WHAT'S NEW - COVID-19 Transmission Update

COVID-19 Transmission Update 

By Sean Derrig

This is an overview of what is currently known about the modes of transmission of SARS-CoV-2, when infected people transmit the virus, and the implications for infection prevention and control strategies. Principal sources are publications in peer-reviewed journals and reviews undertaken by WHO and partners. 

Modes of Transmission 

Despite what you may see or hear in the press, there is still no convincing, direct evidence the spread of this virus is airborne save for some exceptions below. This is partially due to the term ‘airborne’ being used inaccurately. The evidence still reveals controlling droplets is key. COVID isn’t like – for example – measles which is truly airborne and can hang in the air for hours. More below – but focus on droplet precautions. 

Also let’s assume those with symptoms of an active infection will be responsible and stay away from work. This makes it even more important to concentrate on preventing droplets being emitted from potentially asymptomatic carriers so masks, cohorting, distancing, ventilation, assiduous contact tracing if someone becomes sick - and the other measures summarised in the ABC in the Yellow Book all still hold true. 

Contact and Droplet Spread

Transmission of SARS-CoV-2 occurs through direct, indirect, or close contact with infected people through infected secretions such as saliva or respiratory droplets. These are expelled when an infected person coughs, sneezes, talks or sings – or even exhales. The more vigorous the activity, the more droplets. 

Respiratory droplet transmission can occur when a person is in close contact (within 1 metre) with an infected person who has respiratory symptoms (e.g. coughing or sneezing) or who is talking or singing. In these circumstances, respiratory droplets that include virus can reach the mouth, nose or eyes of a susceptible person and can result in infection.

Airborne Spread

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. Respiratory droplets are >5-10 μm in diameter whereas ‘airborne’ describes droplets <5μm in diameter which are referred to as ‘droplet nuclei’ or aerosols. 

Airborne transmission of SARS-CoV-2 can occur during medical procedures that generate aerosols - known as aerosol generating procedures or AGPs. 

The WHO and the scientific community has been actively discussing and evaluating whether SARS-CoV-2 may also spread through aerosols in the absence of aerosol generating procedures, particularly in indoor settings with poor ventilation. 

In all cases where aerosol transmission has been suspected, infections can be explained by a combination of crowded and inadequately ventilated spaces over a prolonged period, close contact environments facilitating transmission from a small number of cases to many other people (‘superspreading’ events), especially if hand hygiene was not performed and masks were not used when physical distancing was not maintained. 

Incidentally viruses do not mutate to ‘become’ airborne. They might mutate to become more efficient at infecting their host – just as this one has – or slightly widening their host range. Mutations in viruses don’t make such wholesale changes – it just doesn’t work like that. So they won’t ‘become’ airborne.

Fomite

Despite consistent evidence as to SARS-CoV-2 ability to contaminate surfaces and the survival of the virus on certain surfaces, there are no specific reports which have directly demonstrated fomite transmission. 

This is because people who come into contact with potentially infectious surfaces often also have close contact with the infectious person, making the distinction between respiratory droplet and fomite transmission difficult to discern. However, fomite transmission is still considered a potential mode of transmission for SARS-CoV-2, given consistent findings of environmental contamination in the vicinity of infected cases and the fact that other coronaviruses and respiratory viruses can transmit this way. 

So, remain vigilant and keep sensible hygiene and disinfection in place. 

Other

SARS-CoV-2 RNA (which is not viable virus) has also been detected in other biological samples, including the urine and faeces of some patients. One study found viable SARS-CoV-2 in the urine of one patient. Three studies have cultured SARS-CoV-2 from stool specimens. To date, however, there have been no published reports of transmission of SARS-CoV-2 through faeces or urine.

How people get infected

People with symptoms

SARS-CoV-2 transmission is principally spread via droplets and close contact with infected, symptomatic cases: in an analysis of 75,465 COVID-19 cases in China over 80% of clusters occurred within household settings, suggesting that transmission occurs during close and prolonged contact.

Outside household settings those who had close physical contact, shared meals, or were in enclosed spaces for approximately one hour or more with symptomatic cases (such as in places of worship, gyms, or the workplace) were also at increased risk of infection.

People without symptoms

It’s important to distinguish between transmission from people who are infected who never develop symptoms (asymptomatic transmission) and transmission from people who are infected but have not developed symptoms yet (pre-symptomatic transmission).

The extent of truly asymptomatic infection in the community remains unknown. 

Multiple studies have shown that people infect others before they themselves became ill. It has been estimated that up to 44% (25-69%) of transmission may have occurred just before symptoms appeared. 

Overall…

Current evidence suggests that SARS-CoV-2 is:

• Primarily transmitted between people via respiratory droplets;

• Transmission is occurring from people who are pre-symptomatic or symptomatic to others in close contact; 

• Close contact is 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. 

Transmission can also occur from people who are infected and remain asymptomatic, but the extent to which this occurs is not fully understood.


Want to know more?  Read our document on the 'chain of infection' and how to break it...