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Original article

Vol. 152 No. 0102 (2022)

Higher viral load and infectivity increase risk of aerosol transmission for Delta and Omicron variants of SARS-CoV-2

  • Michael Riediker
  • Leonardo Briceno-Ayala
  • Gaku Ichihara
  • Daniele Albani
  • Deyan Poffet
  • Dai-Hua Tsai
  • Samuel Iff
  • Christian Monn
Cite this as:
Swiss Med Wkly. 2022;152:w30133


BACKGROUND: Airborne transmission of SARS-CoV-2 is an important route of infection. For the wildtype (WT) only a small proportion of those infected emitted large quantities of the virus. The currently prevalent variants of concern, Delta (B1.617.2) and Omicron (B.1.1.529), are characterized by higher viral loads and a lower minimal infective dose compared to the WT. We aimed to describe the resulting distribution of airborne viral emissions and to reassess the risk estimates for public settings given the higher viral load and infectivity.

METHOD: We reran the Monte Carlo modelling to estimate viral emissions in the fine aerosol size range using available viral load data. We also updated our tool to simulate indoor airborne transmission of SARS-CoV-2 by including a CO2 calculator and recirculating air cleaning devices. We also assessed the consequences of the lower critical dose on the infection risk in public settings with different protection strategies.

RESULTS: Our modelling suggests that a much larger proportion of individuals infected with the new variants are high, very high or super-emitters of airborne viruses: for the WT, one in 1,000 infected was a super-emitter; for Delta one in 30; and for Omicron one in 20 or one in 10, depending on the viral load estimate used. Testing of the effectiveness of protective strategies in view of the lower critical dose suggests that surgical masks are no longer sufficient in most public settings, while correctly fitted FFP2 respirators still provide sufficient protection, except in high aerosol producing situations such as singing or shouting.

DISCUSSION: From an aerosol transmission perspective, the shift towards a larger proportion of very high emitting individuals, together with the strongly reduced critical dose, seem to be two important drivers of the aerosol risk, and are likely contributing to the observed rapid spread of the Delta and Omicron variants of concern. Reducing contacts, always wearing well-fitted FFP2 respirators when indoors, using ventilation and other methods to reduce airborne virus concentrations, and avoiding situations with loud voices seem critical to limiting these latest waves of the COVID-19 pandemic.


  1. Riediker M, Tsai DH. Estimation of Viral Aerosol Emissions From Simulated Individuals With Asymptomatic to Moderate Coronavirus Disease 2019. JAMA Netw Open. 2020 Jul;3(7):e2013807.
  2. Riediker M, Monn C. Simulation of SARS-CoV-2 aerosol emissions in the infected population and resulting airborne exposures in different indoor scenarios. Aerosol Air Qual Res. 2021;21(2):200531.
  3. UK Health Security Agency. SARS-CoV-2 variants of concern and variants under investigation in England [Internet]. 2021 Dec [cited 2021 Dec 16]. Report No.: Technical briefing 31. Available from:
  4. Garcia-Beltran WF, St Denis KJ, Hoelzemer A, Lam EC, Nitido AD, Sheehan ML, et al. mRNA-based COVID-19 vaccine boosters induce neutralizing immunity against SARS-CoV-2 Omicron variant [Internet]. Infectious Diseases (except HIV/AIDS); 2021 Dec [cited 2021 Dec 16]. Available from:
  5. Jones TC, Biele G, Mühlemann B, Veith T, Schneider J, Beheim-Schwarzbach J, et al. Estimating infectiousness throughout SARS-CoV-2 infection course. Science. 2021 Jul;373(6551):eabi5273.
  6. Li B, Deng A, Li K, Hu Y, Li Z, Xiong Q, et al. Viral infection and transmission in a large, well-traced outbreak caused by the SARS-CoV-2 Delta variant [Internet]. Epidemiology; 2021 Jul [cited 2021 Sep 24]. Available from:
  7. Teyssou E, Delagrèverie H, Visseaux B, Lambert-Niclot S, Brichler S, Ferre V, et al. The Delta SARS-CoV-2 variant has a higher viral load than the Beta and the historical variants in nasopharyngeal samples from newly diagnosed COVID-19 patients. J Infect. 2021 Oct;83(4):e1–3.
  8. Wang Y, Chen R, Hu F, Lan Y, Yang Z, Zhan C, et al. Transmission, viral kinetics and clinical characteristics of the emergent SARS-CoV-2 Delta VOC in Guangzhou, China. EClinicalMedicine. 2021 Oct;40:101129.
  9. Pan Y, Zhang D, Yang P, Poon LL, Wang Q. Viral load of SARS-CoV-2 in clinical samples. Lancet Infect Dis. 2020 Apr;20(4):411–2.
  10. Nicas M. The near field/far field model with constant application of chemical mass and exponentially decreasing emission of the mass applied. J Occup Environ Hyg. 2016 Jul;13(7):519–28.
  11. van Doremalen N, Bushmaker T, Morris DH, Holbrook MG, Gamble A, Williamson BN, et al. Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1. N Engl J Med. 2020 Mar 17;NEJMc2004973.
  12. Tsai DH, Lin JS, Chan CC. Office workers’ sick building syndrome and indoor carbon dioxide concentrations. J Occup Environ Hyg. 2012;9(5):345–51.
  13. Allen JG, MacNaughton P, Satish U, Santanam S, Vallarino J, Spengler JD. Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments. Environ Health Perspect. 2016 Jun;124(6):805–12.
  14. SECO. Wegleitung zur Verordnung 3 zum Arbeitsgesetz (SR 822.113; ArGV 3) [Guidance to ordinance 3 of the working law]. Swiss Confederation, State Secretariat for Economics SECO; 2020 Aug p. 316_5. Report No.: Art. 16 Raumklima [indoor climate].
  15. Hamner L, Dubbel P, Capron I, Ross A, Jordan A, Lee J, et al. High SARS-CoV-2 Attack Rate Following Exposure at a Choir Practice - Skagit County, Washington, March 2020. MMWR Morb Mortal Wkly Rep. 2020 May;69(19):606–10.
  16. Jang S, Han SH, Rhee JY. Cluster of Coronavirus Disease Associated with Fitness Dance Classes, South Korea [Internet]. Emerg Infect Dis. 2020 Aug;26(8):1917–20. [cited 2020 May 26] Available from:
  17. Park SY, Kim YM, Yi S, Lee S, Na BJ, Kim CB, et al. Coronavirus Disease Outbreak in Call Center, South Korea. Emerg Infect Dis. 2020 Aug;26(8):1666–70.
  18. Hijnen D, Marzano AV, Eyerich K. GeurtsvanKessel C, Giménez-Arnau AM, Joly P, et al. SARS-CoV-2 Transmission from Presymptomatic Meeting Attendee, Germany. Emerg Infect Dis [Internet]. 2020 Aug [cited 2020 May 15];26(8). Available from:
  19. Mlcochova P, Kemp S, Dhar MS, Papa G, Meng B, Mishra S, et al. SARS-CoV-2 B.1.617.2 Delta variant emergence and vaccine breakthrough [Internet]. Microbiology; 2021 May [cited 2021 Jul 15]. Available from:
  20. Bagheri G, Thiede B, Hejazi B, Schlenczek O, Bodenschatz E. An upper bound on one-to-one exposure to infectious human respiratory particles. Proc Natl Acad Sci USA. 2021 Dec;118(49):e2110117118.
  21. Centers for Disease Control and Prevention (CDC). Laboratory performance evaluation of N95 filtering facepiece respirators, 1996. MMWR Morb Mortal Wkly Rep. 1998 Dec;47(48):1045–9.
  22. Low HT, Chew YT, Zhou CW. Pulmonary airway reopening: effects of non-Newtonian fluid viscosity. J Biomech Eng. 1997 Aug;119(3):298–308.
  23. Halpern D, Grotberg JB. Surfactant effects on fluid-elastic instabilities of liquid-lined flexible tubes: a model of airway closure. J Biomech Eng. 1993 Aug;115(3):271–7.
  24. Johnson GR, Morawska L. The mechanism of breath aerosol formation. J Aerosol Med Pulm Drug Deliv. 2009 Sep;22(3):229–37.
  25. Asadi S, Wexler AS, Cappa CD, Barreda S, Bouvier NM, Ristenpart WD. Aerosol emission and superemission during human speech increase with voice loudness. Sci Rep. 2019 Feb;9(1):2348.

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