During the Covid-19 pandemic, public health and infectious disease authorities struggled to grasp, communicate and act on current evidence about how the SARS-COV-2 virus spreads. These shortcomings resulted in misguided policies and insufficient protection measures, contributing to 15 million potentially preventable deaths worldwide. With ongoing threats like tuberculosis (TB) – South Africa’s leading cause of death – and the need to prepare for future pandemics, it’s time to reevaluate prevention strategies. The World Health Organization (WHO) has made progress by updating definitions but there’s still room for improvement.
The Root of the Problem: Misunderstanding Disease Transmission
The most critical failure during the COVID-19 pandemic was misunderstanding how the virus spreads. Early advice focused on handwashing, surface cleaning, and maintaining physical distance greater than 1.5m – measures grounded in an outdated view of respiratory disease transmission. Health authorities clung to the “droplets vs. aerosols” model, which inaccurately categorized particles as either large droplets that quickly fall to the ground or smaller aerosols that remain suspended in the air for long distances.
In reality, when we breathe, talk, or cough, we produce and emit a wide spectrum of particle sizes. Many of these particles can linger long, and travel far. This makes airborne transmission more significant than close-contact droplet transmission, not only for Covid-19 but for TB.
The failure to recognize COVID-19’s airborne spread resulted in ineffective and wasteful interventions, such as spraying disinfectant in public spaces and empty rugby stadiums.
Image: https://www.youtube.com/watch?v=SXzIz6DrU2o Springboks play the British Lions in an empty stadium, 31 July 2021, Cape Town.
WHO’s New Terminology: A Step Forward
An April 2024 WHO report introduced the term “infectious respiratory particles” (IRPs) to replace the outdated “droplet vs. aerosol” distinction. This reflects the reality that particles vary in size and can stay airborne over different distances. Three transmission modes are described:
- Inhalation, where we breathe in IRPs.
- Direct deposition, where larger IRPs are propelled or sprayed by a cough or sneeze and land on someone’s face or mucous membranes.
- Contact, where we touch contaminated surfaces or people and then touch our face.
The terminology shift acknowledges that airborne disease transmission (inhalation/breathe) happens at both short and long distances, and that it’s a crucial if not dominant mode by which Covid-19 spreads.
However, WHO’s new umbrella term, “transmission through the air,” groups “breathe” together with “spray.” This grouping risks oversimplifying distinct processes and could lead to ineffective control measures. Surfaces too can be contaminated through the air.
Airborne Transmission: Source, Pathway, and Receptor
Transmission of airborne infections can be understood in three parts: source, pathway, and receptor.
Source: An infected person releases respiratory particles when talking, coughing, sneezing, or breathing.
Pathway: For the infection to spread, the agent must stay viable while traveling through the air, surviving environmental conditions over distance and time.
Receptor: The agent must reach and infect susceptible tissue, usually the respiratory tract, to cause disease.
This model helps target interventions. For example, exposure can be reduced by improving airflow (pathway), wearing masks (receptor), or isolating the source.
Determining Airborne Transmission
The source-pathway-receptor model also helps assess whether a disease spreads through the air, guiding public health interventions.
Source: Infectious aerosols must be generated. Strong evidence includes detecting the pathogen in aerosols from infected people. Moderate evidence includes aerosol release during medical procedures. Weaker evidence comes from finding pathogens on nearby surfaces.
Pathway: The pathogen must remain viable while airborne. Strong evidence includes data showing it stays infectious and causes transmission. Moderate evidence includes detecting genetic material in the air. Weak evidence is if the pathogen only survives on surfaces.
Receptor: The pathogen must reach human tissue to cause infection. Strong evidence includes human studies, moderate evidence from animals, and weak evidence involves identifying susceptible tissue.
Diseases like influenza and measles, are airborne, Ebola too, and this list will grow as more evidence emerges and is applied to this framework.
Learning from COVID-19: The Need for Action
The WHO report hasn’t yet translated into updated infection control protocols. It calls for more research to clarify transmission pathways.
However, many experts argue that we already know enough to take action. Delaying could cost lives, particularly in healthcare settings, where hospital-acquired COVID-19 makes up 20-50% of cases. In South Africa, where TB is the leading cause of death, the need for urgent measures is even more critical.
Protecting Healthcare Workers
A tragic outcome of the pandemic was the lack of adequate protection for healthcare workers. Many hospitals failed to provide well-fitted, high-quality masks, leading to preventable infections and deaths. A recent Rutgers University webinar convened experts to discuss airborne transmission and included shared stories of nurses and other frontline workers who were inadequately protected, some losing their lives.
Measuring Ventilation: A Key Step in Protecting Health
Ventilation is one of the most effective ways to reduce airborne transmission of diseases like COVID-19 and TB. Regularly measuring ventilation, especially in healthcare settings, is essential. CO2 sensors offer a simple way to do this. High CO2 levels indicate poor air exchange, suggesting that potentially infectious particles aren’t being cleared fast enough. In poorly ventilated spaces, stagnant air increases the risk of transmission. Routine monitoring helps identify areas for improvement, such as opening windows or adjusting HVAC (Heating, Ventilation and Cooling) systems.
Airflow patterns matter too. Poor flow can trap air (and pathogens) where people gather, instead of directing it toward ventilation or filtration systems. Optimising airflow in healthcare settings ensures that airborne particles are moved away from patients and staff.
The pandemic has driven new ventilation standards that prioritise health rather than comfort; ASHRAE Standard 241 (2023) focuses on “clean air” targets to reduce the spread of aerosols. Recognising clean air as a human right marks a shift in public health priorities.
What Needs to Change
Since COVID-19 and TB spread through the air, we should rethink our approach to respiratory pathogens:
Improving air quality in public spaces, schools, and healthcare facilities is essential. Better ventilation and/or reducing occupancy are first steps. Where this is limited or impossible, germicidal UV lights, air purifiers, and new technologies, can help. For example, daycare centres in Finland used air purifiers to achieve an 18% drop in infections, and a Cambridge ICU successfully removed SARS-CoV-2 and other pathogens from the air.
Healthcare facilities should adopt exposure and risk assessment methods to identify gaps.
Healthcare workers and vulnerable populations need access to high-quality masks like N95s. Surgical masks are useful for source control but aren’t enough for protection from airborne pathogens.
While the WHO calls for more research, we already have enough evidence to act. Delaying prevention updates leaves healthcare workers, patients, and the public, at risk.