The viral life cycle is completely dependent on its host, both for protection and survival. Exposed to the outside environment most viruses* degrade in a matter of hours. Additionally, viral genomes lack the machinery (proteins**) required to manufacture new viral particles to complete the cycle and multiply. For this reason, an important measure of the success of a particular virus is dictated by its transmissibility between hosts. Viruses can be transferred either directly through physical contact or indirectly through airborne transmission and infected surfaces. Understanding viral transmissibility allows us to better fight the spread of infection.
Viruses that do not survive well outside of their host require direct contact for transmission. Most sexually transmitted viruses fall under this category. For example, the Human Immunodeficiency Virus (HIV) retains only 10% infectivity within 1-2 hours on a dry surface [1]. In contrast, the highly infectious poliovirus can persist on surfaces for over 24 hours. As a result, direct contact viruses are not as easily transmissible.
Highly durable viruses can exhibit airborne or indirect contact transmission due to their ability to survive outside the body, or the ability to introduce large numbers of viral particles to the immediate environment. Viral particles that land on and persist on commonly touched surfaces can be transmitted through indirect contact. The infected surfaces are known as fomites. This mode of transmission makes the containment of some viruses very difficult. Tuberculosis and MRSA can persist on surfaces for 4 to 7 months [2]! For this reason, extreme measures were taken to clean surfaces in the early days of the COVID-19 pandemic when it was not yet clear how long the SARS-COV-2 virus could persist on surfaces. Later work demonstrated that SARS-COV-2 persisted for only 2 to 3 hours on surfaces [3].
Still, SARS-COV-2 has other mechanisms of indirect infection. As other coronaviruses, SARS-COV-2 resides primarily in the mucosal linings of the lungs, mouth, and nose. Coughs and sneezes from an infected individual produce thousands of virus-laden droplets and aerosols, some of which can spread as far as 7 meters away [4]. Airborne transmission varies between viruses and is heavily influenced by humidity, air circulation, and viral particle survival. As this appears to be the major method of COVID-19 transmission, current measures to address community spread focus on social distancing and masking.
Furthermore, it is important to understand which phases of the infection allow for the transmission of the virus. In some viruses, shortly after infection, individuals are capable of spreading the virus to others before symptoms are apparent. This is known as asymptomatic transmission. Asymptomatic transmission allows for a much faster spread of infection since seemingly healthy individuals (that are infected) carry on interacting with susceptible individuals for extended periods of time. Likewise, following recovery from infection, some viruses are still able to spread. The complicated nature of the viral life cycle can make regulations and recommendations from the health authorities seem to conflict and contradict themselves regularly, but as the research progresses and the virus becomes better understood, our ability to return to our normal lives becomes more achievable every day.
*Virus - it can be considered a very small and basic life form or an aggregate of molecules that is simpler than a cell. They can be found in the environment or inside living organisms. It has to infect a cell and use its machinery to multiply, since it cannot multiply by itself. Viruses consist of nucleic acids (DNA or RNA), a protein capsid that contains the nucleic acids and, sometimes, an external envelope of lipids. The flu, measles, AIDS and Covid-19, for instance, are caused by viruses.
**Protein - a molecule that forms the structure and gives function to organisms at the most basic level. Proteins consist of amino acids, which can be combined in different sequences to form different protein structures with different functions in cells.
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Written by: Adrian
Edited by: María and Natasha
BioDecoded is a volunteer group committed to sharing accurate scientific information. We cannot offer any specific health advice. If you have any doubts about getting vaccinated due to previous health conditions, please speak with your healthcare professional or family physician. Your doctor can revise your medical history and advise you on the best path to follow. If you have any questions about this topic, please comment or send them to our email.
References:
Moore, B. E. (1993) Survival of human immunodeficiency virus (HIV), HIV-infected lymphocytes, and poliovirus in water. Appl. Environ. Microbiol. 59, 1437–43. Available at: https://pubmed.ncbi.nlm.nih.gov/8100131/
Kramer, A., Schwebke, I., and Kampf, G. (2006) How long do nosocomial pathogens persist on inanimate surfaces? A systematic review. BMC Infect. Dis. 6, 130. Available at: https://pubmed.ncbi.nlm.nih.gov/16914034/
Goldman, E. (2020) Exaggerated risk of transmission of COVID-19 by fomites. Lancet. Infect. Dis. 20, 892–893. Available at: https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30561-2/fulltext
Jayaweera, M., Perera, H., Gunawardana, B., and Manatunge, J. (2020) Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy. Environ. Res. 188, 109819. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7293495/
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