Covid-19: Modes of Transmission
By Gary Ruiz
Highlights:
COVID-19 cases are declining, but continued prevention and caution are necessary to keep the drop consistent.
The two most frequent forms of contact in which COVID-19 can spread through humans are indirect and direct contact.
Keeping the hands clean, using masks, and practicing social distancing are vital in slowing the spread.
Introduction
COVID-19 has by this point become synonymous with the words “pandemic”, “mask”, and, of course, “tiresome.” For more than a year now, the country--and the world--has been at war with COVID-19, and although we have made huge strides towards its containment and prevention, the country still has much more to battle through. According to the CDC (2021b), on average there are more than 10,000 cases each day in the US. Though still a large sum of cases, the country has undoubtedly recovered from the colossal spike back in early 2021. Nonetheless, this decline does not mean the country should lower their guards--and especially not their masks. COVID-19 remains a pestilent and contagious virus that can easily be spread if people are unaware of their actions.
The ways in which the disease can spread are categorized under two main groups: indirect and direct contact. Both can be prevented with proper caution and equipment.
Direct Transmission
The CDC (2021a) states that direct transmission consists of human-to-human transmissions that stem from the spread of respiratory droplets: diminutive liquid particles that are released whenever someone coughs, talks, sneezes, or exhales. Droplets from infected patients often harbor the virus responsible for COVID-19 and thus breathing them can potentially be infectious. A recent study found that larger droplets can travel distances further than 11.5 feet and stay in the air for a few minutes; whilst finer droplets can travel the same distance, they also travel further than 19.7 feet upwards and stay in the air for much longer--up to hours (Renzi & Clark 2020). This suggests that even after leaving rooms for some time, infected people can still leave respiratory droplets behind that can infect other newcomers. Therefore, masks and proper ventilation would be key in lowering the risk of transmissions.
Indirect Transmissions
Indirect transmissions, on the other hand, are caused by touching the face, especially the mouth, eyes, and nose, after coming in contact with contaminated surfaces. Mucous membranes, like the ones previously mentioned, are susceptible to infection and are the gateway for indirect transmissions (Lofti et al., 2020). Many studies have found that on smooth surfaces, the virus can be detected for up to weeks. However, the consensus is that after three days, the virus concentration is low enough for the contamination risk to be minimal. Subsequently, disinfecting commonly used items and isolating potentially contaminated items is helpful in reducing the contamination risk. Though still an influential factor in the transmission of cases, indirect transmissions remain rather seldom.
Blood and Fecal
Blood and fecal matter do not pose a serious threat when it comes to transmissions. Blood is not a carrier for COVID-19 since it is a respiratory virus; hence, no blood transfusion-transmitted cases have been recorded, according to the CDC (2020). On the other hand, fecal matter has been indeed proven to carry COVID-19. Yet, it is not a frequent route of transmission as there have been no recorded fecal-human transmitted cases (Lofti et al., 2020).
Vertical Transmission (Pregnancy)
Vertical transmission is the passage of a pathogen from a mother to a newborn baby immediately before or after birth. Vertically transmitted COVID-19 cases are possible. In a study conducted last September, about 3.28% of babies born from COVID-19 positive women also tested positive (Yuan et al., 2020). Yet, the mechanisms of transmissions could not be determined. It is unknown whether a baby is infected before, during, or after birth. Nor is it known through which route the baby is infected. Nonetheless, there is still a possibility of vertical transmissions between mother and newborn, as suggested by earlier studies.
Animals
The risk of human-to-animal transmissions is possible but considered low. Although it is believed that the COVID-19 virus originated from bats or other closely related species, animals do not play an important role in the current propagation of the virus. However, animals are indeed susceptible to the virus. According to the World Organization for Animal Health (2021), most domestic animals like cats, rabbits, and ferrets report high susceptibility rates whereas dogs have a lower rate. Similarly, transmission rates between the animals also vary; often minks and cats are reported to pass on the virus more frequently than other animals. However, whether the same applies to human-to-animal is still unknown. Though there have been reported cases of animals passing on the virus to humans, they remain rare and happen mostly in close contact scenarios. Likewise, humans are also considered to pose a risk to animals when it comes to transmissions, but it remains low.
Prevention
Prevention is key to contain the ongoing pandemic. The best way all-around to prevent infection is home quarantine. Isolation from people, both infected and healthy-looking, removes any possible threat of transmissions. However, this can be taxing, due to the constant need to stay home and the consequences of seclusion on the body and mind. Therefore, when someone goes outside, whether for work or recreation, the best thing is to always wear a mask, practice social distancing, avoid touching the face, and practice proper hand hygiene, through the usage of 60% alcohol sanitizer or soap and water. An extension of social distancing norms is also advised due to the reach of respiratory droplets; the further from people, the less likely a direct transmission would be. When it comes to animals it is also important to stay away from wildlife to avoid passing on the disease. Similarly, staying away from unknown animals, domestic or not, is advised.
References
Centers for Disease Control and Prevention. (2021a). Scientific Brief: SARS-CoV-2 Transmission. Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/sars-cov-2-transmission.html#anchor_1619805150492.
Centers for Disease Control and Prevention. (2021b). CDC COVID Data Tracker. Centers for Disease Control and Prevention. https://covid.cdc.gov/covid-data-tracker/#trends_dailytrendscases.
Centers for Disease Control and Prevention. (2020). Multistate Assessment of SARS-CoV-2 Seroprevalence in Blood Donors. Centers for Disease Control and Prevention. https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/blood-bank-serosurvey.html.
Lotfi, M., Hamblin, M. R., & Rezaei, N. (2020). COVID-19: Transmission, prevention, and potential therapeutic opportunities. Clinica chimica acta; international journal of clinical chemistry, 508, 254–266. https://doi.org/10.1016/j.cca.2020.05.044
Renzi, E., & Clarke, A. (2020). Life of a droplet: Buoyant vortex dynamics drives the fate of micro-particle expiratory ejecta. Physics of Fluids, 32(12), 123301. https://doi.org/10.1063/5.0032591
World Organisation for Animal Health. (2021). Infection with SARS-COV-2 in animals. Retrieved from https://www.oie.int/app/uploads/2021/03/en-factsheet-sars-cov-2.pdf
Yuan, J., Qian, H., Cao, S., Dong, B., Yan, X., Luo, S., Zhou, M., Zhou, S., Ning, B., & Zhao, L. (2021). Is there possibility of vertical transmission of COVID-19: a systematic review. Translational Pediatrics, 10(2), 423–434. https://doi.org/10.21037/tp-20-144