For SARS-1, it was estimated that four importations were necessary for one transmission chain to be initiated (the other three importations would fail to start epidemics and die out). However, when an outbreak did occur, it was likely to be explosive. For SARS-2, it looks like the variation in R0 is somewhat lower than for SARS-1, so while

In terms of evolutionary biology, the virus has had what is known as an “ecological release.” This refers to the expansion of range and the population explosion that occurs when a species is freed from constraints it previously faced. The typical example of this is invasive species introduced by humans such as the cane toads that overwhelmed

For instance, measles is one of the most infectious diseases known, with an R0 estimated to be 12 to 18 (that is, a single infected person typically can infect somewhere between twelve and eighteen other people). Chicken pox is 10 to 12. Smallpox is 3.5 to 6. Ebola is 1.5 to 1.9. Seasonal influenza ranges from 0.9 to 2.1.

One of these features, paradoxically, was that SARS-1 was actually too deadly. Epidemiologists quantify the lethality of pathogens in two primary ways. The infection fatality rate (IFR) is the probability a person will die if he or she gets infected. The case fatality rate (CFR) is the probability a person will die after being diagnosed with the

Why did the SARS-1 outbreak die out after its extraordinary opening act—featuring rapid global spread, many super-spreader events, and rising alarm—whereas the SARS-2 outbreak did not? It was not just that there was somehow a more efficient public health response in 2003. After all, SARS-1 spread to many countries and it did so in many places, from

While the coronavirus causing the 2020 pandemic is new, we have lived with other coronaviruses for a long time. In our species, four types of coronavirus are endemic, meaning we have reached a détente with these pathogens.

The virus had announced itself with extremely unfortunate timing, right at the start of the annual chunyun (春运) migration in China that was taking place in the run-up to the Lunar New Year festival, on January 25, 2020. During this period, over three billion trips are typically made, a mass movement that puts the annual Thanksgiving travel in the

The latent period is often shorter than the incubation period in SARS-2, but that was generally not the case in SARS-1. The difference between the latent period and the incubation period is sometimes known as the mismatch period; it’s calculated by measuring the incubation period and subtracting the latent period. The difference between these two

The disease also displays a great range of severity across patients. Perhaps half of those infected are entirely asymptomatic. For the remainder, the range of outcomes stretches from a mild illness (in most cases) to hospitalization (in perhaps 20 percent of cases) to death (in perhaps as many as 1 percent of cases).