COVID-19 is most contagious around the time symptoms begin, with the highest contagiousness occurring roughly in the first 5 days after symptom onset. Most transmission happens from about 2 days before symptoms appear to around 3-5 days after symptom onset. Peak viral load, which correlates with contagiousness, tends to occur between 1 to 5 days after symptoms start, often around day 4. Transmission can also occur pre-symptomatically, up to 2 days before symptoms arise. Immunity and vaccination status can influence contagiousness duration, but generally, contagiousness decreases after the first week of symptoms. Summary:

• Most contagious period: From 2 days before symptoms appear to about 3-5 days after symptoms start.
• Peak contagiousness: Usually within the first 5 days of symptoms, often around day 4.
• Transmission possible before symptoms (pre-symptomatic spread).
• Contagiousness wanes after the first week but can last longer especially with severe symptoms or weakened immunity.

This applies to common COVID variants including Omicron, which has a shorter incubation and peak viral load timeline compared to earlier variants.