ABSTRACT
BackgroundThe public and scientific discourse on how to mitigate the COVID-19 pandemic is often focused on the impact of individual protective measures, in particular on vaccination. In view of changing virus variants and conditions, however, it seems not clear if vaccination or any other protective measure alone may suffice to contain the transmission of SARS-CoV-2. MethodsHere, we investigate the effectiveness and synergies of vaccination and non-pharmaceutical interventions like masking, distancing & ventilation, testing & isolation, and contact reduction as a function of compliance in the population. Our new analysis accounts for the practical compliance in the population and for both droplet transmission and aerosol transmission. FindingsFor realistic conditions, we find that it would be difficult to contain highly contagious SARS-CoV-2 variants by any individual measure. Instead, we show how multiple synergetic measures have to be combined to reduce the effective reproduction number (Re) below unity for different basic reproduction numbers ranging from the SARS-CoV-2 ancestral strain up to measles-like values (R0 = 3 to 18). For R0 = 5 as reported for the Delta variant and [~]70% vaccination rate, the synergies of masking and distancing & ventilation with compliances around 30% appear sufficient to keep Re < 1. In combination with 2-3 tests per week, this would work also at lower vaccination rates, e.g., in schools. InterpretationIf the Omicron variant were to reach R0 = 8, it could still be contained with the synergetic measures outlined above. In case of measles-like transmissibilities (R0 = 12 to 18), higher compliances and testing rates or additional measures like general contact reductions would be required. The presented findings and approach can be used to design and communicate efficient strategies for mitigating the COVID-19 pandemic. FundingMax Planck Society. Research in contextO_ST_ABSEvidence before this studyC_ST_ABSStudies on how to mitigate the COVID-19 pandemic are often focused on the impact of individual protective measures, in particular on vaccination. The effectiveness of non-pharmaceutical interventions (NPIs) like masking or distancing & ventilation are often under debate due to a lack of understanding of different transmission pathways (droplet versus aerosol transmission) and protective measures, in particular for the efficacy of masking and contrasting randomized trial results under different conditions (virus-limited vs. virus-rich) and at different levels of practical compliance. Thus, in view of more contagious variants such as Delta or Omicron, it is not clear if vaccination or any other protective measure alone may suffice to contain the transmission of SARS-CoV-2. Added value of this studyOur analysis explicitly accounts for both droplet and aerosol transmission as well as for practical compliance in the population, which is the main reason for divergent results on the effectiveness of the same NPIs in different regions. This was not fully considered before and may have led to misunderstandings and misinformation about the actual effects of preventive measures. For realistic conditions, we find that it would be difficult to contain highly contagious SARS-CoV-2 variants by any individual measure. Instead, we show that combining multiple synergetic measures with realistic compliances can reduce Re below unity without lockdown. Implications of all the available evidenceOur findings and the presented scientific approach can be used to design and communicate efficient strategies for mitigating the COVID-19 pandemic for specific environments like schools as well as on a population level.
ABSTRACT
The role of aerosolized SARS-CoV-2 viruses in airborne transmission of COVID-19 is debated. The transmitting aerosol particles are generated through the breathing and vocalization by infectious subjects. Some authors state that this represents the dominant route of spreading, while others dismiss the option. Public health organizations generally categorize it as a secondary transmission pathway. Here we present a simple, easy-to-use spreadsheet algorithm to estimate the infection risk for different indoor environments, constrained by published data on human aerosol emissions, SARS-CoV-2 viral loads, infective dose and other parameters. We evaluate typical indoor settings such as an office, a classroom, a choir practice room and reception/party environments. These are examples, and the reader is invited to use the algorithm for alternative situations and assumptions. Our results suggest that aerosols from highly infective subjects can effectively transmit COVID-19 in indoor environments. This "highly infective" category represents about one fifth of the patients tested positive for SARS-CoV-2. We find that "super infective" subjects, representing the top few percent of positive-tested ones, plus an unknown fraction of less, but still highly infective, high aerosol-emitting subjects, may cause COVID-19 clusters (>10 infections), e.g. in classrooms, during choir singing and at receptions. The highly infective ones also risk causing such events at parties, for example. In general, active room ventilation and the ubiquitous wearing of face masks (i.e. by all subjects) may reduce the individual infection risk by a factor of five to ten, similar to high-volume HEPA air filtering. The most effective mitigation measure studied is the use of high-quality masks, which can drastically reduce the indoor infection risk through aerosols.
ABSTRACT
Airborne transmission by droplets and aerosols is important for the spread of viruses and face masks are a well-established preventive measure, but their effectiveness for mitigating COVID-19 is still under debate. We show that variations in mask efficacy can be explained by different regimes of virus abundance. For SARS-CoV-2, the virus load of infectious individuals can vary by orders of magnitude, but we find that most environments and contacts are in a virus-limited regime where simple surgical masks are highly effective on individual and population-average levels, whereas more advanced masks and other protective equipment are required in potentially virus-rich indoor environments such as medical centers and hospitals. Due to synergistic effects, masks are particularly effective in combination with other preventive measures like ventilation and distancing. One Sentence SummaryFace masks are highly effective due to prevailing virus-limited environments in airborne transmission of COVID-19.
