RR:C19 Evidence Scale rating by reviewer:
Reliable. The main study claims are generally justified by its methods and data. The results and conclusions are likely to be similar to the hypothetical ideal study. There are some minor caveats or limitations, but they would/do not change the major claims of the study. The study provides sufficient strength of evidence on its own that its main claims should be considered actionable, with some room for future revision.
Since the outbreak of Covid-19 there has been an increasing interest in the modes of transmission of infectious diseases, with a particular reborn focus on airborne transmission.
With the widespread transmission of monkeypox, related to the airborne transmitted smallpox virus, concern for possible transmission by inhalation is obvious. Monkeypox has since previously been considered a pathogen with potential airborne transmission and most infection control guidelines have recommended airborne precautions during hospital care, based on a few outbreak investigations and animal transmission studies.
The present study is the first to conduct air sampling of hospital rooms with patients with active monkeypox diseases, and one of few to investigate contamination of surfaces and PPE.
Five patients were involved in the sampling, of air and surfaces in four different rooms. (The exact number of different patients involved is difficult to extract from the present manuscript). Some clinical data of the patients are included, but the extent of skin lesions and other symptoms is not described.
The result showed widespread surface contamination (66 positive of 73 samples), similar to previous studies. Also no-touch surfaces such as air ventilation outlets were positive.
Five of 15 (or was it 20?) air samples were positive, of which three were collected after bedlinen change. The authors are implicating a risk aerosolisation of virus containing skin particles during this procedure. This finding is in line with a previously reported outbreak investigation were bed linen change was implicated in the transmission to a healthcare worker. The case for virus aerosolisation during bedlinen change is reasonable but not strongly supported with this study. Room air was positive before bed change in both occasions and the small reduction in ct-value in just one case must be interpreted with caution.
Methodologically this study is sound. It is not stated exactly how large surface areas were sampled. Air sampling was performed mostly with a Sartoirus M8 impactor. The particle size limitation for this sampler is not mentioned in the manuscript, but the upper size of particles collected is likely larger than the conservative (and disputed) size limit for true airborne transmission of 5 μm. The study includes air collecting at a distance of 1.5 m from the bed to implicate smaller particles with airborne potential. A reasonable amount of air was collected with the M8 Sartorius, 250-500 liter. Considering the high ventilation rate within the rooms (10 air changes per hour), finding MPXV in the air with the modest volume collected, suggests that non-negligible virus air concentration can arise.
The exact number of air samples collected is unclear in the present manuscript. In the table 20 patient room samples are presented, but in the abstract 15 samples are mentioned. Another 12 air samples were collected in anterooms and corridors, of which one is presented as positive with a high ct-value in the table but not otherwise mentioned in the manuscript. The absence of further comments on this finding is confusing.
The authors also try to ascertain the PCR-positive samples collected at surfaces and in the air may represent infective viruses and not just inert PCR fragments, by culture, studying cytopathic effect and repeated PCR of the supernatant. The result show with reasonable certainty that at least two samples represented infective viruses, by a clear reduction in ct-value during culture, even though the cytopathic effect was less evident. Tecovirimate might affect the ability to cause a cytopathic effect.
To conclude, this is a small and methodologically sound study with only five patients with active monkeypox included, in which a widespread environmental contamination of MPXV is presented. This is in line with previous studies, which only have included 1-2 patients. Contamination of PPE and doffing area is also shown.
In two rooms with two different patients MRXV is also found in air samples, indicating a potential risk for airborne transmission in confined spaces where patients with active skin lesions reside and sleep. As the authors state; the findings support recommendations for healthcare workers interacting with patients with confirmed MPXV infection to use suitable PPE, including respiratory protective equipment. To find a pathogen in the air is not equivalent to significant airborne transmission and a wider interpretation of this study to suggest a more general community risk of airborne transmission of monkeypox should be avoided.
The preprint has now been published in The Lancet Microbe.