After exposure to covid when is someone contagious?

Question

Here it says:

The time from exposure to symptom onset (known as the incubation period) is thought to be two to 14 days, though symptoms typically appear within four or five days after exposure.

We know that a person with COVID-19 may be contagious 48 hours before starting to experience symptoms. People may actually be most likely to spread the virus to others during the 48 hours before they start to experience symptoms.

if symptoms started 2 days after exposure based on above (incubation period), and people are contagious 48 hours before starting to experience symptoms also as stated above, does it mean after exposure to covid person is immediately able to infect others?
After how many days after exposure is someone contagious?

Answer 1

No, and we can state this authoritatively, because to become infectious, the person has be able to put out virus, which means at least a single replicative cycle of the virus before any would be emitted.

For SARS-CoV-2, the generation time is somewhere around 10 hours in cell culture, it would probably take longer in an infection in-vivo. There's a handy info-graphic here¹, with information about the virus and many aspects of infection.

The bit you are looking for is the Replication Timescales section. The eclipse period part is the generation time, and is a measure of the time between entry and production of mature virions.

So, at a minimum, an infected person could potentially be emitting viruses within half a day, but not immediately.

For interpretation of those numbers, as @bryankrause says, you can't simply arithmetically work with them. A symptom onset of 2 days means that they simply show symptoms early, but it still doesn't affect the replication kinetics of the virus.

¹ Bar-On et al., eLife. 2020; 9: e57309.