Surfing the COVID-19 scientific wave

Since the beginning of the COVID-19 pandemic, the number of articles published in scientific journals has skyrocketed; unfortunately, the quality of many of these articles leaves much to be desired.1, 2 We read with interest two publications from the same group3, 4 whose objectives were to demonstrate that normal speech generates droplets that can be suppressed by covering the mouth of a speaker and aerosols that persist for several minutes. Briefly, the authors used fluorescent green light to illuminate particles emitted by a person's mouth when speaking normally in a confined black box and filmed the interior of the black box. The words spoken by the participant were “stay healthy”, chosen by the authors as the “th” sound is known to emit droplets. Unsurprisingly, the authors found that the speaker emitted droplets of various sizes that were suppressed by covering the mouth.³ On the basis of a set of assumptions, the group produced a model suggesting that aerosols smaller than 5 μm were generated. The group concluded that normal speaking is associated with airborne transmission.⁴

We have issues with several assumptions made by the authors.⁴ First, the main assumption in the model is that dehydration is key to reducing the diameter of the expelled droplets, allowing droplets to become aerosols. The experiment was done in an environment with a relative humidity of 27%, which is below the minimum recommended indoor relative humidity of 40%.⁵ Second, the authors assumed an average viral load in saliva of 7 × 10⁶ copies per mL on the basis of a prospective study⁶ wherein viral load was measured in sputum. Thus, they assume that viral load in sputum is the same as in saliva. The group also assume that every RNA copy detected is a potentially infectious virion, without acknowledging that in the cited study samples containing fewer than 10⁶ copies per mL never resulted in a viable virus being isolated. An additional required proof would be to show that the viable virus is infectious and that the load is higher than the infectious dose.⁷

The studies3, 4 have methodological flaws that limit their generalisability. We were surprised that experiments in one person were published in leading scientific journals. No report of the loudness, measured in decibels, was found in either manuscript, although in the videos it seems that in some cases the study participant was shouting, so the claim of normal speech is dubious.3, 4 The air in the black box might have been filtered by a high-efficiency particulate air filter, but in the 2·33 min preceding the beginning of the speech, we counted at least 12 instances where flying particles were observed.⁴ Also, the size of the box was small; the authors did not show that these particles could be found more than 60 cm away from the speaker (the maximum length of the black box). The duration of recorded speech was 25 s, but the results were artificially extrapolated to 1 min.⁴ Also, the presence of a fan at the bottom of the black box during the speech and for 10 s after the end of speech does not represent real-life conditions; a control condition with no fan would have been expected. Neither aspect is discussed.

In the abstract of one of the articles,⁴ it is stated that asymptomatic transmission is plausible, but its role has not been clearly elucidated and indeed is highly disputed.⁸ The authors were mistaken when stating that high viral loads were found in asymptomatic patients while referring to the study by Wölfel and colleagues.⁶ Only one patient reported being asymptomatic in the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak in Bavaria, Germany,⁹ and that patient was not included in Wölfel and colleagues' study,⁶ which included only hospitalised patients.

The title of one of the articles⁴ mentions SARS-CoV-2 transmission, yet the experiment had more to do with sialoquence than with SARS-CoV-2. Although the objectives of these studies are worthy, their findings have no immediate implications.