Cough, fever, and difficulty breathing are all symptoms for Covid-19, but symptoms alone may not qualify someone to be tested for SARS-CoV-2, the virus that causes Covid-19. In the US, only those individuals who meet stringent requirements, such as hospitalization with no other pathogen detected or exposure to an individual with Covid-19, are tested due to the shortage of testing resources. Shortages like this around the world have hampered efforts to understand Covid-19 and to prevent its spread, especially by asymptomatic individuals.
In the US, President Trump has said that testing for novel coronavirus infection will be limited to people who believe they may be infected. But if we only test people who believe they may be infected, we cannot understand how deep the virus has reached into the population. The only way this could work is if those who believe they may be infected are representative of the population with respect to novel coronavirus infection. Does anyone believe this is so?
In order to answer the most pressing questions we have about Covid-19 – how quickly will it spread, how many hospitalizations will be required, how many people may die – we first need to know how many people are infected today. Knowledge of the current prevalence of infection can be incorporated into a model such as the SIR model to project health care needs, as the University of Pennsylvania’s CHIME app does. Unfortunately, the data we have only tells us the number of confirmed cases, not the total number of infections.
How many people are infected now with the novel coronavirus, SARS-CoV-2? How many will be infected tomorrow? These are hard questions because we can’t just see who is infected, and we don’t know how each infected person is interacting with others, either infecting them or not. Here I’ll explain the basic framework used to estimate how many people are likely to become sick, and how many will recover or die. There’s some notation, but I’ll present each equation in words. At this level, we’re reasoning more with logic than with math.
Language is powerful, and how we use it subconsciously shapes our assumptions. Throughout this coronavirus epidemic, the public has been inundated with reports about the virus’s “mortality rate”. The use of this phrase portrays the rate as if it were a real property of the virus itself, embedded in its DNA (or RNA); that somehow it has been programmed to kill X out of every Y people it infects.