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)…
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)…
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)…

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More from @bencowling88

28 Apr
Short thread on #COVID19 vaccines and vaccine hesitancy in Hong Kong. tl;dr summary: vaccines work, we should all choose to get vaccinated so that Hong Kong can get back to normal (1/17)
In Hong Kong, #COVID19 poses just as much threat now as it did a year ago. We have done well to control surges in transmission so far, and have recorded just 11,749 confirmed cases cumulatively (0.15% of the population) (2/17)
High uptake of #COVID19vaccines will allow us to relax the public health measures that have been used either intermittently or continuously in the past year – quarantines, social distancing, mass gathering bans, work-at-home, school closures, mandatory face masks, etc. (3/17)
Read 17 tweets
18 Mar
Hong Kong Citizen News have kindly agreed to publish my commentary on herd immunity. It's a long read, available here, grab a coffee:… I published a similar commentary a few weeks ago in Chinese in Mingpao. I'll give a short summary in a new thread below
Herd immunity describes the situation where a high enough fraction of the population are immune that a large epidemic is no longer possible. Immunity can be acquired through infection or through vaccination (2/10)
In some parts of the world, herd immunity will be reached mainly through large numbers of infections. In other places like Hong Kong where infection rates have been kept low by successful public health measures, herd immunity could be achieved through vaccination (3/10)
Read 11 tweets
16 Mar
As more and more people received #COVID19 vaccines in Hong Kong, it will soon be time to think about how immune status could modify our test-and-trace procedures locally (1/17)
tl;dr summary: I think vaccinated persons should still be isolated if testing positive, while quarantine duration could be reduced or even converted to medical surveillance for some fully vaccinated persons (2/17)
If you have respiratory symptoms, you should be tested for COVID, whether or not you have previously been infected, and whether or not you have been vaccinated. Testing is also done on contacts of known cases, as well as people living or working near known cases (3/17)
Read 17 tweets
17 Sep 20
Our work on superspreading has just been published…, the preprint was posted a few months ago.
Key finding - between 23 January and 28 April 2020 in Hong Kong, we analysed contact tracing data and estimated that 19% of cases seeded 80% of all local transmission. For those interested in the dispersion parameter k it was 0.33 (95% CI: 0.14–0.98).…
Read 5 tweets
6 Aug 20
Here is an update on the #COVID19 situation in Hong Kong, following from my thread last week (1/18)
(2/18) We have seen daily case numbers decline in the past few days, and hopefully we have now passed the peak in our "third wave" (which is really a second wave here, because what is being referred to as the "first wave" in Hong Kong was not a wave ...).…
(3/18) However, the local reproductive number has not dipped very far below 1. A reproductive number of exactly 1 would mean on average one case is infecting one further person, and daily numbers of cases would stay the same from week to week. The lower R is below 1, the better.
Read 18 tweets
30 Jul 20
(1/9) Good news about COVID-19 suppression in Hong Kong, today we estimate that the daily reproductive number Rt dropped below 1 on 22 July, around a week ago
(2/9) There is a delay in estimates of Rt due to a 10-day lag between infections and laboratory confirmations, including an average of 5 days incubation period (time from infection to illness onset) and another 5 days on average between illness onset and laboratory confirmation.
(3/9) Since Rt fell below 1 on 22 July, we would expect the epidemic curve of cases by illness onset to reach a peak approximately 5 days after that (27 July), and the daily number of laboratory-confirmed cases to reach a peak within 7-14 days after 22 July.
Read 9 tweets

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