People who catch SARS-CoV-2 can be placed into four broad categories:
(1) Sub-clinical cases, who are asymptomatic
(2) Those with infection in the upper respiratory tract, who have mild fever and a cough, and possibly other milder symptoms like headache or conjunctivitis.
(3) Those with more severe fever and cough, as well as flu-like symptoms that would cause them to present to hospitals and surgeries, and keep them off work.
(4) Those who develop severe illness with features of pneumonia.
In Wuhan, it worked out that from those who had tested positive and had sought medical help, roughly 6% had a severe illness.
Why has SARS-CoV-2 spread more readily than the SARS Virus?
A number of factors, some specific to the virus and some purely down to “luck”, are responsible for the much bigger patient numbers with COVID compared to SARS. The main feature of SARS-CoV-2 that makes it more easily transmissible than the SARS virus is the fact that it undergoes significant replication in the upper airways. This means that it can be spread readily by infected individuals by breathing, talking, or especially by coughing or sneezing. This can happen even if they are asymptomatic, or during the 5-14 days before they develop symptoms. A single cough will release about 3,000 droplets, and these travel at approximately 50 miles per hour. Most of these droplets are large, and fall quickly, but many stay in the air, and can travel, for example, across a room in a few seconds. A single sneeze releases about 30,000 droplets, with droplets traveling at up to 200 miles per hour. Most of these are small, and travel significant distances. If a person is infected, the droplets in a single cough or sneeze may contain as many as 200 million SARS-CoV-2 particles!
Other factors that increased the spread from Wuhan were: (1) the fact that international flights from China are now 9 times commoner than they were at the time of SARS, and (2) the timing of the Chinese New Year in relation to the start of the outbreak.
COVID: Much more than a Respiratory Infection:
The experience of SARS led doctors to expect COVID to be primarily a respiratory infection. Common symptoms of SARS include fever, cough, body pains, headache and shortness of breath. It can sometimes cause diarrhoea. COVID has turned out to be a much more complex illness (see my next post), but the effects of SARS-CoV-2 on the lungs are the ones that are most severe, and most deadly. The mortality rate of SARS was around 10%. Although COVID’s mortality is probably around 1%, the vastly greater number of COVID patients makes even this 1% highly significant.
COVID Lung Disease:
Like everything else about COVID, the lung disease it produces has turned out to be much more complex than we expected. Doctors thought that most patients with COVID-19 pneumonia would meet the typical criteria for acute respiratory disease syndrome (ARDS). Such patients often have stiff lungs (reduced pulmonary compliance), and often require mechanical ventilation. It became apparent early on that many of the COVID-19 pneumonia patients had severe reduction in blood oxygen with relatively normal lung compliance: so-called “silent hypoxaemia”. Some have suggested using the term Type 1 for the patients with near normal lung compliance, and Type 2 for those with decreased compliance. Others have pointed out the current lack of patient data, and experimental evidence to support this model. Management of these different groups of patients has certainly presented major challenges for intensive care units. An additional problem is the fact that if ventilators are not used appropriately in these patients the machines can injure the lungs themselves.
Effects of COVID on Coagulation:
Part of the explanation for the unexpected features of COVID lung disease may be related to the effects of the virus on blood vessels, leading to clot formation (see next blog post). These clots can form in arteries and veins, including those in the lungs (pulmonary thromboembolism). Clots in the pulmonary circulation can cause a low blood oxygen level due to a phenomenon called ventilation-perfusion mismatching.