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Coronavirus (COVID-19) information

 

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SARS-CoV-2 (the virus) and COVID-19 (the Disease)  

SARS = Severe Acute Respiratory Syndrome). COVID-19 + Coronavirus Disease 2019

 

This is an enveloped RNA virus of the coronaviridae family – the envelope is a lipid coating and is affected by alcohol based hand rub. Some circulating members of this family cause the common cold while others have caused the 2003 SARS outbreak in humans (from horseshoe bats via a civet cat intermediary) and MERS (Middle Eastern Respiratory Syndrome) which is also most likely from bats but via a camel intermediary). The current SARS-CoV-2 which causes COVID-19 would have undergone a series of mutations that would eventually allow ease of transmission human to human. MERS and SARS outbreaks had case fatality rates (CFR) approaching 10% while that for COVID-19 is possibly 0.2% - 1%. With diligent contact tracing, both SARS and MERS were contained. If there were no intervention with the current pandemic, then the R0 (reproduction number) would be at least 2.2 - 2.6, meaning one person on average could transmit to 2.6 others. Our current efforts have sought reduce this to below 1.

 

Evidence appears to show that this infection is transmitted by virus in muco/salivary droplets. These need to reach the nasopharynx of a susceptible host in sufficient numbers to overcome that host’s defences. This is most likely to occur over a period of sustained contact e.g. 15 minutes face to face or 2 hours in the same room. Coughing, sneezing and talking may all transmit sufficient virus to the nearby susceptible person as does indirect contact e.g. touching the mouth after touching a contaminated surface.

 

When the infectious person coughs, the larger, viral laden droplets (5-10 microns in size), fall 1 – 2 metres, while the smaller droplets with less virus fall further away. The evidence shows that it is being closer and for long enough that increases risk. The much smaller droplets may evaporate to form droplet nuclei and be carried on air currents, not settling for up to an hour. If these are inhaled, the risk is lower because an infectious dose is less likely to reach the nasopharynx. Influenza is also transmitted by droplet while measles is airborne spread meaning it is transmitted by the much smaller particles further away. All may be transmitted by contact as well.

 

Each virus has a target cell that it initially infects. Some respiratory viruses e.g. rhinoviruses that cause the common cold, prefer the cooler temperature of the nose and are not likely to move lower down the respiratory tract to cause pneumonia. SARS-CoV-2 initially infects the nasopharynx but can move further down to the lungs to cause pneumonia.

 

To get an infection you need:

 

      • To be susceptible i.e. non immune
      • Get an infectious dose i.e. enough pathogen to overcome host defences
      • The pathogen to gain access to the target site

       

This is not a difficult virus to inactivate and most commonly used household disinfectants are considered effective at achieving an acceptable 4 log reduction i.e. reducing infective virus from 10,000 particles to 1 as long as:

      • The surface is cleaned first
      • The stated concentration is used
      • Wet contact is maintained for the specified time (usually a minute)

       

How long a surface remains infective varies with the surface itself ie. Less time for fibre/paper (hours) and longer for smooth surfaces (up to 2 days). However, whether enough viable particles remain after a day when other factors such as temperature and humidity are factored in is unknown. Placing of hand sanitizer everywhere and regular cleaning of high touch surfaces in public areas e.g. handles/switches/buttons reduce this risk.

 

In the 24-48 hours before symptoms develop (the pre-symptomatic phase), viral shedding commences then increases just before symptoms emerge. In order of most effective droplet transmission is sneezing (not that common with COVID-19) followed by coughing then talking (especially explosive consonants p,b,s,t).

 

All viral illnesses have some asymptomatic cases so this more than likely for COVID-19. Random swabbing of the population has been perfumed and shows this to be around 20 per 150,000. Asymptomatic or pre-symptomatic cases would not be as efficient as symptomatic cases at transmitting the infection because they are not coughing etc. The standard test is a RT-PCR test that detects SARS-CoV-2 RNA and is extremely sensitive and specific. Quicker tests for the antigen/viral RNA are now available in an hour. In time there will be a reliable antibody test where we can take blood and estimate who has had the infection.

 

Over the 18 months ie until late 2021 that this pandemic is likely to run, second and third wave may occur when virus is reintroduced e.g. travel or when precautions are relaxed.