The study is based on the assumptions that SARS-CoV-2 transmission is reduced by about 70% following full vaccination, and by about 80% and 99% for surgical masks and N95 or FFP2 masks, respectively, if worn universally.
Keeping a two-meter distance between people and ensuring proper ventilation indoors reduces droplet and aerosol transmission by 90%, though droplet transmission is mostly reduced by distancing and aerosol-mediated spread by good ventilation.
Reducing the number of people that encounter a potentially infected individual brings down the effective reproduction number (Re) in direct proportion.
What are the findings?
The study shows that for each individual measure, the extent of compliance determines the Re reduction. For instance, if 70% of the eligible population are vaccinated, the Re drops to 2.5, from the original reproduction number R0 of 5.
Even if the whole population was vaccinated, say the scientists, the Re would be above 1, indicating continued spread, and in fact, it could be as high as 1.5, triggering further exponential spread.
Thus, even though the vaccines do protect against the disease and its severe outcomes, they do not contain the transmission of the virus by themselves.
At such high R0 values, the vaccination coverage would have to be more than 85%, for a vaccine with 95% efficacy against infection and transmission, to reduce the Re to below 1 and thus prevent the expansion of the pandemic. The scientists consider that only about half of the population complies with social distancing, masking, and ventilation norms, meaning that this would leave the Re above 1.
With a combination of all these measures, even at 50% compliance the Re comes below 1 if N95 or FFP2 masks are used, and close to 1 if surgical masks are used. The use of the N95 or FFP2 masks along with social distancing and ventilation would still keep the Re below 1 with 50% compliance. Note that this is brought about without vaccination.
Without universal mask use, the reduction in Re with only distancing and ventilation at a vaccination rate of 70% would be inadequate to prevent future waves of infection. With high rates of compliance with mask use, however, highly vaccinated populations could break the chain of transmission. Conversely, moderate vaccination coverage, universal masking, social distancing, and ventilation must all be complied with at high rates to achieve a Re less than 1.
If all measures are complied with at about 30% each, this too will effectively prevent further waves of COVID-19. These results not only confirm earlier studies but offer a useful model for combined protective measures. They could also be used to explain why and how masks are useful in stopping aerosol and droplet transmission, and how different masks can be used in different situations, and how they are also useful outdoors.
Ventilation of indoor spaces could also be improved using relatively inexpensive methods like exhaust fans and carbon dioxide sensors, while testing and isolation protocols should be implemented more extensively at workplaces and schools.
With high surges of new infections, higher compliance rates or intensive reductions in contacts (lockdowns) will be required to effectively break viral transmission. Importantly, for schools, doing two to three tests a week along with social distancing, masking, and ventilation can help contain the viral spread.
Overall, the strong dependence of Re on compliance highlights the importance of situations where masking, distancing & ventilation or isolation are not possible, impractical, or ineffective.”
What are the implications?
The study finds that viral transmission cannot be contained effectively using individual interventions at realistic levels of compliance. However, the use of multiple measures – physical distancing, masking, indoor ventilation, and test-isolation protocols – can push up the effectiveness of intervention and work synergistically with each other as well as with vaccination, to push the Re below 1.
The investigators caution, “We are not suggesting to promote these physical measures without vaccination, which would also be missing the benefit of reducing both the transmission of the virus and the severity of the disease by immunization.” The findings do show the marked benefits of applying purely physical measures to reduce the reproduction number.
Such lower levels of compliance are considered realistic in keeping with recent modeling and real-life studies, both due to practical constraints, as with eating or drinking outside or in schools, as well as the use of masks with varying levels of filtration efficiencies, and similar difficulties with physical distancing. Ventilation norms may be non-feasible in common indoor environments.
Thus, even at 50% compliance, these interventions in combination act synergistically to reduce the Re to well below 1. A greater number of measures, or higher compliance rates, would be needed if newer variants with higher R0 values emerge.
We suggest that the presented approach and results can be used to design and communicate efficient strategies for mitigating the COVID-19 pandemic.”
Moreover, the simplicity of the modeling methods, coupled with the easily understandable graphs, could help educate the public about the importance of cooperating with such interventions.
Additionally, the need to comply with the whole set of measures, without excluding anyone, can be communicated by showing how the combination is required to effectively break or suppress the wave of infections and thus mitigate the pandemic.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.