droplets

…the reported number and size of released droplets vary significantly. In terms of the total mass of saliva, 1.1-6.7 mg of saliva were collected on a mask during a single cough, and 18.7 mg were collected while counting from 1-100.

There were 1-320 droplets per liter of exhaled air found for breathing, 24-23,600 found for coughing, and 4-600 found for speaking.

…individuals infected with influenza virus produce a significantly greater volume of aerosol during clinical illness compared with during the asymptomatic stage (P = .0143). This enhancement in aerosol generation during illness may play an important role in influenza virus transmission.

Two reviews

highlight the limited evidence base supporting the efficacy of face masks in reducing influenza virus transmission. They suggested that surgical masks may reduce infectiousness, rather than protect against infection, especially when airborne transmission is important. Influenza viruses (with sizes in the 80- to 120-nm range) and other viruses of similar size are capable of penetrating the mask in either direction. The N95 respirators are efficient in removing very fine droplet nuclei, but face masks are not. However, face masks, if worn by an infected person, can suppress the expired jets (Fig 2A) and reduce the close contact transmission via both the droplet-borne and short-range airborne routes.

growing evidence covid-19 is (aerosol) airborne

A group of 239 scientists from 32 countries have written an open letter to the World Health Organization arguing that covid-19 can be transmitted through the air. You might think we know that already, but most current guidance is based on the idea that covid-19 is transmitted via droplets expelled from an infected person’s nose or mouth. The thought is that these larger respiratory droplets quickly fall to the floor. That's the position the WHO has taken from early on in the pandemic, and that’s why we have been keeping at a distance from one other. However, the signatories of the open letter say the organization is underestimating the role of airborne transmission, where much smaller droplets (called aerosols) stay suspended in the air. These aerosols can travel farther than droplets and linger in an area even when an infected person has left.

What’s the evidence? The letter says multiple studies “have demonstrated beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air.” It says these microdroplets “pose a risk of exposure at distances beyond 1 to 2 m from an infected individual.” An early laboratory studycarried out by the US National Institutes of Health found that the coronavirus can linger in the air for up to four hours in aerosol form. The coronavirus was also detected in aerosols collected at two hospitals in Wuhan, China, according to a study published in Nature in April. And superspreading events add to the weight of evidence: for example, after a choir practice in the US nearly 50 people were infected even though they kept a safe distance apart.

The implications: If airborne transmission is a route for the spread of the virus, it could lead to changes in the current advice. It would suggest that social distancing may be insufficient, especially indoors. This may place yet more importance on mask-wearing around people who are not part of your household if you meet them indoors, even if you are distancing, and increasing ventilation in enclosed areas. It could make air-filtering systems more important to try to cut down on the recirculation of air. And it might mean health-care workers caring for coronavirus patients need the highest grade of mask—N95—to filter out the smallest droplets.

truth

The naked figure of Truth is held aloft by her father, Time, who with his scythe subdues Falsehood, in her fine apparel and dissembling mask, while the baleful figure of Discord and Envy recedes, protesting. The subject may have held a personal significance for the artist; the day after he completed it he committed suicide. Yet, as the collector Caylus recalled: ‘there is no sense of his alienation of spirit in the work, it is even one of his most beautiful cabinet pictures’.

Truth is the property of being in accord with fact or reality.[1] In everyday language, truth is typically ascribed to things that aim to represent reality or otherwise correspond to it, such as beliefs, propositions, and declarative sentences.

Truth is usually held to be the opposite of falsity. The concept of truth is discussed and debated in various contexts, including philosophy, art, theology, and science. Most human activities depend upon the concept, where its nature as a concept is assumed rather than being a subject of discussion; these include most of the sciences, law, journalism, and everyday life. Some philosophers view the concept of truth as basic, and unable to be explained in any terms that are more easily understood than the concept of truth itself.[2] Most commonly, truth is viewed as the correspondence of language or thought to a mind-independent world. This is called the correspondence theory of truth.

Various theories and views of truth continue to be debated among scholars, philosophers, and theologians. There are many different questions about the nature of truth which are still the subject of contemporary debates, such as: How do we define truth? Is it even possible to give an informative definition of truth? What things are truthbearers and are therefore capable of being true or false? Are truth and falsity bivalent, or are there other truth values? What are the criteria of truth that allow us to identify it and to distinguish it from falsity? What role does truth play in constituting knowledge? And is truth absolute, or is it merely relative to one's perspective?

Mathematics of Testing

Coronavirus Antibody Tests Have a Mathematical Pitfall

The accuracy of screening tests is highly dependent on the infection rate

With a test that is not 100 percent accurate, there are four possible outcomes for each individual:

• you are positive and test positive

• you are negative and test negative

• you are positive but test negative (a false negative)

• you are negative but test positive (a false positive)

If a test has a 95 percent specificity and a 95 percent sensitivity, that means it correctly identifies 95 percent of people who are positive and 95 percent of those who are negative. Even with very effective screening tests, depending on the infection rate in the population, and individual’s test result may not be reliable.

If a test with 95 percent specificityand 95 percent sensitivity is used in a community of 500 people with a 5 percent infection rate, the results look like this:

In this scenario, an individual who tests negative has a 99.8 percent chance of actually being negative. But an individual who tests positive has only a 50 percent chance of being positive.

If an equally accurate test is used on a group of 500 people with a 25 percent infection rate, the results may look like this:

In this scenario, and individual who tests negative has a 98.3 percent chance of actually being negative. And and individual who test positive has an 86 percent chance of actually being positive.

Sensitivity and specificity

Sensitivity and specificity are statistical measures of the performance of a binary classification test, also known in statistics as a classification function, that are widely used in medicine:

• Sensitivity (also called the true positive rate, the epidemiological/clinical sensitivity, the recall, or probability of detection[1] in some fields) measures the proportion of actual positives that are correctly identified as such (e.g., the percentage of sick people who are correctly identified as having the condition). It is often mistakenly confused with the detection limit[2][3], while the detection limit is calculated from the analytical sensitivity, not from the epidemiological sensitivity.

• Specificity (also called the true negative rate) measures the proportion of actual negatives that are correctly identified as such (e.g., the percentage of healthy people who are correctly identified as not having the condition).

The terms "positive" and "negative" do not refer to the value of the condition of interest, but to its presence or absence; the condition itself could be a disease, so that "positive" might mean "diseased", while "negative" might mean "healthy".

COVID-19 antibody test

“A positive test result shows you may have antibodies from an infection with the virus that causes COVID-19. However, there is a chance a positive result means that you have antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold.”

Common Human Coronaviruses

Common human coronaviruses, including types 229E, NL63, OC43, and HKU1, usually cause mild to moderate upper-respiratory tract illnesses, like the common cold. Most people get infected with one or more of these viruses at some point in their lives. This information applies to common human coronaviruses and should not be confused withcoronavirus disease 2019 (formerly referred to as 2019 Novel Coronavirus).

last reviewed:  February 13, 2020

Test for Past Infection

Antibody tests check your blood by looking for antibodies, which may tell you if you had a past infection with the virus that causes COVID-19. Antibodies are proteins that help fight off infections and can provide protection against getting that disease again (immunity). Antibodies are disease specific. For example, measles antibodies will protect you from getting measles if you are exposed to it again, but they won’t protect you from getting mumps if you are exposed to mumps.

Except in instances in which viral testing is delayed, antibody tests should not be used to diagnose a current COVID-19 infection. An antibody test may not show if you have a current COVID-19 infection because it can take 1–3 weeks after infection for your body to make antibodies. To see if you are currently infected, you need a viral test. Viral tests identify the virus in samples from your respiratory system, such as a swab from the inside of your nose.

What do your results mean?

If you test positive

• A positive test result shows you may have antibodies from an infection with the virus that causes COVID-19. However, there is a chance a positive result means that you have antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold.

• Having antibodies to the virus that causes COVID-19 may provide protection from getting infected with the virus again. If it does, we do not know how much protection the antibodies may provide or how long this protection may last.

• Talk with your healthcare provider about your test result and the type of test you took to understand what your result means. Your provider may suggest you take a second type of antibody test to see if the first test was accurate.

• You should continue to protect yourself and others since you could get infected with the virus again.

  • If you work in a job where you wear personal protective equipment (PPE), continue wearing PPE.

• You may test positive for antibodies even if you have never had symptoms of COVID-19. This can happen if you had an infection without symptoms, which is called an asymptomatic infection.

If you test negative

• You may not have ever had COVID-19. Talk with your healthcare provider about your test result and the type of test you took to understand what your result means.

• You could still have a current infection.

  • The test may be negative because it typically takes 1–3 weeks after infection for your body to make antibodies. It’s possible you could still get sick if you have been exposed to the virus recently. This means you could still spread the virus.

  • Some people may take even longer to develop antibodies, and some people who are infected may not ever develop antibodies.

If you get symptoms after the antibody test, you might need another test called a viral test​.

Regardless of whether you test positive or negative, the results do not confirm whether or not you are able to spread the virus that causes COVID-19. Until we know more, continue to take steps to protect yourself and others.

Learn more about using antibody tests to look for past infection.

last reviewed:  June 30, 2020

Development of Face Processing

…The development of face processing during infancy and childhood is one of the most extensively researched topics in perceptual development (6, 32). There is general agreement that face processing begins at birth because newborns: prefer face-like stimuli (see figure 2) to non-face stimuli (4); imitate another’s facial movements (33); prefer familiar over unfamiliar faces (34, 35); and attend to attractive over unattractive faces (36). However, some of these preferences at birth may reflect newborns’ general tendency to look at certain stimulus-general configurations that happen to be similar to faces (37). Despite this early start, researchers also agree that face expertise development is protracted with some investigators arguing that children reach adult levels only after puberty (38). More recently, however, Crookes and McKone (39) suggested that the face processing system was already very much mature by 5-6 years of age. The extensive literature notwithstanding, surprisingly, the role of experience in the development of face processing ability has until recently received little attention.

…This review of the developmental literature reveals early competence in face processing abilities, with infants presenting a preference for face stimuli and facial discrimination using featural, configural, and holistic cues. This early competence is then later refined as evidenced by age-related changes throughout childhood. Some of the refinements are likely due to the development of general cognitive abilities, whereas some others (e.g., configural processing) may be face-specific.

Although biological factors may initially play some role in biasing the newborn’s visual system towards faces in their environment, the existing evidence overwhelmingly suggests the important role of experience in the development of face processing expertise. The role of experience has been implicated in all aspects of the development of face processing expertise and from infancy through childhood. For example, it is evident that experience plays a crucial role in infants’ discrimination and children’s identity judgments for different categories of faces (i.e., species, race, gender, age), with better recognition abilities for the more familiar face categories (i.e., own-species, own-race, female, and own-age). However, considering the quasi-experimental nature of the existing studies, well-controlled experiments that directly manipulate experience (e.g., training studies) are needed to establish fully the causal linkage between differential experience and the development of face processing abilities.

The present review of the existing literature on the development of face processing has also revealed significant gaps in our research endeavors. While most of the recent exciting discoveries have been made with infants in all aspects of face processing, relatively limited knowledge has been gained about childhood face processing abilities, except for the development of facial configural processing. Future studies need to examine how children’s classification of faces at the basic and various subordinate categorical levels develops with age and to what extent such classification is related to their increasing abilities to process faces at the individual level, which in turn will provide a more comprehensive developmental account of the formation of face processing expertise.