The SARS-CoV-2 Virus

About the Coronavirus Family

Coronaviruses are complex pathogens which can infect multiple host species. Coronaviruses are a cause of zoonotic infections due to human – wild animal interactions.

A novel coronavirus was discovered in China after genome sequencing of broncho-alveolar-lavage specimens from patients with unexplained pneumonia. Examination of the 2019 novel Coronavirus (2019-nCoV) under a transmission electron microscope revealed distinctive spikes that create the impression of a solar corona.

The SARS-CoV-2 Virus

The typical coronavirus (CoV) genome is a single-stranded, non-segmented RNA genome, which is approximately 26–32 kb and is coding for structural and non structural proteins.

SARS-CoV-2 differs from the original SARS-CoV by 380 amino acid substitutions including 5-6 vital amino acids in the receptor binding domain. SARS-CoV-2 is closely related to SARS-CoV (about 79%) and less to MERS-CoV (about 50%). Phylogenetic analysis suggest that bats are the natural reservoirs and source of the virus, with an unknown intermediate host, and humans as terminal hosts.

The Virus Receptor

ACE2 is a functional receptor for SARS-CoV

The receptor binding domain of the spike protein of SARS-CoV-2 forms a complex with ACE2.  ACE2-binding is more stable and has increased affinity compared to SARS-CoV.

The virus genome is changing: 93 mutations recently described including also the spike surface glycoprotein which can lead to changes in antigenicity.

The Host

The Host – Human to Human Transmission

  • Viral load peaks during the first week of illness – individuals can be most infectious during initial phase of the disease, this might account for the high transmissibility of SARS-CoV-2.
  • SARS-CoV-2 has been detected in faeces, blood, and urine samples (schematic).
  • Viral RNA can still be detected in posterior oropharyngeal saliva samples for 20 days or longer – a challenge for isolation of patients.
  • Viral load correlates with age - severe disease in older patients with SARS-CoV-2 (see graph, schematic representation needed).
  • IgG and IgM antibodies started to increase on around day 10 after symptom onset, and most patients had seroconversion within the first 3 weeks.

COVID-19 – The Disease

COVID-19 is a multi-organ disease, which can potentially cause significant or life-threatening damage, especially in the lung. 50-75% virus-positive individuals remain asymptomatic.

  • The vast majority of individuals with symptoms and more severe disease course had comorbidities like hypertension, diabetes, and cardiovascular disorders, with elevated case fatalities among elderly and frail patients.
  • Common symptoms are fever, dyspnoe, and cough. Headache, nausea, diarrhea, and myalgia are less common (<10% of cases).
  • Increased inflammation indices (reduced procalcitonin, increased CRP) are associated with clinical severity.