• Stresses the importance of following health guidelines

As Sri Lanka sees an activation of a new cluster which recorded the highest number of cases within a day at 293 on 10 July, it is important that we remind ourselves of how Covid-19 could transmit within communities. The World Health Organisation (WHO) in their latest scientific brief addressed “Transmission of SARS-CoV-2: Implications for infection prevention precautions”, in which the WHO together with the scientific community, has been actively discussing and evaluating whether SARS-CoV-2, from which Covid-19 derived, may also spread through aerosols in the absence of aerosol-generating procedures, particularly in indoor settings with poor ventilation. At this point the biggest concern is the ability of the virus to transmit through air which is referred to as airborne transmission. Airborne transmission is defined as the spread of an infectious agent caused by the dissemination of droplet nuclei (aerosols) that remain infectious when suspended in air over long distances and time. Airborne transmission of SARS-CoV-2 can occur during medical procedures that generate aerosols (“aerosol-generating procedures”). Recent clinical reports of health workers exposed to Covid-19 index cases, not in the presence of aerosol-generating procedures, found no social transmission when contact and droplet precautions were appropriately used, including the wearing of medical masks as a component of the personal protective equipment (PPE). The observations suggest that aerosol transmission did not occur in this context. However, WHO noted that further studies were needed to determine whether it is possible to detect viable SARS-CoV-2 in air samples from settings where no procedures that generate aerosols are performed and what role aerosols might play in transmission. Outside of medical facilities, some outbreak reports related to indoor crowded spaces have suggested the possibility of aerosol transmission, combined with droplet transmission, for example, during choir practice, in restaurants, or in fitness classes. In these events, short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out. However, the detailed investigations of these clusters suggest that droplet and fomite transmission could also explain human-to-human transmission within these clusters. Furthermore, the close-contact environments of these clusters may have facilitated transmission from a small number of cases to many other people (e.g. superspreading event), especially if hand hygiene was not performed and masks were not used when physical distancing was not maintained. Current evidence suggests that SARS-CoV-2 is primarily transmitted between people via respiratory droplets and contact routes – although aerosolisation in medical settings where aerosol-generating procedures are used is also another possible mode of transmission – and that transmission of Covid-19 is 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 metre and for prolonged periods of time), when not wearing appropriate PPE. Transmission can also occur from people who are infected and remain asymptomatic, but the extent to which this occurs is not fully understood and requires further research as an urgent priority. The role and extent of airborne transmission outside of healthcare facilities, and in particular in close settings with poor ventilation, also requires further study. As research continues, WHO expects to gain a better understanding about the relative importance of different transmission routes, including through droplets, physical contact, and fomites; the role of airborne transmission in the absence of aerosol-generating procedures; the dose of virus required for transmission to occur, the characteristics of people and situations that facilitate superspreading events such as those observed in various closed settings, the proportion of infected people who remain asymptomatic throughout the course of their infection; the proportion of truly asymptomatic persons who transmit the virus to others; the specific factors that drive asymptomatic and pre-symptomatic transmission; and the proportion of all infections that are transmitted from asymptomatic and pre-symptomatic individuals. Main findings

• Understanding how, when, and in what types of settings SARS-CoV-2 spreads between people is critical to develop effective public health and infection prevention measures to break chains of transmission
• Current evidence suggests that transmission of SARS-CoV-2 occurs primarily between people through direct, indirect, or close contact with infected people through infected secretions such as saliva and respiratory secretions, or through their respiratory droplets, which are expelled when an infected person coughs, sneezes, talks, or sings
• Airborne transmission of the virus can occur in healthcare settings where specific medical procedures, called aerosol-generating procedures, generate very small droplets called aerosols. Some outbreak reports related to indoor crowded spaces have suggested the possibility of aerosol transmission, combined with droplet transmission, for example, during choir practice, in restaurants, or in fitness classes
• Respiratory droplets from infected individuals can also land on objects, creating fomites (contaminated surfaces). As environmental contamination has been documented by many reports, it is likely that people can also be infected by touching these surfaces and touching their eyes, nose, or mouth before cleaning their hands
• Based on what the WHO knows, transmission of Covid-19 is primarily occurring from people when they have symptoms, and can also occur just before they develop symptoms, when they are in close proximity to others for prolonged periods of time. While someone who never develops symptoms can also pass the virus to others, it is still not clear to what extent this occurs and more research is needed in this area
• 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

To prevent transmission, WHO recommends a comprehensive set of measures including:

• Identify suspect cases as quickly as possible, test, and isolate all cases (infected people) in appropriate facilities
• Identify and quarantine all close contacts of infected people and test those who develop symptoms so that they can be isolated if they are infected and require care
• Use fabric masks in specific situations, for example, in public places where there is community transmission and where other prevention measures, such as physical distancing, are not possible
• Use of contact and droplet precautions by health workers caring for suspected and confirmed Covid-19 patients, and use of airborne precautions when aerosol-generating procedures are performed
• Continuous use of a medical mask by health workers and caregivers working in all clinical areas, during all routine activities throughout the entire shift
• At all times, practice frequent hand hygiene, physical distancing from others when possible, and respiratory etiquette; avoid crowded places, close-contact settings, and confined and enclosed spaces with poor ventilation; wear fabric masks when in closed, overcrowded spaces to protect others; and ensure good environmental ventilation in all closed settings and appropriate environmental cleaning and disinfection

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