Introduction

With a sharp rise in Covid-19 infections and deaths globally, the mode of transmission and precautions to minimise the spread of the disease need serious consideration. Earlier, the primary route of transmission of the virus was considered to be through larger respiratory droplets and direct contact with the infected people or surfaces. However, the recent surge in the cases is severe and researchers are speculating the chances of another mode of transmission, which involves inhalation of the airborne particles.

Preventive and control measures are necessary to stop the spread of infection. The current knowledge about the transmission points towards the airborne transmission of the virus through an aerosol generated from an infected person. When the patient exhales or sneezes or coughs, the virus in the aerosol may stay in the air for a long time. Here, we discuss the various modes of transmission of COVID-19 and the proper steps to minimise them, especially the indoor transmission of the virus.

Modes of transmission of the COVID-19 virus

While the COVID-19 infection affects your respiratory system causing severe illness and sometimes even death, some people might be asymptomatic after getting infected. The possible modes of transmission as reported by the World Health Organization (WHO) are as follows:

• Airborne transmission

The virus spreads through the air in the form of aerosolised infected particles, generated by an infected individual or during medical procedures and affects individuals coming in contact with these particles. Here, the particles are smaller than 5 μm in diameter, can remain in the air for long periods and transmitted to others over distances greater than 1m.   

Appropriate measures include sufficient and effective ventilation, possibly enhanced by particle filtration and air disinfection, avoiding air recirculation and avoiding overcrowding.

Dr. M.G. Kartheeka, MBBS, MD

• Droplet transmission

When a person gets infected with the SARS CoV-2 virus, it makes its presence in the saliva and other respiratory secretions or droplets expelled out through coughing, sneezing, talking or singing. Suppose a healthy person comes in contact with the infected person; the virus from the respiratory droplet may reach the nose, mouth or eyes of the healthy person in the vicinity, thereby spreading the infection. Droplet transmission occurs when a person is in close contact, within 1 meter of the infected person.  

• Surface transmission

The respiratory secretions or droplets may fall on the surfaces and contaminate them. If a healthy person touches this surface and then touches his/her mouth or nose, there are chances of infection. Although few reports suggest this type of transmission in hospitals or healthcare facilities treating COVID-19 patients, there is a possibility of a high viral load on the table surfaces, doorknobs, stethoscopes, etc.    

• Other modes of transmission

Although there are reports of the SARS CoV-2 virus in stool, urine, blood and plasma samples, there is no substantial evidence to support the fecal-oral, urine, blood or mother-fetus transmission.

Transmission of the COVID-19 virus can occur by direct contact with infected people and indirect contact with surfaces in the immediate environment or with objects used on the infected person (e.g., stethoscope or thermometer), endotracheal intubation, bronchoscopy, open suctioning, administration of nebulized treatment, etc. are commonly implicated in the causation of airborne transmission.

Dr. Ashish Bajaj – M.B.B.S, M.D.

Key differences between airborne and droplet transmission

An infected person can generate aerosolised droplets nuclei that measure less than 5 µm in size in the air, during talking or exhaling. If healthy people come in contact with the aerosolised particles with a high viral load, they may get infected by inhaling the nuclei directly into the lungs. Furthermore, the aerosolised nuclei may remain suspended for an extended time in the air owing to their small size. However, in droplet transmission, the droplets generated are typically between 5-10  µm in size and can affect an individual only in a close radius of the infected person. 

Indoor transmission of COVID-19

Several reports show indoor transmission of Covid-19 through the aerosolised droplets of sub-micron size present in the air.  

• Indoor vs. outdoor transmission

The environmental conditions affect the viability and infectivity of the virus. Indoor transmission occurs in closed places such as daycare centres, universities, schools, offices or hospitals, where people spend longer durations in close contact with each other. On the other hand, the outdoor risks of transmission include viral load in the particulate air and wastewater system. In the outdoors, the virus may remain viable for hours and days on the surface. Although both environments pose a risk for transmission of COVID-19, indoor transmissions are considered more risky than outdoor due to the chances of crowding in a limited space and poor ventilation.

• Studies conducted for indoor transmission

The indoor transmission of Covid-19 is reported in some research studies. Here are a few findings:

1. One such study shows an indoor spread of the infection in a 2.5 hour long Skagit Valley Chorale choir practice held in Washington state, where 53 of 61 members were infected.
2. Another study shows the indoor transmission of the virus in a bus that had recirculation of air. Out of the 67 passengers who rode a bus for a worshipping event in an eastern Chinese province, 24 were infected.  

• Factors that boost the indoor spread

Research further shows that certain factors may increase the indoor spread of the COVID-19 infection:

1. Poor ventilation.
2. Faulty air filtration units in hospitals, buildings or restaurants.
3. Facing towards airflow from the air-conditioner.
4. Crowding and not maintaining proper distance.
5. Infected surfaces at the workplace.

Measures to minimise indoor transmission of Covid-19

To reduce the transmission of COVID-19, the following actions are suggested:

• Segregate infected individuals: Immediately isolate the infected person from others. Every member of the family should wear a mask, even at home, if any member is infected. 
• Ventilation: Avoid staying in closed and poorly ventilated areas. Keep the doors and windows open for sufficient and adequate ventilation.
• Clean air: Use portable air cleaners.
• Avoid the use of air-conditioners: Air gets recirculated in the air conditioner; its dirty filters can transmit the virus. Wherever possible, use natural ventilation. Hospitals use air conditioners as they are equipped with a proper air engineering system that maintains lower pathogen load in the air.
• Disinfection: Use local air cleaning and disinfecting devices to disinfect the premises periodically.
• Avoid crowding: Minimise the number of people in the same room.

Conclusion

Follow hygiene practices such as healthy and safe disposal of PPE kits and waste management.  If indoor environmental control recommendations are followed strictly, coronavirus transmission rates can be drastically minimised. In addition to the air control and ventilation measures, people must follow some essential hygiene etiquettes.

Maintenance of proper physical distance and personal hygiene and use of masks is a must. When coughing or sneezing, one must cover the mouth and nose with tissue paper, which should be properly disposed of after use. In absence of tissue paper or handkerchief, one should cough or sneeze into one’s elbow, not hands.

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