Analysis puts the “reproductive number” of this virus at 2.2 or higher, meaning each infected person infects at least two other people. This is an explosive number. By comparison, the median value of studies of seasonal and pandemic influenza viruses put the reproductive number of ordinary influenza, which can sicken 10 to 20 percent of the population any given winter, at 1.28, with the 1918 pandemic virus at 1.8 — which was enough to kill between 50 million and 100 million people in a world population that is about a quarter of today’s population.
To contain the Wuhan virus, its reproductive number must be brought below 1. Is this possible? The answer depends on whether this virus transmits more like severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), or more like influenza.
The virus which caused the SARS outbreak also had a high reproductive number. But SARS required close contact for transmission, the incubation period seemed to have been longer than the Wuhan virus, and, most important, people with SARS were most infectious when they were already too ill to mix with the public. (That’s why in Hong Kong, Singapore, and Taiwan, upwards of 90 percent of transmission occurred in hospitals.) Those factors allowed medical interventions — rapid case detection, isolation, contact tracing and stringent infection control in hospitals — to eradicate the disease in humans.
MERS has a high reproductive number and a case mortality rate of roughly 40 percent, but it has not spread widely in the general population because it does not transmit easily and, like SARS, cases are frequently clustered in hospitals. It can be controlled, if not eliminated, by the same methods that worked against SARS.
By contrast, none of those control measures can contain influenza. In influenza, aerosol transmission is most important, though the virus can survive on a hard surface for hours and so also be transmitted by touch. Also, the incubation period can be as short as one day and the disease can be transmitted by people before they have any symptoms at all, making contact tracing and even quarantine ineffective.
Perhaps the most relevant data comes from U.S. Army camps during World War I; in some camps, soldiers were inspected twice a day and immediately isolated if they showed any flu symptoms, and if a unit had more than one symptomatic soldier, the entire unit was quarantined. In camps where measures were rigidly enforced, disease transmission was slowed, but quarantine had no impact on rates of infection or death; where the measures were less than rigidly enforced, they had no impact whatsoever. If quarantine could not work in the military during wartime, one can hardly imagine it working in a civilian community during peacetime.
What will work for the Wuhan virus? Reports, so far, suggest that close contact is not necessary for transmission and, while the information is not definitive, health authorities in China have said that asymptomatic individuals can infect others — making it much more like influenza than either SARS or MERS.
All that makes containment look impossible, but there is encouraging news. The spatter of cases outside China has not led to any spread in general populations. At this writing, there are 142 people infected in 22 locations outside mainland China — almost all of whom had traveled to the mainland. The fact that only one U.S. case has yet surfaced among someone who did not come from China seems inconsistent with what’s happening there. If this disease were that easy to transmit there should have been many more cases by now.
Can it be contained? Again, if the disease transmits like influenza, it appears that containment is all but impossible. Eventually, and more likely sooner rather than later, it will spread in countries around the world.
The three most important remaining questions involve morbidity (what percentage of a population will get sick), virulence and how well humans develop an immune response. Since no solid numbers exist as to how many people have become infected — we only know how many have sought treatment and tested positive — we have no idea of the morbidity, and the current estimate of 2 percent case mortality is not reliable. Two percent is a frightening number if the disease spreads widely; that was the approximate case mortality for the 1918 influenza pandemic. We do, however, know that 11 percent to 15 percent of those hospitalized die — suggesting a lethality roughly double seasonal influenza, which kills anywhere from 3,000 to 61,000 Americans each year.
Longer term, if this virus does become endemic, will a vaccine protect people against infection? Some vaccines, such as against measles, provide almost 100 percent protection. But the effectiveness of influenza vaccines varies year to year, and have ranged from 10 percent to 62 percent effective. That, in the long run, may be the most important unknown.