Coronavirus: what is it?

Coronavirus: what is it?

Coronaviruses are named for the corona, or crown, of surface proteins that the virus uses to penetrate the cells of its host. They originate among various species of bats, and have mostly transferred to humans in the last 100 years.

Four seasonal variants of coronavirus are endemic in human populations and cause millions of infections each year with low severity. Versions of coronavirus cause about 10% of cases of the common cold, which notoriously has never been cured. A cold is relatively harmless if uncomfortable. Infectious bronchitis is another, slightly more severe variant

The dangerous variants of coronavirus have included

·         SARS (10% fatality rate, killed 774 in 2003, spread from exotic mammals in wet markets)

·         MERS (35% fatality rate, killed 858 in 2012, found in 95% of dromedary camels, rarely transferred between humans).

The new variant SARS-CoV-2 causes the disease COVID-19. This has a much lower fatality rate than the killers SARS and MERS, but it spreads far more rapidly than these other killers, and has very quickly developed pandemic status. Eighty percent of CV cases are mild or symptom-free, especially among the young. One could cynically say the virus is the perfect Nazi  ‘grim reaper’ disease as it is rarely fatal for young 'socially productive' people, while those over 80 have a 15% death rate –Nature’s unforgiving way of correcting our increased longevity.

Comparison with influenza has been inevitable, since the symptoms and the means of spread through aerosol droplets are similar. The community has developed resistance to influenza and fatality levels are under 1% (considerably lower than this in developed countries). Influenza also spreads extremely rapidly, so that 10% of unvaccinated adults are infected with ‘flu each year. It kills 300,000 to 650,000 people annually. Sometimes major outbreaks of new variants such as Spanish ‘flu can have a 15% death rate.

Infection 

Initially, it was thought that COVID-19 was less infectious than 'flu, but the extremely rapid rate at which it has moved through European populations has given this the lie.The R0 statistic that measures how many people are infected on average by each infected person has been set as high as 3 for  COVID-19, whereas it is around 1.5 for most influenza strains. This is partly because influenza is endemic so that some 'herd immunity' has developed, and it no longer has the virulence of a fresh strain like CV. 

Other historical plagues have been far more deadly - smallpox kills between 30% to 90% of new populations it encounters, bubonic plague has killed 20% to 90% of even previously exposed populations, and typhoid, which is bacterial, has a 25% death rate.

In many diseases the mortality rate is much lower in populations that have been exposed for an extended period. Malaria has a fatality rate of 0.4% in Africans, but about 3% in tourists. Because of its frequency, it remains the world’s biggest killer, with 400,000 deaths annually (peaking at a million in 2004), mostly young children. As these now occur largely in Africa they tend to go unnoticed.

Onset

It’s the great irony of the Twitter age that we know too little about the novel coronavirus as we drown in information updates about it. 

COVID-19 was first noticed amid the background of many other respiratory diseases, because of the large number of cases of atypical pneumonia it caused. According to data from the Chinese epidemic, about 21% of CV patients become severely ill.

SARS lung infection honeycomb pattern

CV begins to show symptoms in an average of 5 days, though cases taking as long as 14 days to appear are known. Mild symptoms can then persist for several weeks. This is accompanied by heavy infestation  of the virus in the mouth and upper bronchial area, during which time the patient is extremely infectious.

The novel coronavirus rapidly invades human lung cells - particularly the cilia cells that clean the airways. These slough off and fill he lungs with debris. The immune system is then activated and attempts to deactivate the bad cells. Sometimes the immune system goes haywire and starts to kill good cells as well, filling the lungs with even more debris. SARS (1 and 2) can punch holes in the lungs, giving them a unique honeycomb- like appearance, and causing 'ground glass opacity'. The holes are an attempt by the immune system to protect and stiffen the lungs. In severe cases inflammation can also fill the lungs with fluid, effectively drowning the patient.  

In other cases the immune system can attack the major organs instead in a 'cytokine storm', giving a complete system failure. This may be related to the presence of other conditions - possibly explaining why so many of the deceased in Italy had other severe conditions or 'comorbidities' such as hypertension, diabetes and cancer. 

https://jamanetwork.com/journals/jama/fullarticle/2761044
https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30243-7/fulltext

Time to outcome 

The median time from onset to hospitalization in Wuhan was 7 days, and 8 days to acute respiratory distress. The median hospital stay for those discharged was 10 days. The median time from onset to death was  18 days. People may however lose the fight as long as three weeks after they are placed on a ventilator. 

Sources: https://www.nationalgeographic.com/science/2020/02/here-is-what-coronavirus-does-to-the-body/. Much of this material has been extrapolated from SARS-1, which appears to be quite similar if less infectious and more lethal.
https://jamanetwork.com/journals/jama/fullarticle/2761044

Summary of cases 

Severity of cases detected in Wuhan, by severity. Source: Lancet article

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