A while ago, a study published in November 2020 that involved the testing of over 9.8 million persons in Wuhan, China, was mentioned. According to the paper, during the period 14 May 2020 to 1 June 2020, mass testing found merely 300 asymptomatic cases and no positive cases amongst their close contacts.
In other words, although a possibility, asymptomatic transmission was not detected.
Whilst that study has a massive sample, it is limited to one city with its particular circumstances. But fret not, there is a systematic review published in December 2020 by Oyungerel Byambasuren et al titled “Estimating the extent of asymptomatic COVID-19 and its potential for community transmission: Systematic review and meta-analysis” that include studies from seven countries.
The overall sample was 21,708 at-risk persons tested, of which 663 tested positive, including 111 asymptomatic cases (16.7% of positive cases). As for asymptomatic transmission, there were five studies that reported data.
The asymptomatic transmission rates ranged from none to 2.2%, whereas symptomatic transmission rates ranged between 0.8% and 15.4%. Cycle threshold from real-time RT-PCR assays or the viral load did not differ between asymptomatic and symptomatic individuals in three of the studies (10, 14, 16). Overall, the RR of asymptomatic transmission was 42% lower than that of symptomatic transmission (pooled RR 0.58, fixed-effects 95% CI 0.335 to 0.994, p = 0.047; RR 0.38, random-effects 95% CI 0.13 to 1.083, p = 0.07; I2 = 43.4%).
In other words, sweet not much of sweet not much is… well, sweet not much.
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