A lot of people are asking how "backward tracing" is any different to standard contact tracing.

It’s not obvious, but there are major differences, and it’s one of the factors cited as being key to Japan’s success at keeping the virus under control.

Here comes a quick thread: https://twitter.com/paulwaugh/status/1280142453614563336

1) Standard contact tracing moves *forward* from the positive case, asking who they have come into contact with *since they got the virus*, seeking to find out who they could have passed it onto, so those people can be isolated.

2) Backward tracing is the other way around: it starts with the positive case and works *backwards*, seeking to find out who could have infected *them*.

This is key with a virus like Covid-19 where most people don’t infect anyone else, but a few people infect *lots* of people.

3) Imagine one person infected 30 others at an event. Only half of them develop symptoms. They test positive, contact tracing & isolation stops their contacts infecting others.

But the 15 who were infected but had no symptoms keep moving around, meeting people, infecting others

4) With backward tracing, you ask the people who tested positive who they’d been in contact with *before* they developed symptoms. You notice many of them were at this event, suggesting it was a super-spreading event, so you contact everyone else there and ask them to isolate.

5) You’ve now found and isolated 15 asymptomatic cases you wouldn’t otherwise know about, and stopped them spreading it further.

This is why backward tracing is potentially much more effective than standard tracing alone for a virus where super-spreading events are common.

6) This all stems from Covid-19’s very low "k-factor" of dispersion: the vast majority of people with Covid-19 go on to infect nobody, but a few cases infect lots of people.

See this from @kakape, featuring @AdamJKucharski https://www.sciencemag.org/news/2020/05/why-do-some-covid-19-patients-infect-many-others-whereas-most-don-t-spread-virus-all

7/7 Standard post-infection tracing can stop people becoming super-spreaders, but backward tracing can find super-spreading events you’d otherwise have missed, hopefully in time to isolate & quarantine only attendees & their contacts rather than their contacts’ contacts’ contacts