Thread by @bencowling88

HK is currently testing all 370,000 Foreign Domestic Helpers (5% of local pop) within 10 days. Here's a short thread on interpretation of mass testing. tl;dr be cautious because many of the positives could be “long-term intermittent shedders” (1/5) scmp.com/news/hong-kong…

When interpreting the testing results of the 370,000 Foreign Domestic Helpers, it’s helpful to look back at the last mass testing exercise in September 2020 when 1.7 million local residents were tested (2/5)
scmp.com/news/hong-kong…

In total that exercise picked up 40 positives in 1.7 million people tested. In our analysis (not yet published), those 40 broke down as follows:
8 were "long-term intermittent shedders"
13 were newly-identified asymptomatic cases
19 were newly-identified symptomatic cases (3/5)

Strictly speaking, symptomatic persons should not have been tested in that scheme. A few of the 19 symptomatic cases might have developed symptoms in the interval between the sample being collected and when the test result was available. (4/5)

What's my point? When doing mass testing in a location with very little active transmission, like HK right now, a considerable fraction of positives will actually be long-term intermittent shedding in people who have already recovered from infection (5/5) thelancet.com/journals/ebiom…