World Health Organisation (WHO) has said that airborne transmission of coronavirus cannot be ruled out in health care settings where specific medical procedures generate very small droplets called aerosols. In its scientific brief on July 9, WHO also points to some reports to suggest that indoor crowded spaces - restaurants, fitness classes etc - also have higher risk of aerosol transmission, combined with droplet transmission.
The latest version of the scientific brief titled 'Transmission of SARS-CoV-2: implications for infection prevention precautions' admits that many unanswered questions about transmission of SARS-CoV-2 remain, and research seeking to answer those questions is ongoing. However, its says that current evidence suggests that SARS-CoV-2 is primarily transmitted between people via respiratory droplets and contact routes.
Aerosolisation in medical settings where aerosol generating procedures are used has been highlighted as another possible mode of transmission. The transmission of COVID-19 occurring from people who are pre-symptomatic or symptomatic to others in close contact (direct physical or face-to-face contact with a probable or confirmed case within one meter and for prolonged periods of time), when not wearing appropriate PPE seems to be the most common route.
The WHO brief also says that transmission can occur from people who are infected and remain asymptomatic, but the extent to which this occurs is not fully understood and requires further research on priority. It says that the role and extent of airborne transmission outside of health care facilities, and in particular in close settings with poor ventilation, also requires further study.
This scientific brief is not a systematic review, rather, it reflects the consolidation of rapid reviews of publications in peer-reviewed journals and of non-peer-reviewed manuscripts on pre-print servers, undertaken by WHO and partners, WHO cautions.
The WHO brief concludes that urgent high-quality research is needed to elucidate the relative importance of different transmission routes, the role of airborne transmission in the absence of aerosol generating procedures, the dose of virus required for transmission to occur, the settings and risk factors for superspreading events and the extent of asymptomatic and pre-symptomatic transmission.
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