The Covid Watch app shows you the date, duration, and signal strength of all exposures to people who are infected even if the contact was brie for the Bluetooth signal was weak.
For each of these exposures, the app combines two different factors–Transmission Risk Level of the infected person you were in contact with and Attenuation Durations.
Transmission Risk Level refers to how infectious the person was when you came into contact with them. People are most infectious on the day their symptoms start. They are less infectious before and after that onset date, but they can infect as early as 2-3 days before symptoms begin, and they can also transmit even if they don’t show symptoms. We use this information on the timing of infectiousness to score from the lowest Transmission Risk Level of 1 to the highest Transmission Risk Level of 6.
Attenuation Durations tell you how many minutes your phone registered Bluetooth signals of different strengths issued by the phone of someone who was infectious. Roughly speaking, strong signals mean the two phones were closer to one another. Sometimes you will get a weak signal even from someone in another room, other times, if phones are in bags, the signal may be weak even if you were in close contact. You will see these listed as numbers separated by commas(e.g. 0m,5m,15m), This would mean your phone registered a strong Bluetooth signal for 0 minutes, a moderate signal for 5 minutes, and a very weak signal for 15 minutes. More minutes at a close distance are riskier. The maximum duration for each of these numbers is 30 minutes. So if you sit on a couch with someone and watch a movie it might read 30m, 0m,0m, even though it was longer than that. This is to protect privacy.
The Attenuation Durations are combined with the Transmission Risk Level to determine if a single Possible Exposure OR a combination of Exposures add up to the level of Significant Exposure. If you get a Significant Exposure you should quarantine.
Sometimes you will see exposures here on the Possible Exposures page even when the app registers "No Significant Exposure". This means that your regional public health authority doesn’t think your exposure was dangerous enough to require you to quarantine or take other action. It does indicate that people around you are both using the app and testing positive.
When exposure is Significant, the screen will change from what it looks like on the left to the one on the right. Specific follow up actions will be listed in the Coivd Watch app based upon your selected region.