SARS-CoV-2 and COVID-19 disease first emerged as a cause of severe respiratory  infection in Wuhan. The first two cases in the United Kingdom were seen in late  January 2020. In March 2020, The World Health Organization declared a SARS-CoV 2 pandemic.  

In adults, the clinical picture varies widely. A significant proportion of individuals are  likely to have mild symptoms and may be asymptomatic at the time of diagnosis. Symptoms are commonly reported as a new onset of cough and fever, but may  include headache, loss of smell, nasal obstruction, lethargy, myalgia, rhinorrhoea, taste dysfunction, sore throat, diarrhoea, vomiting and confusion. Fever may not be  reported in all symptomatic individuals. Progression of disease, multiple organ failure  and death will occur in some individuals. 

As with other coronaviruses, SARS-CoV-2 is an RNA virus which encodes four major  structural proteins.  

Most vaccine candidates focus on immunisation with the spike glycoprotein, which is  the main target for neutralising antibodies following infection. Neutralising antibodies  that block viral entry into host cells by preventing interaction between the spike protein  and the host cell are expected to be protective.  

The Pfizer, BioNTech vaccine 
The Pfizer, BioNTech vaccine is a lipid nanoparticle-formulated mRNA vaccine. The  mRNA encodes the SARS-CoV-2 full length spike protein. The mRNA in this vaccine  is translated and prescribed by the body to produce the spike protein. The protein  then acts as an intracellular antigen to stimulate the immune response. The mRNA in  the vaccine is normally degraded within a few days and cannot incorporate into the  host genome. Data from the Pfizer, BioNTech vaccine trials undertaken in over  40,000 individuals indicate high vaccine efficacy, with no serious safety concern  observed.  

AstraZeneca COVID-190 vaccine  
The AstraZeneca COVID-19 vaccine uses the replication deficient chimpanzee  adenovirus as a vector that encodes the full length SARS-CoV-2 spike protein. Chimpanzee adenoviruses are non enveloped viruses, meaning that the glycoprotein  antigen is not present on the surface of the vector, that is only expressed at high levels  once the vector enters the target cells. Genes are deleted from the adenovirus to  render the virus replication incompetent and to enhance immunogenicity.

Once the vector is in the nucleus, mRNA encoding the spike proteins reduce but then  enters the cytoplasm. This leads to translation of the target protein, which acts as an  intracellular antigen. Data from vaccine trials undertaken indicate high vaccine  efficacy with no serious safety events related to the vaccine.  

The Moderna Vaccine  
This works by injecting a small part of the COVID-19 virus genetic code, which triggers  an immune response and creates antibodies in the human body able to fight the virus. The dosage for this specific vaccine requires two doses to be given. It is  recommended to administer the second dose 28 days after the first. It is approved for  use in patients aged 18 and over and it can be used by pregnant and breastfeeding  women following a discussion with their health care provider. 

Dr Paul Ettlinger 

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