How long can the new coronavirus survive in droplets and on surfaces?
A recent study has explored how long SARS-CoV-2 remains infectious outside the human body, either in droplets or on contaminated surfaces. Two key parameters were measured: the half-life of the virus, which is the time taken for 50% of the viruses to be no longer infectious, and the maximum time at which viable viruses could be recovered. Evidence collected for SARS-CoV-2 showed that viruses in droplet aerosols (a fine mist) had a half-life of just over an hour but some could survive for three hours or more. Infectious virus could be detected on copper surfaces for up to four hours, on cardboard for up to 24 hours, and on plastic and stainless steel for at least 72 hours. These observations of virus persistence underline the value of regular disinfection of surfaces and attention to hand hygiene in controlling the spread of infection. A limitation of these studies is that they have been performed under a single set of conditions (indoors with constant temperature and humidity), and with a single initial dose of virus. It is likely that virus persistence will vary in different indoor and outdoor environments, and the length of time a surface remains contaminated will depend on the initial dose of virus to which it is exposed.
Can coronavirus be detected on surfaces in healthcare settings?
Coronavirus contamination of surfaces in healthcare settings was studied in Wuhan, China, during the COVID-19 outbreak. Commonly used objects in hospital wards, such as medical equipment and personal protective equipment worn by healthcare workers, were swabbed and tested for virus. Researchers found that the most contaminated zones within the hospital were in the intensive care unit; the highest levels of contamination were found on desktops/keyboards (16.8% of total swabs taken were positive), doorknobs (16%), and hand sanitizer dispensers (20.3%). Virus was detected on gloves (15.4%), eye protection and face shields (1.7%). This information gives an indication of where decontamination practises should be focussed to decrease the risk of virus transmission in these settings.
How does coronavirus spread through the air?
COVID-19 virus chiefly transmits through liquid droplets containing viruses. These droplets are often generated by infected patients through coughing and can transmit the virus to uninfected individuals by direct inhalation, or by contaminating nearby surfaces. These larger droplets tend to fall close to where they were released, which is why social distancing measures have been enforced.
If SARS-CoV-2 could travel in the air, outside liquid droplets, it might be carried longer distances on air currents. There is currently no robust scientific evidence to support airborne transmission of SARS-CoV-2. However, following a hospital-associated outbreak of SARS in Hong Kong in 2003, a study examined the contribution of the ventilation system to the spread of the virus through the hospital ward. During that outbreak, the virus spread to patients and medical students who were in close contact with the index patient, as well as to people in distant areas of the ward, suggesting possible airborne transmission. The pattern of new infections was associated with both the distance and the air flow from the room containing the index patient to other areas of the ward. This finding suggested, but did not prove, that the ventilation and air conditioning systems contributed to the apparent airborne transmission of SARS.
In a study conducted during the COVID-19 outbreak in Wuhan, researchers used pumps to sample air from several locations in two hospitals, a general hospital treating acute COVID-19 patients and a field hospital repurposed from an indoor sport facility used to quarantine and treat mild cases of COVID-19. They looked for traces of the virus (specifically the RNA that makes up its genetic material) within different sizes of airborne droplets. Virus RNA was detected in aerosols in the intensive care unit of the general hospital, within staff changing areas, and in a small, temporary, unventilated toilet. These were areas of the hospital used by large numbers of patients and healthcare workers, or communal areas in which activities allowed virus-containing droplets to be re-suspended in the air, such as the removal of contaminated personal protective equipment or toilet flushing. Virus RNA was not detected in areas with strict disinfection procedures or sophisticated ventilation, suggesting that such measures might limit the generation or movement of small droplets containing the virus. While these experiments only detected viral RNA, which does not necessarily imply the presence of infectious virus, they do suggest that SARS-CoV-2 has the potential to be transmitted via aerosols.
Studies into the timing of new infections during localised outbreaks of COVID-19 on cruise ships, at restaurants and at call centres suggest that large droplet and contaminated surface transmission are the main routes of infection. For example, during the COVID-19 outbreak on the Diamond Princess cruise ship, isolation measures were imposed that confined passengers to their cabins for significant periods of time. Direct contact with others was strictly limited and increased hand hygiene was enforced; these measures led to reduced rates of new infections. This suggests that the outbreak was driven by close contact transmission rather than long-range transmission between rooms via the air conditioning system.
Understanding how respiratory viruses are distributed in droplets and in the air, and whether ventilation systems play a role in their spread, can help to inform containment practices in health care environments.