numeracy and statistics
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Aerosol generating procedures (AGPs) and their mitigation in international guidelines: Fallow time
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The recommendations for fallow time in dental practice following an AGP appears almost random in its application across half of international guidelines. Its basic idea appears to be centred on a single precautionary principle based on simulation studies that are only weakly supported by evidence. Policy makers will need to take a more balanced approach when assessing the benefits and harms fallow time creates.
With the publication of, ‘Aerosol Generating Procedures and their Mitigation in International Dental Guidance Documents – A Rapid Review’ by the COVID-19 Dental Services Evidence Review (CoDER) Working Group (Clarkson et al., 2020) it was interesting to note their finding regarding fallow time for non Covid-19 patients:
- 48% of the guidelines recommend having a fallow period.
- The amount of time recommended varied (2-180 mins) between guidelines and also within guidelines, depending on environmental mitigation.
- None of the fallow period recommendations referenced any scientific evidence.
The fallow period is the ‘time necessary for clearance of infectious aerosols after a procedure before decontamination of the surgery can begin’ (FGDP, 2020), and this has caused considerable discussion/stress amongst the dentist returning to practice after lockdown in the UK (BAPD, 2020, Heffernan, 2020).
The fallow times following an AGP for both Covid-19 negative patients and Covid-19 positive patients are presented in Figure 1 and Table 2 using the data from the CoDER rapid review:
Figure 1. Stacked bar chart representing fallow times for both Covid negative and positive patients
Since the fallow times are not normally distributed the median value was utilised and the 95% confidence limit approximated according to Hill (Hill, 1987).
Table 1. Median fallow time
|Aerosol generating procedure||Median Fallow time (mins)|
|Non Covid-19 patient||15 (95% CI: 15 to 30)|
|Covid-19 positive patient||20 (95% CI: 10 to 60)|
Much of the theory around the need for fallow time is based on the aerosol transmission of the Coronavirus SARS-CoV-2 and the need to allow the aerosol to settle or be physically removed from the room via ventilation or filtration. Initially the WHO supported the idea that spread was predominantly caused by large droplets and contact (WHO, 2020a) but under increased lobbying from the scientific community to include airborne transmission as a significant factor (Morawska and Milton, 2020) the WHO amended their position (WHO, 2020b) on the 9th July.
The evidence for airborne transmission of Coronavirus SARS-CoV-2 is uncertain and mostly based on mathematical modelling (Buonanno et al., 2020) but where there has been limited observational data Hota and co-workers concluded that the virus was not well transmitted by the airborne route compared to measles, SARS-1 or Tuberculosis (Hota et al., 2020).
The reason there is no strong scientific evidence for fallow time and airborne transmission is possibly because it is based on two conceptual arguments:
The Precautionary Principle (PP)
The precautionary principle (PP) states that if an action or policy has a suspected risk of causing severe harm to the public domain (affecting general health or the environment globally), the action should not be taken in the absence of scientific near-certainty about its safety. Under these conditions, the burden of proof about absence of harm falls on those proposing an action, not those opposing it (Taleb et al., 2020). The confusion at the moment is that the PP has been reversed and the action is evidence-based ‘normal/enhanced PPE and cross infection policy’ rather than the imposition of untested application of ‘fallow time’ in general practice.
The Independent Action Hypothesis (IAH)
The IAH states that each virion has an equal, non-zero probability of causing a fatal infection especially where airborne spread via small droplets (5μm) is the proposed method of transmission (Stadnytskyi et al., 2020). The reality of the IAH is that evidence supporting this theory is oversimplified and indirect, based mainly on small sample studies of moth larvae, and tobacco mosaic virus (Zwart et al., 2009, Cornforth et al., 2015).
The main problem for anyone challenging the PP and IAH is having to prove a ‘near certainty of safety’ when there are many confounding factors in play, and the application of a ‘non-zero’ probability of a single inhaled virus causing death results in an ill-defined probability of risk (Taleb et al., 2020). Unfortunately, once the PP was been invoked designing challenge studies to create the scientific proof that the action is safe can be unethical in humans or experimentally impossible due to the effect of low viral prevalence in the community on statistical power.
One solution may be to rapidly assess the retrospective infection rates associated with the provision of dental care in those countries with extended fallow times and compare then to countries of similar demographics that do not have fallow time (Table 1). Policy makers may need to revise their current interpretation of the precautionary principle and IAH based on the fact that in a pandemic situation there may be multiple interacting factors that can cause significantly more second and third order harm to the public domain than the virus itself.
Table 2. Countries with/without(bold) fallow time. ND – no data
|Country||Non Covid-19||Covid-19||Country||Non Covid-19||Covid-19|
|Dominican Republic||10||10||Costa Rica||ND||ND|
|UK – NI||60||60||Slovakia||ND||ND|
|UK – Wales||60||60||Slovenia||ND||10|
|Paraguay||180||180||UK – Scotland||ND||ND|
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