Tuesday, 12 May 2020

Plain soap and water is enough

When SARS-CoV-2 began to spread in England, antibacterial/antiviral hand wash and hand sanitizer became a target of panic buyers. While hand sanitizer with at least 70% alcohol to kill germs is important when we are outdoors and have no proper facilities to wash our hands, have you ever wondered whether it is necessary for us to use hand washing liquid, or soap with antibacterial/antiviral ingredients, at the sink or basin? Well, the U.S. Food and Drug Administration (FDA) says no to this.1 Instead, they said "plain soap and water" is enough to kill germs.

In 2016, the U.S. FDA announced the banning of 19 common ingredients,2 including triclosan and triclocarbon, in "antibacterial" soaps and body washes that are used with water. The FDA were concerned about the effects of these antibacterial ingredients in hand soaps and body washes when they are used on a long-term regular basis by consumers.

According to the FDA, "the benefits of using antibacterial hand soap haven't been proven." In 2013, the FDA proposed a rule requiring safety and efficacy data from manufacturers, consumers, and others if they wanted to continue marketing antibacterial products containing those ingredients, but very little information has been provided.

What is more, according to the FDA, the "wide use" of antibacterial products "over a long time has raised the question of potential negative effects on your health." "The manufacturers have not proven that those antibacterial active ingredients-including triclosan and triclocarban-are safe for daily use over a long period of time." Triclosan is an ingredient of concern to many environmental, academic and regulatory groups. Animal studies have shown that triclosan alters the way some hormones work in the body and raises potential concerns for the effects of use in humans. "We don't yet know how triclosan affects humans and more research is needed." In addition, laboratory studies have raised the possibility that triclosan contributes to making bacteria resistant to antibiotics. This resistance may have a significant impact on the effectiveness of medical treatments, such as antibiotics.

As "there isn't enough science to show that antibacterial soaps are better at preventing illness than washing with plain soap and water," in the undesirable event of another pandemic, we don't need to panic or feel desperate if no antibacterial/antiviral hand wash or antibacterial soap can be found on the shelves of the shops. Plain soap is enough to kill germs if you wash your hands properly. And if you don't have alcoholic hand sanitizer with you, just simply stay at home as much as possible. Unnecessary outdoor activity exposes you to the virus environment and increases your risk of being infected whether or not you have alcoholic hand sanitizer with you.


1. "Antibacterial soap? You can skip it, use plain soap and water" The US FDA. https://www.fda.gov/consumers/consumer-updates/antibacterial-soap-you-can-skip-it-use-plain-soap-and-water
2. "Consumer antiseptic wash final rule questions and answers. Guidance for industry" The US FDA. https://www.fda.gov/media/106652/download

Saturday, 9 May 2020

Wash away soap or detergent after every use

While we are washing hands frequently in order to stop the spreading of viruses, please remember to try your best to wash away the soap or detergent from your skin completely. Do not think that leaving some soap or detergent residue on the skin could be helpful in protecting your skin. Most hand wash contains sodium lauryl sulfate (SLS, also known as sodium dodecyl sulfate, SDS), which can damage the skin if left for a prolonged period of time, making the skin more vulnerable to the invasion of bacteria and viruses.

The harm SLS causes to humans

You may be familiar with SLS as it is a well-known ingredient in shampoo that can cause hair loss. However, SLS can also cause irritant skin. In fact, the SLS is such a well-known irritant that it is used as a standard irritant in the positive control in dermatological tests.1 According to research studies, SLS causes irritation to the skin if it is left for a prolonged period of time.1,2,3

Researchers from Germany found 42% of 1600 tested patients had an irritation due to SLS.1 Skin irritation is usually assessed by the changing level of redness of the skin, stratum corneum thickness, and the level of transepidermal water loss (TEWL) before and after SLS treatment. Research studies on Caucasian and Japanese populations found significant erythema, stratum corneum dehydration, and elevated TEWL in a dose-dependent manner, when 0.025% to 0.75% of SLS was applied and retained in the forearm for up to 24 hours.1,2,3 When SLS was applied repeatedly, the levels of erythema and TEWL were augmented and the reactions developed more quickly.3

There are two main ways the SLS triggers skin irritation. One way is by physiologically damaging the skin. Our skin is protected by layers of cells that are composed of oil and protein. Prolonged exposure to SLS can disrupt the natural oil in lipid membrane that protects skin and thus damages the skin. This results in cracked, dry and tender skin which makes it irritant.4 More importantly, this also reduces the ability of the skin to keep out bacterial and viral invasions.

Additionally, SLS triggers skin irritation on the biological level. A study using confocal Raman microscopy reported that SLS can penetrate into human skin.5 Research studies showed that SLS triggers the expression of two inflammatory mediators, IL-1alpha and PGE2, upon topical application of SLS on keratinocytes.6,7 Given the recent findings that SARS-CoV-2 can also trigger a hyperinflammatory response in severe cases,8 we can imagine what would happen if the SARS-CoV-2 virus invaded cells which had been penetrated by SLS.

Why SLS is commonly used

SLS, with the formula CH3(CH2)11SO4Na, is a *surfactant (surface active agent), a substance made from molecules that have hydrophobic ("grease loving, water hating") groups as tails and hydrophillic ("water loving") groups as heads. Soap, an alkaline salt of fatty acids, is the oldest known surfactant.

Surfactant is the main ingredient in hand soaps, and is added into face washes, shaving creams and toothpastes because it lathers up to generate cleansing foam. The lather-creating feature also enables the core ingredients of the products to be dispersed effectively across the entire cleaning surface. Because of its amphiphillic property, surfactant is added in cosmetic products, dermatological products, and cleaning products, to help mixing oily ingredients with aqueous ingredients. However, when it is left in contact with the skin, the hydrophobic tail of the molecule can disrupt the lipid structure of the skin cell and causes skin damage.

SLS is a commonly used surfactant because of its easiness to produce. It is made by combining lauric acid (from coconut oil) with sulphuric acid (from petroleum) and sodium carbonate. Moreover, it has higher efficacy in generating lather, which is important for removing dirt. As an anionic surfactant, this means SLS has higher ability to solubilize fats and oil. However, unfortunately, this also means that SLS is more harmful than the other surfactants in causing more skin irritancy than non-ionic, or amphoteric surfactants.

What can we do to protect ourselves from the effects of SLS

SLS is not only commonly found in hand wash, it is also found in shampoo, toothpastes, and cleaning detergents. It is very hard to avoid using products with SLS. As there is no scientific evidence that it can cause cancer, and the skin irritation no longer exists once exposure to SLS has ceased,3 we do not need to worry overmuch about the use of SLS-containing products. However, as prolonged exposure to SLS damages the skin and make it vulnerable for virus invasion, it is important to avoid leaving SLS on the skin each time we finish using the relevant products, especially during the pandemic period.

There are a few practical tips, that you may have missed in your daily routine, to minimize the chance of leaving SLS on ourselves, apart from using SLS-free products.
1. While we should use warm water with soap for cleaning in order to increase the lather and thus increase the cleaning power to remove dirt, it is best to avoid using excessively hot water for cleaning or showering, as a high water temperature damages the skin.9
2. Put on gloves while we wash dishes to avoid direct contact with the detergent. Or simply use a dishwasher to do the washing.
3. All of our cells, not only the skin cells, are protected by cell membranes composed of fat which can be disrupted by the SLS. SLS left on the kitchenware or the kitchen utensils which will be used later for food will come into our body. Therefore it is really important to rinse these things in running water to get rid of the soapy water from the cleaning.

There are many milder alternatives available (eg. sodium lauryl phosphate, **sodium laureth sulfate, alkyl phenol ethoxylate, fatty alcohol ethoxylate, or fatty acid alkoxylate) to replace SLS in cosmetic and cleaning products. You can seek advice from your pharmacist or GP on the usage of these products if you think SLS might be the cause of your dermatitis or worsening of your eczema.10,11

*Surfactant comes from the name surface active agent which means a substance that can lower the surface tension of a liquid. When surfactant is dissolved in water, the surfactant molecules orientate at the surface so that the hydrophobic regions are away from the aqueous environment. The surfactant molecules thereby adsorb at the water surface and weaken the forces between water molecules. The contraction force in the water thereby is reduced and thus the spreading and wetting properties of an aqueous solution are increased.
In water, micelles of surfactants are formed by aggregates of surfactant molecules in a way that the hydrophobic tails are directed inwards and the hydrophillic heads are directed outwards. In this way, the aggregates will form balls, cylinders or laminar layers depending on the concentration of the surfactant. When added into an aqueous solution containing oil, surfactant molecules aggregate around the oil so that the oil or fat molecules will be totally incorporated inside micelles. This way, the fat is dispersed into very small particles.
When added into non-aqueous solvent, the surfactant molecules aggregate the other way around, where hydrophilic heads form the core of the aggregate and hydrophobic tail are in contact with the surrounding fat/oil. The surfactant will act in such a way that they will disperse the water-soluble material, in the solvent, into very small parts by creating aggregation around the particle and forms a micelle. This makes it possible to remove the water-soluble material from substrates in solvent using surfactant.

** Sodium laureth sulfate, SLES, is a compound derived from SLS by introducing ethylene oxide through a process called ethoxylation. SLES is safe to use in bath and body care products and is gentler to skin than SLS. The compound won't aggravate your skin or strip any excess moisture off. On the other hand, SLES will be just as cleansing, foaming and emulsifying as SLS.


1. J. Geier, W. Uter, C. and Pirker, et al. Patch testing with the irritant sodium lauryl sulfate (SLS) is useful in interpreting weak reactions to contact allergens as allergic or irritant. Contact Dermatitis, 2003 Feb;48(2):99-107.
2. J. Aramaki, S. Kawana, and I. Effendy, et al. Differences of skin irritation between Japanese and European Women. Br J Dermatol., 2002 Jun;146(6):1052-1056.
3. Nara Branco, Ivy Lee, and Hongbo Zhain, et al. Long-term repetitive sodium lauryl sulfate-induced irritation of the skin: an in vivo study. Contact Dermatitis, 2005 Nov;53(5):278-284.
4. A. di Nardo, K, Sugino, and P. Wertz, et al. Sodium lauryl sulfate (SLS) induced irritant contact dermatitis: A correlation study between Ceramides and in vivo parameters of irritation. Contact Dermatitis, 1996 Aug;35(2):86-91.
5. G. Mao, C.R. Flach, and R. Mendelsohn, et al. Imaging the distribution of sodium dodecyl sulfate in skin by confocal Raman and infrared microspectroscopy. Pharm. Res. 2012, 29, 2189-2201.
6. C. Cohen, G. Dossou, and A. Rougier, et al. Measurement of inflammatory mediators produced by human keratinocytes in vitro: a predictive assessment of cutaneous irritation. Toxicol. Vitr., 1991, 5, 407-410.
7. S. Gibbs, H. Vietsch, and U. Meier, et al. Effect of skin barrier competence on SLS and water-induced IL-1? expression. Exp. Dermatol., 2002, 11, 217-223.
8. P. Mehta, D. F. McAuley, and M. Brown, et al. COVID-19: Consider Cytokine Storm Syndromes and Immunosuppression. Lancet, 2020 Mar 28;395(10229):1033-1034.
9. E. Berardesca, G.P. Vignoli, and F. Distante, et al. Effects of water temperature on surfactant-induced skin irritation. Contact Dermatitis, 1995 Feb;32(2):83-87.
10. M. Tsang, and R.H. Guy. Effect of aqueous cream BP on human stratum corneum in vivo. Br. J. Dermatol., 2010 Nov; 163(5): 954-958.
11. N. Kuzmina, L. Hagstromer, and M. Nyren, et al. Basal electrical impedance in relation to sodium lauryl sulphate-induced skin reactions--A comparison of patients with eczema and healthy controls. Skin Res Technol. 2003 Nov; 9(4): 357-362.

Saturday, 2 May 2020

How do soap molecules form

After writing a blog post about how soap can kill viruses and how it can remove viruses from the skin, I started to examine how soap is produced, as each soap molecule contains two completely different chemical properties in its two ends (one end is "water loving" while the other end is "water hating" or grease loving). I learned about the function of soap molecules when I was in high school studying chemistry, but at that time, I was not thinking about how soap is made. So a few days ago, I started searching for the answer.

There are a few websites that I found interesting, and I would like to share with you some information from these sources. In general, the basic ingredients of a soap are oil (vegetable oil or animal fat), water, and lye. Lye is an alkaline salt which can be either sodium hydroxide or potassium hydroxide. Sodium hydroxide is used to make a hard soap while potassium hydroxide is used to make a soft soap. A combination of the two is used to make a cream soap.1,2

When oil and lye are mixed together, soap molecules (fatty acid salts) are formed in a chemical reaction called saponification.3 During the reaction, oil that contains fatty acid ester linkages undergoes alkaline hydrolysis with the metal hydroxide.

Triglyceride + 3 sodium hydroxide (or potassium hydroxide)>>>> glycerol + 3 soap molecules

Although water molecules are not involved in the saponification, water is an important mediator to mix the oil and the alkaline salt together. This is used to create the lye solution that is mixed into the oil. The correct proportion of water for saponification is crucial, as too much of it results in too soft a bar of soap. The majority of the water evaporates out of the soap as it cures and ages.

Soap has been made for thousands of years, and the basic recipe has not very much changed. Nowadays, with the help of the highly developed chemical industry, it is easy to get all the three ingredients to make soap. However, we may wonder how and where did the ancient people get the alkaline salt. The most ancient recipe for soap which was found on Babylonian clay containers dated at 2800 B.C. gives us a clue.4 Inscriptions on the containers showed that they used wood ashes as a source of alkaline salts.4,5 Lye is formed when wood ash (mainly potassium carbonate) is mixed with water.

You may like to make an organic, purely natural handcrafted soap by yourself once you know the basic ingredients. However, it is extremely important to remember that lye is a corrosive strong alkaline of pH 13. You can get serious burns if you don't handle the lye solution carefully. If you start by mixing wood ash with water, the mixture can turn your skin into soap once it comes in contact with your skin and absorbs the oil in your skin. Moreover, inhalation of the lye vapour will cause serious damage to your respiratory system and can be fatal.


1. "Soap ingredients" https://www.soapguild.org/consumers/soap-ingredients.php
2. "Why do we use soap?" Live Science, 5th March, 2020. https://www.livescience.com/57044-science-of-soap.html
3. "Saponification definition and reaction" ThoughtCo. https://www.thoughtco.com/definition-of-saponification-605959
4. "Who invented soap?-About soap inventors" http://www.soaphistory.net/soap-history/who-invented-soap/
5. "How to Make Homemade Lye Using Two Ingredients" ThoughtCo. https://www.thoughtco.com/make-homemade-lye-using-two-ingredients-608276

Tuesday, 28 April 2020

How soap can kill viruses: 20 seconds, lather and warm water

Since the outbreak of coronavirus in the UK, the government has been encouraging people to wash their hands frequently with soap in order to stop spreading the virus. The government's promotion successfully made people do this for a full 20 seconds by asking them to mentally sing "Happy Birthday" twice while washing our hands. (Those who don't like that particular song can now find suggestions for many other 20-second "hand-washing songs" online.) While it is easy to understand the reason for washing hands long enough, we may also like to know how we can optimise our hand washing efficiency using soap. An article with interviews from two professors gives us the answer: bubbles and foam, and warm water.1

Before we go into what the two professors had said, we can have a brief understanding of the structure of an encapsulated virus (such as coronavirus) and how this kind of virus attaches to our hands. An encapsulated virus consists of three key components: genetic material made up of ribonucleic acid (RNA), proteins, and lipids (basically called fat). The virus replicates in the host and makes lots of these components which then self-assemble to form complete viruses. The proteins and lipids form the virus envelope (including a lipid bilayer and membrane proteins), giving protection and attachment ability to the virus when it is outside the host cell. When an airborne virus comes in contact with the skin, the envelope of the virus interacts with proteins and fatty acids in the dead cells on the skin's surface. However, the bondings that hold the three key components in the virus, and the bonding between the virus and the skin's dead cells, are not strong.*

Soap molecules have a special property that disrupts those weak bondings. Each soap molecule has two ends: a hydrophillic ("water-loving") head, and a hydrophobic ("water fearing") tail that is attracted to grease. When soap is applied, the hydrophobic tail of the soap molecule is drawn to the fatty outer layer of the virus, competing with the lipids in the membrane and thus prying it open. Once the virus splits open, its contents is spilled out into the soapy water and "dies", or, more correctly, becomes inactive.**

The combination of water and scrubbing with hands using soap creates more soap bubbles. You may wonder if these have an actual function. The foam or bubbles created can disrupt the chemical bonds that allow a virus to stick to the skin's surface. After 20 seconds of thorough scrubbing to get into every crack and crevice of your hands and fingers, all the viruses that have been damaged, or inactivated, by soap molecules are washed away when you rinse your hands. Therefore, according to Professor William Schaffner of the preventive medicine and infectious disease department at Vanderbilt University of Medicine in Nashville, the "bubbles and foam literally pick germs up and wash them down the drain." It is "an indication that the soap is ... trying to encapsulate the dirt and the bacteria and the viruses in them," said Dr. Bill Wuest, an associate professor at Emory University who studies disinfectants.1

Dr. Wuest said that warm water can make everything bubbly more easily than can cold water: "Cold water will work, but you have to make sure you work really vigorously to get a lather and get everything soapy and bubbly." We may need to sing the "Happy Birthday" song three times instead of two. "Warm water with soap gets a much better lather, more bubbles."1

Therefore next time you wash your hands with soap, please remember that besides 20 seconds, warm water and a lot of bubbles are also essential to destroy the viruses and wash them away from the skin effectively.

*The lipid bilayer membrane is assembled by noncovalent bonds, which are weak bonds that hold molecules. The proteins and lipid molecules in the virus membrane are held together by Van der Waals forces, which also hold together the hydrophobic tails of soap molecules, and hydrogen bonding, which also binds the hydrophilic heads of soap molecules with water. These weak bondings help to stabilize the lipid bilayer structure of the virus.
Washing off the virus with water alone, therefore, is not enough, as water cannot eliminate the interaction between the virus and the dead cells on the skin's surface. Coronavirus covered with a membrane of lipid bilayer, the "greasy" virus, thus cannot be simply separated from the dead cells of the skin by water. Soap and ethanol are the best tools to do that.
**Almost all sanitizer, which typically contains 60%-80% ethanol, "kills" viruses in a similar fashion as soap. Both dissolve the greasy coating of the enveloped virus.


1. "Why soap, sanitizer and warm water work against COVID-19 and other viruses" CNN Health, 24th March, 2020. https://edition.cnn.com/2020/03/24/health/soap-warm-water-hand-sanitizer-coronavirus-wellness-scn/index.html

Thursday, 23 April 2020

Coronavirus (9) Warning from WHO on using antibody tests

Besides the problems with sensitivity and reliability of antibody tests for COVID-19,1,2 scientists from WHO issued a warning on the use of these tests, as they thus far do not have enough evidence that a person could be risk free from reinfection even if a positive antibody test result for COVID-19 is correct. This was announced in a news conference at WHO's Geneva headquarters. Their concern is based on two factors, for which they do not yet have answers because the disease has just newly emerged and has been threatening humans for only a few months.3,4

One concern is what level of blood antibody is enough for COVID-19 immunity and protection from reinfection. As mentioned in my last blog, people with severe illness tend to have a quicker and stronger antibody response, while people with mild or no symptoms tend to develop the antibodies slowly and in lower quantities. However, even among the people who developed a strong antibody response and have over 90% chance to be detected by an antibody test, that level of antibodies "does not mean that somebody is immune" said Dr. Maria Van Kerkhove, head of WHO's emerging diseases and zoonosis unit. These tests can detect serological antibodies if they reach the sensitivity level of the test. However, "right now, we have no evidence that the use of a serological test can show that an individual is immune or protected from reinfection."3

The other concern is the duration of protection the antibodies can give to a person who has been infected with SARS-CoV-2. Dr. Mike Ryan, executive director of emergencies program of WHO, said that "nobody is sure whether someone with antibodies is fully protected against having the disease or being exposed again," after his saying that scientists are still determining the length of protection antibodies might give to a person with coronavirus infection history. "With regards to recovery and then reinfection, I believe we do not have the answers to that. That is an unknown."3

The level and the duration of the serological antibody which can protect a previously infected person during the second time of SARS-CoV-2 infection are crucial knowledge in protecting patients. It will take a long time to find out the answers from bigger studies amid the increased number of the cases. Therefore it is better if countries or peoples do not use antibody tests as an indication of immunity before clearer answers emerge. Moreover, even if your antibodies do protect you from becoming sick, you may still harbour the virus and pass it to others.


1. "Coronavirus (5) What is RNA test? Antigen test? Or antibody test?" https://yunwenincambridge.blogspot.com/2020/04/coronavirus-5-what-is-rna-test-antigen.html
2. "Coronavirus (7) Abandonment of home-based antibody tests" https://yunwenincambridge.blogspot.com/2020/04/coronavirus-7-abandonment-of-home-based.html
3. "WHO warning: No evidence that antibody tests can show coronavirus immunity" CNBC, 17th April, 2020. https://www.cnbc.com/2020/04/17/who-issues-warning-on-coronavirus-testing-theres-no-evidence-antibody-tests-show-immunity.html
4. "Coronavirus: Double warning over antibody tests" BBC news, 18th April, 2020. https://www.bbc.co.uk/news/uk-52335210

Tuesday, 21 April 2020

Coronavirus (8) Why is it difficult to have a high accuracy home-based antibody test?

In my last blog post, I mentioned about the technical problems that prevent the home-based antibody test from giving an accurate and sensitive result. Two of the reference articles I cited1,2 tell us additional reasons that make antibody tests for COVID-19 difficult to apply at this moment. This is precious knowledge that we cannot learn from textbooks but only from experienced experts. Let us have a closer look at what they said.

Thus far, it has been found that people infected with COVID-19 can develop different degrees of symptoms for the disease.3,4,5 Some can be seriously ill, while some can show little or no symptoms at all. Based on the patient samples analysed by research team of Dr. David Da-i Ho, a leading scientist at Columbia University in New York who invented cocktail therapy for HIV and is now leading a team to look for COVID-19 treatment, the patients with severe illness tend to develop a faster and stronger antibody response, while the ones with milder symptoms tend to develop the antibody response more slowly. This means that the one with severe illness will have a positive antibody test result at an earlier stage of the disease development compared with the one with milder symptoms, yet it is not possible to predict how long it's going to take for the body to develop enough antibodies to give a positive result in the antibody test. This highly variable immunity response across the population makes the development of a simple, reliable home-based antibody test considerably challenging. 1

Moreover, the level of antibody production for the asymptomatic or mildly symptomatic cases remains low even after two weeks of antibody response. This can result in only about half of the cases being detected. Additionally, it is still unclear whether the antibody level will increase if these people are tested one or two weeks later. Prof Marion Koopmans from the Erasmus University Medical Centre in Rotterdam did not think that the detection rate can reach the levels seen in severe cases. She said that the claiming of over 90% sensitivity by the home-based test is likely to be based on tests in patients recovering in hospital, who represent the most severe cases. Therefore, for the people with low or no symptoms, which accounts for about 85% of the cases, the sensitivity of the home-based antibody test will "end up around 50%-60%", which is highly unreliable.1

A shortage of blood samples from people recovered from COVID-19 is another reason that holds up the development of an antibody test. This convalescent blood is necessary for academia and industry to develop and validate the tests. The shortage is due to the administration in the UK making it difficult to get patient samples. "Access to patient samples has been a longstanding issue for the industry, and that is highlighted now when companies desperately need them" said Dr. Doris-Ann Williams, chief executive of the British In Vitro Diagnostic Association. The other reason for the shortage of convalescent blood is simply because the disease is newly emerged. "People can have two weeks of disease and then three weeks to mount this antibody reponse, but if you look back five weeks or so, there was hardly anyone in Britain with the disease" said Prof John Bell from Oxford University.2

From these experts in the field, we now understand that the big range in immune response for a disease seen across the population makes it difficult to develop a simple home-based test which is reliable over an entire population. The shortage of blood also hampers the development and validation of the antibody test. As the problem arising from a wide range of immune response can never be changed, and the shortage of blood for antibody development will not be solved in the near future, I would not recommend the use of antibody test to check the history of infection of a person. The false results could be disastrous.


1. "Coronavirus 'game changer' testing kits could be unreliable, UK scientists say" The Guardian news, 5th April, 2020. https://www.theguardian.com/world/2020/apr/05/coronavirus-testing-kits-could-be-unreliable-uk-scientists-say
2. "UK COVID-19 antibody tests not ready until May at earliest" The Guardian, 8th April, 2020. https://www.theguardian.com/society/2020/apr/08/uk-covid-19-antibody-tests-not-ready-until-may-at-earliest
3. Qian G, Yang N, Ma AHY, et al. A COVID-19 Transmission within a family cluster by presymptomatic infectors in China. Clin Infect Dis 2020. Epub March 23, 2020.
4. Du Z, Xu X, Wu Y, Wang L, Cowling BJ, Meyers LA. Serial interval of COVID-19 among publicly reported confirmed cases. Emerg Infect Dis 2020. Epub March 19, 2020.
5. Kimball A, Hatfield KM, Arons M, et al. Asymptomatic and presymptomatic SARS-CoV-2 infections in residents of a long-term care skilled nursing facility-King County, Washington, March 2020. MMWR Morb Mortal Wkly Rep 2020. Epub March 27, 2020.