Wednesday, July 29Over the last two days, we have covered how to estimate the total number of infections from the confirmed cases and the test positivity, and how to calculate Rt, and what each number means. Today, we will go over the process to answer the question “When will it be safe for me to do X?”This is a general infectious disease epidemiology problem, and the answer is rather straightforward, and also rather complicated. Each activity has its own inherent risk of transmission. For COVID-19, the risks seem to center on prolonged contact (15 minutes or more) within six feet. The CDC uses this criteria for contact tracing.If someone you know tested positive, you would be a contact if you were within six feet of them for more than 15 minutes while that person was transmissible. Risks increase if you are indoors, not masked, and if you are breathing heavily or singing. We need to know how much transmission we are we to expect if someone transmissible is engaged in this activity.The second thing we need to know is the level of community transmission. If it is lower, the probability of transmission in our activity scales down. For example, how bad will it be to have a one-hour lecture in a room of 30 undergraduates if one person in a million is transmissible in the community?How about one in a hundred? One in 10? It should be easy to see that someone would feel comfortable if community transmission were very low, and nervous if it were high.The third things we need are mitigation and surveillance. In our activity, we need to reduce the potential for transmission, and we need to monitor it. Testing and isolation of positives and contacts acts as both surveillance and mitigation if done properly. In major league baseball, each player and staff member tests every two days, and contacts and positives must isolate. If the teams stick to the protocol, they should make it through the season. You can also mitigate risk by being outdoors, wearing masks, staying more than 6 feet apart, and not having activities with prolonged, greater than 15 minute, exposure.To determine if we can hold an activity, we want to know what the inherent risk in our activity is, how we will mitigate and surveil that risk, and what the level of community transmission is. The Harvard Global Health Institute has defined different zones for community transmission. They are green, yellow, orange and red. In terms of cases, these are:Red: over 25 new cases per day per 100,000 peopleOrange: 10-25 new cases per day per 100,000 peopleYellow: 1-10 new casesGreen: Under 1 caseNow, we can pull the data for the Central Savannah River Area. We have, in the past week, 1,683 cases over our 768,000 population, which is 31.3 cases per day per 100,000 people. We are in the red zone and community transmission is high. I can also estimate the proportion that are positive today in the CSRA using my formulas, and it is about 1.5 percent.And back to our premise for today: When can we resume an activity? When the community transmission level is low enough, and there is enough mitigation and surveillance in the activity to prevent spread of COVID-19. If you know of an activity resuming soon, you should compare the planned, communicated plan for mitigation and surveillance against the level of transmission in the community, and the inherent potential for transmission in their activity. If they don’t match up, there is potential for outbreaks to spark in that environment.I am closing today with a link to that Harvard Global Health website. It has a neat map that shows county by county case levels:https://globalepidemics.org/ Contact David Blake at Send Email to Dave Blake.