Thread by @MarionKoopmans

Ok, finally figured out how to make a tweetorial... Here we go: interesting paper describing the mass screening effort pulled off by our Chinese colleagues in Wuhan. Important to read carefully for all those planning mass screening. nature.com/articles/s4146…

Wuhan had been under lockdown measures from January 23 until April 8, 2020. During the first 2 months after city’s reopening, there were only 6 sporadic COVID-19 cases in Wuhan.

Nonetheless, there was uncertainty about reopening businesses. Therefore, Wuhan decided to test….everyone in two weeks from May 14, 2020 to June 1, 2020.
All city residents aged six years or older were eligible and 9,899,828 (92.9%) participated. A 93% response rate.

That response rate is something any epidemiologist would envy and may be difficult to do unless mandatory.

No new symptomatic cases and 300 asymptomatic cases (detection rate 0.303/10,000, 95% CI 0.270–0.339/10,000) were identified. Contact investigation: no positive tests amongst 1,174 close contacts of asymptomatic cases.

Of the 34,424 participants with a history of COVID-19, 107 tested positive again, giving a repositive rate of 0.310% Mind you, this is > 6 weeks after the control of the pandemic wave.

Testing was done by two PCR’s targeting different parts of the SARS COV 2 genome, to reduce likelihood of false positives. Virus cultures were negative for all asymptomatic positive and repositive cases, indicating no “viable virus” in positive cases detected in this study.

All positives were put in strict isolation for two weeks (China still does this for any incoming traveller)

Less asymptomatic cases in children up to the age of 18 ((0.124/10,000), and the highest among those over 60 years of age (0.442/10,000). More positive women than men. Difference in rate in urban versus suburban areas (lower in the latter)

What does it tell us: massive effort, low yield but some important findings: high level of asymptomatic positives, but no secondary cases from those, lower rate of positives in <18 year olds, some areas with more transmission that others.

Question raised: how asymptomatic is asymptomatic? This was cross sectional, authors say they did not do follow up.
But yes...impressive nature.com/articles/s4146…