In determining the potential danger of a viral pandemic, two variables are used:
Transmissibility- the number of cases spread by a single infected individual
Severity- how ill the individual becomes with the infection, including the need for hospital care, and possible need for intubation for respiratory failure.
Currently it is too early to know what effect COVID 19 will have in the US population, but preliminary numbers and comparison with prior viral pandemics are being made.
Transmissibility is high with this virus, as is obvious by the rapidity of it’s spread throughout the world.
Experts use the Ro(R naught) or Reproduction number to define a virus’ ability to spread. It is the average number of cases of disease an infected person will transmit during the period of their infection. When this value is less than one the virus is likely to phase out over time. Values above one mean the virus will be able to continue to spread, with higher numbers indicating greater spread.
We are not yet certain of the current Ro for this Corona virus, a recent study put it at 2.2. However, other numbers of Ro such as from the Imperial College place the value at 1.5 to 3.5.
These are high enough values to be of concern.
Severity of this Corona virus derived from the Chinese CDC is divided into three groups:
1. Mild disease- 81% experience little to no symptoms.
2. Severe disease-14%, also WHO estimate, indicates low oxygen with findings if “pneumonia” on chest x ray. Admission to the hospital is likely.
3. Critical disease- 5% indicates respiratory failure and the likely need for intubation to maintain the patient’s oxygen level, as well as septic shock, and the dysfunction of body organs.
The current case fatality rate is 2.1%, however this may be high because at the beginning of a pandemic the most severely ill patients are encountered initially.
We might have been lulled into complacency by the behavior of other viral pandemics over the past 17 years which had low transmission rates, but high severity rates. Examples include:
SARS-CoV(2003)
MERS-CoV(2012 to current)
For comparison sake, seasonal influenza infections have Ro of roughly 1.5, and are responsible for 3000 to 49000 deaths per year(1976 to 2007).
The 1918 influenza pandemic had Ro 1.34 to 3.21, and lead to 675,000 deaths in US of 103 million US population (0.65% of the population then). No vaccination was available, medical treatment has advanced greatly, and quarantining was attempted.