Researchers have been able to isolate virus from air samples collected 2 to 4.8 metres away from the patients. The viability of the virus present in air was proved by culturing the virus. The genome sequence of the virus isolated from air is identical to patient samples.
Researchers had so far been able to collect novel coronavirus from the air and from surfaces. But until now, aerosol transmission of the virus could not be confirmed as the virus was not isolated and cultured or the attempts did not succeed. Instead, the presence of the virus collected from the air was confirmed through RT-PCR. But the detection of viral RNA through RT-PCR does not automatically mean that infectious viruses are present and can be transmitted. But now, a preprint posted on medRxiv server has found strong evidence for aerosol transmission of the virus. Preprints are yet to be peer-reviewed and published in scientific journals.
The study that collected air samples from two hospital rooms found the presence of virus. The researchers were able to isolate viable virus from air samples collected 2 to 4.8 metres away from the patients. The over two metres distance between the air-samplers and the patients “suggests that the virus was present in aerosols”.
However, the amount of airborne virus detected per litre of air was small, the researchers found. It is unclear if the small amount of airborne virus is dependent on the infectious phase of the patient or issues with collection and extraction of virus samples from the air.
Viability of the virus
The viability of the virus was proved by culturing the virus isolated from the air samples. According to the preprint, the viable viral concentrations ranged from 6 to 74 TCID50 units/L of air. Fifty-percent tissue culture infective dose (TCID50) is the measure of infectious virus titer.
The team led by Dr John A. Lednicky from the College of Public Health and Health Professions, University of Florida, Gainesville, also sequenced the genomes of the virus collected from the air, virus isolated in cell culture and from nasopharyngeal swab samples.
They found the genome sequence of the virus strain collected from the air was “identical” to the ones isolated from the nasopharyngeal swab from the patient with active infection.
Evidence for aerosol transmission
Based on these findings, the researchers claim that patients with respiratory manifestations of COVID-19 “produce aerosols even in the absence of aerosol-generating procedures” and such viruses are also viable. Thus aerosols “may serve as a source of transmission of the virus”.
“Unlike previous studies, we have demonstrated the virus in aerosols can be viable, and this suggests that there is an inhalation risk for acquiring COVID-19 within the vicinity of people who emit the virus through expirations including coughs, sneezes, and speaking,” they write.
The World Health Organization had on July 9 finally acknowledged that novel coronavirus can be airborne in closed settings and spread from one person to another. WHO stated that aerosol transmission “cannot be ruled out”, particularly in indoor locations that are crowded and inadequately ventilated and where infected persons spend long periods of time with others. However, it insisted that more “studies are needed to investigate such instances and assess their significance for transmission of COVID-19”. But several virus outbreaks in enclosed spaces with poor ventilation and on prolonged exposure provide sufficient evidence to suggest that aerosol transmission might be one of major routes of virus spread.
Implications of the findings
Based on the findings of aerosol-based transmission, the researchers caution that non-pharmaceutical measures such as physical distancing by six feet might not be helpful in an indoor setting. This incorrect notion of physical distancing to prevent virus transmission in such settings might, in fact, provide a false-sense of security. “With the current surges of cases, to help stem the COVID-19 pandemic, clear guidance on control measures against SARS-CoV-2 aerosols are needed,” they write.
This study adds to the evidence for universal masking especially in enclosed spaces with poor ventilation involving prolonged contact.