Airborne transmission of MPXV and its aerosol dynamics under different viral load conditions

Since May, 2022, outbreaks of human mpox (formerly known as monkeypox) have occurred in many countries across Europe and the Americas, which are outside the typical mpox-endemic countries located in the west and central Africa.1, 2, 3, 4 As of Jan 15, 2023, 84 733 infections have been reported globally.¹ The mode of transmission for human monkeypox virus (MPXV) has thus far been considered to be direct physical contact with lesions or the fluids contained within them during intimate contact.² However, ongoing outbreaks since 2022 have triggered new debates regarding roles of other possible transmission modes of MPXV, such as the airborne route.²

The recent experimental results and analytical studies must be considered when discussing this topic. Bruno Hernaez and colleagues⁴ reported that in their study of 44 patients with symptomatic mpox at two health centres in Spain, high loads of MPXV DNA were identified in 35 (85%) of 41 saliva samples and that infectious MPXV was recovered from 22 (67%) of 33 saliva samples positive for MPXV DNA. The reported maximum viral load in saliva samples of patients reached approximately 1×10⁸ copies of viral genome per mL,⁴ and the minimum size of respiratory particles carrying the virus in those viral load conditions was calculated to be more than 25 μm, by use of aerosol dynamics theories and the reported viral load (appendix).4, 5, 6, 7

Hugh Adler and colleagues⁸ reported on the viral loads (denoted by Ct values) of MPXV identified in the upper respiratory tracts of seven patients in the UK. The viral loads of the patients, based on these Ct values, were estimated to be less than 10⁶ copies per mL.4, 8 The minimum size of respiratory particles carrying MPXV virions was estimated to be more than 100 μm using aerosol dynamics theories and the aforementioned viral loads.4, 5, 6, 7, 8

If respiratory particles (ie, particles emitted from respiratory tracts) are larger than the cutoff size between aerosols and droplets, they are categorised as droplets (ie, large earthbound respiratory particles), whereas if they are smaller than the cutoff size, they are catagorised as aerosol particles (ie, tiny airborne particles; appendix). The cutoff size between aerosols and droplets is highly dependent on the surrounding airflow conditions, including relative humidity and airflow velocity.⁶ The standard cutoff size between aerosols and droplets, which has been under debate in aerosol dynamics, ranges between 5 μm and 100 μm.6, 7, 9

Large-sized respiratory particles (>200 μm) are destined to be earthbound owing to gravitational sedimentation time and moisture evaporation period;6, 9 thus, airborne transmission of viruses inside these large respiratory particles is hardly possible. However, viruses in transition-sized respiratory particles (between 5 μm and 200 μm) can transform into viral aerosols under adequate conditions, such as high-speed airflow with low humidity. Therefore, airborne transmission of viruses inside these transition-sized respiratory particles is possible under the adequate conditions (appendix).

According to medical mathematics analysis, high viral loads in respiratory fluids reduce the minimum size of respiratory particles that can transmit the virus.6, 7 Thus, a high viral load condition in respiratory fluids is the prerequisite for generating viral aerosols and airborne transmission of viruses (appendix).6, 7 The estimated minimum sizes of respiratory particles carrying MPXV, based on reported viral loads in respiratory fluids, are sufficiently small to generate viral aerosols.

This analysis based on aerosol dynamics with viral load data is in harmony with two sets of experimental findings in The Lancet Microbe. Susan Gould and colleagues³ detected infectious MPXV in air samples collected during a bedding change for patients in a UK hospital. The authors also detected MPXV DNA in air samples collected at distances of more than 1·5 m from the beds of patients.³ Furthermore, Hernaez and colleagues⁴ reported that from 40 to about 9×10³ MPXV genomes per m³ were detected in air samples collected at two health centres in Spain. Therefore, these novel findings and the analysis with aerosol dynamics show that aerosols carrying MPXV could be present in environments where patients have resided and that airborne transmission of MPXV can occur. As to the stability of the virus in air, it was reported that the viability of the airborne MPXV was maintained for 90 h under artificial test conditions in a rotating chamber.¹⁰

Although the binding affinities of MPXV to human cells are unclear, the airborne transmission route must be considered as a possible transmission mode under the conditions of current experimental and analytical findings. Therefore, monitoring airborne viruses as well as non-pharmaceutical interventions (ventilation, aerosol control, or respirator), will help control the spread of MPXV.

I declare no competing interests.