To question 1, infection of others before symptoms of the carrier
Yes, carriers of Covid-19 can infect others before developing noticeable symptoms themselves.
There is a study to a very traceable transmission chain in a company in Germany.
Person 1 was infected from a from China comming
person 0 after January 19, 2020.
person 3 had contact only with
person 1 on January 20 and 21. However,
person 1 had first symptoms on the January 24th, so the infection occurred 3 days before symptoms of the carrier.
person 4 had also contact only with
person 1 from January 21 to January 24. This infection occurred within 0 to 3 days before
person 1 first symptoms.
This study can be read in detail here: https://www.nejm.org/doi/10.1056/NEJMc2001468.
There exist also an interview with
person 1, which states that he met
person 0 on Monday (January 20).
Person 0 doesn't show symptoms of illness during their presence. Symptoms were established on their return trip on January 22, so
person 1 was infected 1 to 2 days before symptoms of
The interview is readable here: https://www.br.de/radio/bayern1/coronavirus-in-bayern-100.html.
To question 2, duration until the carrier becomes contagious
- The above study shows that
person 1 infected
person 3 already after 1 to 2 days after infection themselves.
In addition, there is an study to 312 infections in Austria about the serial interval which refers to the time between successive cases of transmission.
- According to this study, carriers of the virus contribute significantly to an infection already after day 1. The risk of transmission is highest on day 3 and after 10 days the risk of transmission drops significantly, but is still present.
You can see this study here: https://www.ages.at/en/wissen-aktuell/publikationen/schaetzung-des-seriellen-intervalles-von-covid19-oesterreich