Gates Package — page 978

many people. ted, these at all.)16 But if you do pass it along to someone else, you probably pass it along to many people. For reasons we don't entirely understand, 80 percent of COVID infections with early variants came from just 10 percent of the cases.17(These numbers could be different for the omicron variant--as I write this, we don't have enough data to know.) So with a virus like COVID, using the conventional approach means you'll spend a lot of time finding people who wouldn't have infected anyone else--epidemiologically speaking, you'll find yourself in a lot of cul-de-sacs. What you really want to do is find the main thoroughfares, the relatively small number of people who are causing the most infections. Understanding this limitation, a few countries tried a newer approach to contact tracing.18 Instead of working forward in order to find out whom they might have infected, these countries worked backward--identifying contacts up to fourteen days before the person started feeling sick. The goal was to find out who infected the patient, and then see who else that person may have passed the virus to. Backward contact tracing is hard to pull off unless you have widespread testing, fast results, and a system for contacting people quickly. But where it was practical--like in places where people could use smartphone apps to notify them if they'd been exposed--the approach worked really well. It was used at locations in Japan, Australia, and other countries, and it proved quite effective at finding individuals who were superspreading the early COVID variants. One study found that it could prevent two to three times more cases than the traditional approach.19 Good ventilation matters more than you might think.