In this video, Dr Sam Bailey, along with Dr Robin Wakeling, state that there is no evidence for asymptomatic or pre-symptomatic transmission of SARS-CoV-2. (She also claims there is no evidence for any kind of SARS-Cov-2 transmission, but that's not the main question here.)

She cites correspondence from Wakeling with NZ official health authorities that they (both) dismiss as failing to provide credible references to their evidence (5 papers).

  • Low et al.; dismssed in the video as providing "no actual evidence [...] simply speculation based on modelling";

  • Byambasuren et al. dismissed as "amusing" because it's described as having found that only one case ("out of 305") was traced to an asymptomatic index, thus described as a statistical fluke;

  • Lee et al. dismissed as "the methodlogy in this study is clustering" and "in no way is it evidence that transmission has taken place";

  • Barnaby, Leo et al. dismissed for PCR case results like "a trumpet's walk", i.e. oscillating over time.

  • Bailey also dismisses the paper of Lau et al., including the idea that PCR cycle thresholds correlate with viral load; she also links and quotes from a published criticism of this paper in the same journal.

At 11m52 seconds she summarises:

My personal thoughts on asymptomatic transmission is that it has been promoted alongside a dazzling array of buzzwords and mantras such as "flatten the curve", "social distancing", "superspreaders" and so forth. All of them have had dramatic effects in scaring people into compliance, but have no sound scientific basis in how they have been applied in the current situation.

Is there evidence to support asymptomatic and pre-symptomatic transmission of the virus behind COVID-19?


2 Answers 2


There is actual scientific evidence for asymptomatic Sars cov 2 transmission.

I found about 50 scientific publications in the peer-reviewed literature that clarify this information (in a 10-minute search), so there is clearly some evidence which is much more than none.

Below are three of them with their conclusions quoted:

Our study confirms asymptomatic and human-to-human transmission through close contacts in familial and hospital settings.

  • Li, Chunyang, et al. "Asymptomatic and human-to-human transmission of SARS-CoV-2 in a 2-family cluster, Xuzhou, China." Emerging infectious diseases 26.7 (2020): 1626.

Five of the studies that conducted detailed contact investigations provided enough data to calculate a secondary attack rate according to the symptom status of the index cases (Fig 3) [36,65,66,90,111]. The summary risk ratio for asymptomatic compared with symptomatic was 0.35 (95% CI 0.1–1.27) and for presymptomatic compared with symptomatic people was 0.63 (95% CI 0.18–2.26) [66,90]. The risk of bias in the ascertainment of contacts was judged to be low in all studies.

  • Buitrago-Garcia, Diana, et al. "Occurrence and transmission potential of asymptomatic and presymptomatic SARS-CoV-2 infections: A living systematic review and meta-analysis." PLoS medicine 17.9 (2020): e1003346.

Asymptomatic persons seem to account for approximately 40% to 45% of SARS-CoV-2 infections, and they can transmit the virus to others for an extended period, perhaps longer than 14 days.

  • Oran, Daniel P., and Eric J. Topol. "Prevalence of asymptomatic SARS-CoV-2 infection: a narrative review." Annals of internal medicine 173.5 (2020): 362-367.
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    Commented May 11, 2021 at 15:46

There are many techniques that have been used to demonstrate the importance of asymptomatic or pre symptomatic transmission of SARS-CoV-2. Many of these are population/statistically based, which you seem to reject.

On an individual level whole genome sequencing is the gold standard test for person to person transmission. Because the virus acquires mutations at a high rate we can sequence the genome of the virus in the suspect source and target of the transmission, and a close match demonstrates that the suspicion is correct.

One such study that used this methodology in a healthcare situation where the contacts were known is here. They examined 3 instances of suspected transmission between healthcare professionals and patients where the source was either pre-symptomatic or asymptomatic. In all three cases whole genome sequencing confirmed 0 single nucleotide polymorphism differences between the patient and healthcare professional’s specimens.

This demonstrates with a very high confidence that the transmission occurred between the individuals tested. Because the situations were well monitored, we can know with a very high confidence that the source was either pre-symptomatic or asymptomatic. This demonstrates that such transmission occurs.


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