New Delhi: When the Covid pandemic struck in early 2020, health agencies around the world had come out with prevention guidelines stressing the need to disinfect surfaces, leading to rigorous disinfection drives.
There was widespread belief that surfaces host infectious germs or microbes and one can contract Covid-19 by touching them.
However, by early 2021, scientific evidence started to point to the fact that Covid is much likelier to spread through air than by surface or fomite contact.
Soon, both the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC) accepted the aerosol theory, which meant there was a much stronger chance of you getting Covid-19 if you talk to a carrier than touching a public elevator button.
A recent study by the US National Institutes of Health (NIH), which compared the effects of transmission through different routes, has now found that airborne transmission is more “efficient” than fomite (surface) transmission.
The study done on Syrian hamsters shows that airborne transmission is also associated with more severity of the disease.
"Considering the scale of the COVID-19 pandemic, it remains unclear to what extent the different routes of exposure contribute to human-to-human transmission and how the exposure route affects disease manifestation," said the NIH study published in Nature Communications journal last week.
The study was conducted by scientists from the NIH's National Institute of Allergy and Infectious Diseases, and they used different mechanisms of exposure like intranasal inoculation and aerosol and fomite exposure techniques to determine the severity of the disease associated with each route.
The inoculation of SARS-CoV-2 was performed on four to six-year-old female Syrian hamsters. One part of the study was to check how different exposure routes impacted disease development, and the other part was making a comparison between animal-to-animal transmission of virus by airborne mechanisms and in infected cage environments or fomites.
Airborne And Fomite Transmission Route Experiments
Four animals were housed together for the experiment. Droplets were loaded with the SARS-CoV-2 virus in aerosol exposure, and the hamsters were exposed to them for ten minutes. Then, the amount of inoculum (a pathogen or its parts having the ability to cause infection in an organism) was determined from the respiratory minute volume rate of the animal, which is the volume of gas inhaled by or exhaled from an animal's lungs per minute.
Equal number of infected and non-infected hamsters were placed on either side of the same cage, with a divider between them. This was to prevent direct contact between the infected (donor) hamsters and the non-infected (sentinel/recipient) hamsters. Intranasal inoculation was used to infect the donor hamster. Normal airflow conditions were allowed within the cage, with the airflow being directed either from the donor to the sentinel, or the other way round, the study report said.
A SARS-CoV-2 contaminated dish was placed in the animal cages. Two animals were placed in one cage for four days and were made to undergo fomite exposure.
Air was made to flow at a constant rate, and an inlet was introduced on the infected hamster side, along with sample ports at each end of the cage. This arrangement was meant to measure the virus-loaded droplets in the air. After the hamsters had been infected, swabs were collected from the animals and the cage. The viral load was calculated from the standard curve.
What Did The Scientist Find?
It was found that there is a direct connection between disease severity and exposure route. Aerosol exposure resulted in the SARS-CoV-2 virus being deposited deep in the lungs, while fomite exposure caused the virus to be initially replicated in the nose, and at later stages, in the lungs.
The severity of lung damage was higher in aerosol exposure than in fomite exposure. Aerosol exposure resulted in increased weight loss, while fomite exposure caused milder respiratory pathology, and delayed viral shedding pattern (release of virus offspring after reproduction in the infected host cell). The onset of viral shedding impacts disease severity.
There is deposition and enhanced replication of the virus in the upper and lower respiratory tracts due to aerosol exposure, while less severe lung pathogenesis and delayed replication in the respiratory system occurs as a result of fomite exposure, the study found.
Also, the immune response in the lungs, which is a measure of cytokine production, is less in fomite exposure. The early shedding patterns help predict the severity of disease and associated immune response. It was also found that airborne transmission is more efficient than fomite transmission in the Syrian hamsters.
This suggests airborne droplets are a significant SARS-CoV-2 transmission route. When airflow is directed from donor to sentinel, transmission efficiency is more, compared to the case in which airflow is reversed. Thus, airborne transmission is dependent on the direction of airflow.
Since SARS-CoV-2 is a rapidly evolving virus, its mechanisms of infecting the host cell keep changing. Hence, the scientists suggested, Covid transmission models must be carefully devised so they can be used to assess the infection-causing patterns.