By Vidya Rajan, Columnist, The Times
News of the novel coronavirus outbreak has exposed our vulnerability to two things: fear and contagion. I have had conversations with people, admittedly not science-aware types, who think that countries should be providing antibiotics prophylactically, or frustrated a vaccine is taking so long to make. I have heard people conflating the virus that causes COVID-19, a type of severe pneumonia, with the influenza virus and the common cold virus. Before my head falls off from all the shaking I am subjecting it to, I thought to produce a short primer to this contagion, and how to best protect yourself. Knowledge is power, and hopefully it will also prevent you from getting sick. In this article, I will review some of the terms associated with contagion, mode of transmission, prevention and treatment, and draw a contrast between viral and bacterial pathogens.
Contagion is simply “catching something”. Something that is contagious spreads, such as ideas, fashion, and diseases. The speed of spread depends on how many who catch it go on to spread it to others in their network. This speed of dissemination is known as R-naught, abbreviated R0. The higher the R0, the more the number of individuals infected by a single transmitter. So an R0 of less than one, say 0.1, means that only 1 in 10 people contacted gets infected. An R0 of 1 means that each person infects someone, and an R0 of 2 means, of course, 2 people for every transmitter. You can see that contagions greater than 1 will spread through the system with greater and greater speed proportional to the R0 number. The novel coronavirus, 2019-nCov (now renamed SARS-CoV-2 due to its resemblance of the Severe Acute Respiratory Syndrome from 2002) which originated in Wuhan province of China, is thought to have an R0 of somewhere between 3-5. That is quite a contagious virus. But, for reference, measles has an R0 of 12-15, and the scary, scary Ebola outbreak of 2013-2016 had an R0 of 1.5-2.5. It makes more sense to get vaccinated against the measles than to worry about COVID-19 emerging in your backyard.
What other factors matter in a contagion? Of course, something that spreads through the air rather than through kissing is going to have a bigger R0. A sneeze in an outdoor park will constitute a lower R0 risk than than a sneeze in an airplane or elevator due to dose dilution. Objects which are contaminated can be a source of infection, so simple measures like washing your hands, using a hand sanitizer, and also not touching your face, are effective at limiting its spread. Consider this Emergency Room scenario. On presentation in the ER, the sick patient produces an insurance card, and is given a sheet of information to read, and then sign. But that card or pen or table or document is a really overlooked “fomite”, an object that may transmit the virus from the patient to the next person who touches it. The intake person at the desk in the ER doesn’t triage before pushing out documents to sign, or wipe down counters after each patient. Infection control requires thought. Bathrooms, doorknobs, computer keyboards, chairs, coats… how many more fomites can you list? It’s relevant. For infections, prevention is more effective than treatment. The Centers for Disease Control has some terrific information on these topics [1].
Treatment is based on the disease or the symptoms. Of course, a person’s immune competence, health status and age are relevant to infection outcomes. Since there are no vaccines yet available for SARS-CoV-2, treatment is designed to buy the body time to mount its own defences. Time is a key factor. A highly virulent pathogen at a high infective dose can replicate fast, overwhelming the body quickly. But if the dose or virulence is lower, supportive treatment can buy time for the body to kick in with its immune arsenal. I discussed immunity in an earlier article [2] and, given time, the immune system can recognize and respond to any shape of molecule, including new and emerging pathogens. Vaccines also take time to make in the lab, because the pathogen has to be analyzed, potential immunogenic domains identified, and then a domain selected that efficiently primes the immune system. The vaccine then has to pass regulatory safety and efficacy tests, and then be mass-produced, purified, and formulated for delivery. These factors are hurdles to speed of production. If you would like large amounts of a safe vaccine that works, well, that takes time.
What do the words endemic, epidemic and pandemic mean, and what differentiates them? Endemic diseases circulate in a defined area. For example, malaria is endemic in some parts of the world. Endemics are problematical but are limited in geographical impact. Epidemics have a wider spread – the Ebola virus epidemic in Western Africa (2013-2016) is a case in point. Quick containment action and an effective treatment and vaccination paradigm quenched it before it spread widely. A pandemic is defined as an infectious disease that spreads to every part of the world. By this definition, COVID-19 is possibly a pandemic, albeit one that is not yet particularly devastating given its relatively low mortality rate, estimated at this time to be less than 5% compared to 20% for the Spanish flu. Of course, this may change in either direction, to less spread or more spread, less virulence or more virulence. We still have to wait to see if we are in a truly troublesome pandemic or a world-wide paranoia.
On to viruses vs. bacteria. Bacteria are actually living cells which contain DNA and RNA, ribosomes and other organelles, which metabolize and cycle energy. Viruses are not cells, nor are they living, so antibiotics do not work on them. They are just a collection of replicating chemicals, containing either DNA or RNA with a protein shell, but no ribosomes or other organelles. Animal viruses may also have an envelope that they fashion from the membranes of the cells they infect. Since viruses do not contain organelles to grow and replicate, they are adept at hijacking a living cell to copy themselves. In some ways, computer viruses are a great analogy to biological viruses – they too are just software codes that replicate by hijacking a computer’s hardware to copy themselves, and then propagate through the population by infecting other computers. Viruses can be crystallized and stored without losing potency. Viruses are smaller than bacteria, on average 1/1000th the size of a bacterium; bacteria average 1/1000th the size of human cells. That makes a virus 1/1,000,000th (one-millionth) the size of a human cell. A human cell that is infected by a virus allows the virus to replicate inside, releasing many hundreds of thousands of tiny, infective viruses from each cell. The tissues viruses infect are very specific to the virus type because the outer layers of viruses contain proteins that dock with tissue-specific receptor molecules. Thus, respiratory viruses affect mucus membranes, sometimes causing both breathing issues and diarrhea. A respiratory virus that gets on your hand won’t cause an infection until your hand transfers it to mucus membrane. But it’s important to note an active infection stresses the whole body and, in extremis, can lead to organ failure.
Viruses are a fascinating group. No one seems to be able to answer even seemingly simple questions about viruses: Are they the earliest replicators, or a recent escapee from living cells?; How are they related to cells and to each other?; Why do some have DNA and others RNA?, How do some viruses so readily cross species barriers?, How long can they last in the environment? As to the last of these questions, a 30,000-year old virus that infects amoebas was recovered from Siberian permafrost and shown to still be infective [3], and a sampling of ice cores in Tibet revealed 28 previously unknown viral groups in 15,000-year old ice [4]. Will the thaw that global warming is causing resurrect other prehistorical or historical blights like the 1918 Spanish Flu, or perhaps a virus that may have caused mammoths to go extinct?
Then the question will be: How do we protect ourselves, avoiding contagion and fear?
- CDC. Coronavirus Disease. 2020; Available from: https://www.cdc.gov/coronavirus/2019-ncov/about/prevention-treatment.html.
- Rajan, V., Inner Nature: Immunity – Autosurveillance, in The Unionville Times, https://www.unionvilletimes.com/?p=43053. 2019: Unionville, PA.
- Legendre, M., et al., In-depth study of Mollivirus sibericum, a new 30,000-y-old giant virus infecting Acanthamoeba. Proceedings of the National Academy of Sciences, 2015. 112(38): p. E5327-E5335.
- Zhong, Z.-P., et al., Glacier ice archives fifteen-thousand-year-old viruses. bioRxiv, 2020.
