Virulence depends upon whether a virus is both: 1) contagious, and 2) deadly. These two properties can be plotted in a Cartesian coordinate system that places the most contagious at the right and the most deadly at the top.
The reason that recent outbreaks like SARS, MERS, and Ebola were so frightening is that they are so deadly, not because they were highly contagious. By contrast, the reason that COVID19 has motivated so many cities to shutdown is that it is highly contagious, albeit not as deadly.
Data from South Korea, which has done extensive testing, indicates that estimates of COVID-19 contagion must be revised upwards, which would move the orange rectangle in the figure above down and to the right--closer to chicken pox and further from Spanish Flu. We know now that asymptomatic children and young adults who were previously thought to be uninfected are likely to be carrying the disease without detection. The real number of cases in countries like the United States, which has not implemented comprehensive testing programs, may be 100x more than the reported.
Public health strategies to combat COVID-19 can work along either or both of the 'contagious' and 'deadly' axes by slowing the rate of transmission, or by reducing fatalities. Strategies such as social distancing, travel bans, quarantine, and self isolation all act to reduce exposures, and therefor reduce the rate of transmission. These are called "suppression" strategies in a recent Imperial College London study (Ferguson et. al, 2020).
Strategies that seek to reduce fatalities among the infected include expanding health care infrastructure, such as increasing available ventilators and intensive care unit admissions, as well as building immunity in the general population. The Imperial College study calls these strategies "mitigation".
The problem with mitigation is that treatment of vulnerable populations is expensive, resource intensive, and takes a long time. The number of cases requiring intensive care in Wuhan, China necessitated 50,000 additional health care workers and the construction of 14 temporary, new emergency hospitals. These resources will not be available in many regions as the pandemic becomes geographically widespread, and infection rates grow exponentially. Consequently, a million or more Americans may die from complications associated with COVID-19, should suppression fail.
Almost all of those fatalities will be among patients over 60 years old. The fatality rates estimated in Table 1 of the Imperial College study below are consistent with data from Italy, and they show that risks of infection to those younger than 50 years is minimal. In fact, few children and young adults are likely to require hospitalization. Fewer still will require critical care, and the vast majority of those will recover when they get the care they need.
To protect the most vulnerable segment of the population, public health officials in major US cities like San Francisco, Boston, New York City, and Los Angeles, have shut down businesses, churches, schools, and other places where people gather in large numbers. These policy prescriptions exemplify suppression, and they are characteristic of those that prevented COVID-19 from escaping Wuhan to infect other major Chinese cities like Shanghai and Beijing.
The advantage of suppression is that it has been proven to reduce the peak load of new cases at the onset of the pandemic. The figure below shows a hypothetical intensive care case load curve under mitigation (top curve, solid line) and suppression (bottom curve, dashed line) scenarios. The mitigation peak is higher, representing a faster rate of transmission to vulnerable population. Not even additions to medical treatment capacity (blue dashed line) can avoid the necessity of rationing medical treatment in such a way that some people who need it at the peak of the crisis must be denied. Fatalities among this group will large, even though the total case load drops fast. By contrast, suppression slows the rate of transmission, allowing more time to add additional medical capacity (blue, dashed line) to ensure that all patients needing intensive care can be served. Under suppression, the pandemic will be longer-lasting and less deadly.
The problem with suppression, according to the Imperial College study, is this:
"The major challenge of suppression is that this type of intensive intervention package – or something equivalently effective at reducing transmission – will need to be maintained until a vaccine becomes available (potentially 18 months or more) – given that we predict that transmission will quickly rebound if interventions are relaxed."
The COVID-19 virus is so contagious, and will reside in asymptomatic cases for so long, that even after the number of new cases dwindles and suppression tactics are relaxed, a resurgence of the virus in inevitable. The 1918 Spanish Flu pandemic presents a convincing example, in which deaths peaked in Oct 1918 (Lai 2015). In fact, the Second Deadly Wave is characteristic of many of the viral pandemics of the Industrial Age. The figure below shows data from four different pandemics that is consistent with the 1918-1919 pattern (B, upper right) showing that the deadliest months occur during subsequent waves. In northern latitude cities, these waves always occur during the Fall and Winter months. There are several reasons for it, including the fact that people tend to crowd indoors when the temperatures drop, but one of the most important reasons is that the lack of sunlight causes a Vitamin D deficiency that compromises the immune system. Winter is "flu season" because crowding indoors makes a viral infection more contagious, while Vitamin D deficiency makes it more deadly.
What the figures do not explain is how these pandemics ever came to a close. That is, how were these viruses eventually stopped?
The answer is that eventually, enough people in the general population became exposed in ways that allowed them to build antibodies. In other words, they developed immunity. This is exactly how vaccines work.
The first vaccine, for smallpox, was introduced in 1798 by Dr. Edward Jenner, who observed that milk maids who had been exposed to a similar, less dangerous virus called cowpox developed immunity to the more deadly smallpox virus. Dr. Jenner reasoned that intentionally inoculating patients with a similar, but safe, form of the smallpox virus may stimulate the immune system to develop antibodies that are effective against the more dangerous forms. The polio vaccine works in this way. The vaccine is an inactive form of the polio virus that stimulates production of antibodies that grant immunity to polio.
Such immunity might last for several months, as is the case with the common cold, or for several decades, as is the case with chicken pox. We don't have enough experience with COVID-19 to know how long immunity will last. If it is like the common cold, then seasonal outbreaks of COVID-19 will remain a reality for decades. Smallpox it took two centuries after discovery of the vaccine to eradicate smallpox. It stands to reason that when a COVID-19 vaccine is discovered, eradication will require a similar, long-standing effort over several decades.