection rate was 6%, higher infection rate among friends (22%) and household (18%), and main risk factors include contact in household (13%), transport (11%), dining (7%) [BL:  note that only 18% of household contacts were infected!]

[In another study] 445 close contacts of 10 #COVID19 cases were followed up, of those 54 (12%) developed symptoms, suggesting secondary attack rate of 0.45%, household attack rate of 10.5%. No other close contacts incl community members, HCWs were positive.

Based on 1286 close contacts of 319 #COVID19 cases [in another study]; household and transport contacts had higher risk of transmission (80% of infections caused by 9% of cases), household attack rate of 11.5%, children were as likely to be infected….

Among 392 household contacts of 105 index #COVID19 cases, overall household attack rate was 16%, the secondary attack rate was highest in spouse (28%), all adults (17%) and was lower in <18 age group (4%)…

In summary: While the infectious inoculum required for infection is unknown, these studies indicate that close & prolonged contact is required for #COVID19 transmission. The risk is highest in enclosed environments; household, long-term care facilities and public transport.

High infection rates seen in household, friend & family gatherings, transport suggest that closed contacts in congregation is likely the key driver of productive transmission. Casual, short interactions are not the main driver of the epidemic though keep social distancing!

Although limited, these studies so far indicate that susceptibility to infection increases with age (highest >60y) and growing evidence suggests children are less susceptible, are infrequently responsible for household transmission, are not the main drivers of this epidemic.

Finally, these studies indicate that most transmission is caused by close contact with a symptomatic case, highest risk within first 5d of symptoms.