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Vineet Bhola

19 Jan, 17 tweets, 12 min read

Many research papers as well as #RKI, Germany assert average start of symptoms for #COVID19 around 5th-6th day of infection and peak of infectiousness between 2 days before to 1 day after onset of symptoms.. 🧵1/. #2ndlevel #contacttracing #coronavirus

As thumbrule it takes 10-12 days for #lockdown to impact the spread of #COVID19, so duration between infection till confirmation is on avg. 10 days. This is reasonable as people would wait 1-2 days with symptoms, book a test appointment and get the results so 6+2+1+1=10 days 🧵2/

So if Person A gets infected with #COVID19 today, he will be reported as a confirmed case after 10 days. But he might have infected person B after 4 days. Person B might have infected person C after 4+4 = 8 days from today, or 2 days before person A is reported. 🧵3/

When we start tracing only the direct contacts of person A on the 11th day and inform/test person B but not person C, we let the #COVID19 infection chain grow exponentially, if the #ReproductionNumber is >1. That means we let more infected people roam freely than traced . 🧵4/

Some numbers as example :
Let's say on 01.01 in 🇩🇪 20,000 people got infected with #COVID19. With an R=1.1, on 05.01 there will be 22,000 new infected people and on 09.01 there will be 24,400 new infected people. On 11.01 we got the test results of 20,000 cases from 01.01🧵5/

So on 11.01 we start tracing direct contacts of 20,000 people let's say on an average 10 contacts per confirmed case in last 2 weeks would need testing of 200,000 people out of which we will find those 22,000 infected people from 05.01. But if we want.. 🧵6/

To trace 2nd level contacts of confirmed cases before getting the results of direct contacts, we need to trace 10*200,000= 2 Million people. With conventional contact tracing that needs lot of manpower and that's the reason why no one is talking about 2nd level contact but..🧵7/

With contact tracing apps, this can be done efficiently, using information about distance, duration and day of contact using Bluetooth technology, maybe we can even filter out 5 high risky contacts per confirmed #COVID19 case instead of 10. But you might ask why..🧵8/

Waiting for the results of direct contacts of confirmed #COVID19 cases is too late even when we need to trace contacts of 22,000 instead of 200,000? Because the results will come by 13.01 and by that time those 24,400 infected from 09.01 would have already infected 26,600 🧵9/

2nd level infected will always be exponentially higher than infected from direct contacts. This is the reason why without #lockdown no country has managed to keep R for #COVID19 <1. If we want to reduce the cases and bring R<1 without implementing strong lockdown measures🧵10/

We need to think and act faster than the transmission of #COVID19 and that means contact tracing of 2nd level of contacts using bluetooth based apps. Coming back to the numbers, for 20,000 confirmed cases on 11.01 using apps we have 5*20,000= 100,000 direct contacts and🧵11/

Only 0.5*5*100,000 = 250,000 2nd level contacts. Why did I use an additional factor of 0.5? Because if we trace contacts from last 10 days for 1st level, we can reduce it to contacts from last 5 days for 2nd level. This way even with limited testing capacity.. 🧵12/

We can trace people with higher probability of being infected with #COVID19 for 2 levels of contacts via Bluetooth based apps. #vaccine are available but that would need months if not years to be available for everyone. with #Mutations from #UK, #SouthAfrica and #Brazil ,🧵13/

Instead of just relying on vaccines, we need a support of #contacttracing apps to keep the #COVID19 numbers lower, helping the #economy by trying to come out of #lockdown as soon and as safely as possible. Data protection can't have priority than life protection!!! 🧵14/

If #Masks can be mendatory, shops and businesses can be closed, #lockdown can be enforced, why can't #contacttracing apps be enforced? If not enforced atleast encourage them. e.g. shops can open if they check status of the app at entry or tax benefits to them etc. #COVID19 🧵15/