The debate around the effectiveness of masks in preventing the spread of respiratory viruses, including during the COVID-19 pandemic, has been complex and has seen shifts in scientific consensus over time. Here's a detailed look at the evolution of the evidence and the associated controversies:
Initial Stance in 2019:

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  Before the onset of the COVID-19 pandemic, the evidence for mask-wearing by the general public to prevent transmission of influenza and other respiratory viruses was mixed. A 2019 WHO review concluded there was "no evidence" that masks were effective in reducing transmission of influenza in community settings (). This was largely due to the lack of robust randomized controlled trials (RCTs) demonstrating clear benefits.
  
  

Shift During the COVID-19 Pandemic:

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  As SARS-CoV-2 emerged, the nature of the virus, including its transmission characteristics (like asymptomatic and pre-symptomatic spread), prompted a re-evaluation of mask use. The urgency of the situation led to different types of evidence being considered:
  
  
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    Mechanistic Evidence: There was a strong theoretical basis for masks preventing transmission by blocking respiratory droplets. Studies and reviews began to argue from this mechanistic perspective, suggesting masks should work based on the physics of how viruses spread ().
    
    
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    Observational Studies: With the onset of the global health emergency, observational studies started to accumulate, showing correlations between mask mandates or widespread mask use and reduced transmission rates of COVID-19. For instance, research indicated that mask wearing was associated with declines in the daily growth rates of COVID-19 cases in the U.S. ().
    
    
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    Real-World Data: Countries and regions with early mask adoption often reported better control over the spread of the virus, which influenced public health recommendations. This was supported by studies examining the impact of mask mandates in various settings ().
    
    
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    RCTs and Systematic Reviews: While initial RCTs on mask use for respiratory viruses were inconclusive, later reviews and studies, some of which included non-RCT data, began to suggest that masks could reduce transmission when used correctly and widely (). However, there were criticisms about the design and applicability of these RCTs, especially when applied to a novel virus like SARS-CoV-2 ().
    
    

Conclusion:

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  The shift from "no scientific support" for masks to a broad endorsement wasn't due to corruption but rather a combination of evolving scientific understanding, new types of evidence (observational, mechanistic), and the unique challenges of an ongoing global health crisis. The scientific community has learned more about SARS-CoV-2 transmission, leading to updated recommendations. However, the debate reflects the complexities of public health science where evidence must sometimes be acted upon before it's fully definitive.