People are talking about *forward* and *backward* #ContactTracing #COVID19

What do they mean?

I made a case study for the @GOARN @WHO contact tracing group

A fictional contact tracer’s tale

My slides are here:…

Short thread (1/7) Forward and backward contact tracing for COVID-19, slideshow
Most #ContactTracing seeks contacts from 2 days before the infected person developed symptoms

#SARSCoV2 can be transmitted before symptoms emerge.
That is looking *forwards*

Who might the index case have infected?

Contacts in quarantine soon enough won’t infect others

Forward contact tracing can break onward transmission chains.

But it doesn’t find the source who infected the index case

To find the source, we need to go *backwards*

Index cases are more likely to have been infected by a source, who also infected others in a cluster

An important characteristic of #SARSCoV2 shows why going *backwards makes sense

It doesn’t transmit randomly. Best shown here, by @jburnmordoch

And shown v early on

So, #BackwardContactTracing focuses on high-risk potential #superspreading events

Look at our index case's garden party

After finding a cluster, you need to trace *forward again
Needs speed, effort, communication, cross-checking records, #TestTestTest and quarantine

Mathematical modelling finds #BackwardContactTracing identifies more infected contacts per index case and reduces effective reproduction number more than #ForwardContactTracing alone: @medrxivpreprints……
When #COVID19 cases are increasing rapidly, @c_drosten says «Looking back is more important than looking forward»

We should try #ClusterBusting in countries as well as Japan and South Korea

Let's evaluate it too…


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More from @nicolamlow

6 Oct
@nataliexdean @EpiEllie Ooh, yes. You should judge risk of bias in #LivingSystematicReviews as you would a standard one. @jhjelliott says, „We propose living systematic review as a contribution to evidence synthesis that combines currency with rigour“… 1/n
@nataliexdean @EpiEllie @jhjelliott Julian Elliott really started the movement with a project in @cochranecollab… and followed up with a series in J Clin Epidemiol (pt1 here…) 2/n
Read 6 tweets
20 Aug
This paper is (mostly) being interpreted as showing that #SARSCoV2 viral load is higher in children than adults. But is it?… @EricTopol @DoctorYasmin @angie_rasmussen @skepteis @apsmunro @mugecevik My pedantic post-publication peer review thread (1/n)
The headline result on Tweets is that VL is higher in children than adults. But the same p-value seems to appear with different comparisons in different parts of the manuscript. Have a look. NB: I am not a fan of p-values or sub-group comparisons (2/n)
Abstract: “Nasopharyngeal viral load was highest in children in the first 2 days of symptoms, significantly higher than hospitalized adults with severe disease (P = .002).

So, in the first 2 days, when there were 2 adults and 9 children?

Read 12 tweets
4 Aug
The #COVID19 data problem in Switzerland. As ⁦@sbonhoeffer⁩ says, we need to analyse #ContactTracing data to look at where coronavirus is, but they still aren’t available... 1/n…
The point is to have info to break transmission chains. Showing that families account for a large proportion of #COVID19 is one thing. But it doesn’t show the source of infection. Where did the family member get infected? #ContactTracing should joint the dots... 2/n
No identifiable #COVID19 source outside the family is a warning ⚠️ That means #CommunityTransmission not controlled. And that means we need to #TestTestTest to find infections, then #TestTraceIsolateQuarantine to stop transmission ...3/n
Read 5 tweets
2 May
This piece on #SARSCoV2 viral load in children and adults has been highly influential.
Can you extrapolate from “Data on viral load” that “we have to caution against an unlimited re-opening of schools and kindergartens in the present situation”
I have serious concerns. 1/7 Response to Christian Drosten group report of viral load by age group
There are two issues: science and politics
On the science
1. There is no methods section about how study population was selected and who they represent
– yes, I know it is a bunch of samples tested in a virology lab - with no denominators about how many samples tested by age 2/7
2. The very low number of samples from children already says a lot about selection into the study. There is no information about their clinical characteristics, stage of infection, etc.
And unequal numbers across the groups makes them very difficult to compare, even visually 3/7
Read 7 tweets

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