“SARS, caused by the virus known as SARS-CoV-1, appeared in 2002 and spread to 30 countries. But it infected only 8,422 people and killed just 916 before being declared “contained” by the World Health Organisation eight months later. On the other hand, the virus behind covid-19, SARS-CoV-2, has infected more than 18m people and killed more than 700,000 so far.
On the surface, the pathogens have several aspects in common, beyond belonging to the same family of coronaviruses (and having genetic sequences that are 79% identical). Both emerged in China in late autumn and were noticed in a place where wild animals and people were in close proximity: a seafood market in Guangdong for SARS, and a similar market in Wuhan for covid-19. Both produced respiratory ailments and could be lethal. Their transmissibility or “reproduction number” (the now famous R0) was also roughly the same, with each victim infecting about three other people, on average.
But that is where similarities end. The virus behind SARS had intrinsic qualities that made it harder to spread and easier to control, compared to the one behind covid-19, which has overwhelmed the world. This is because of their respective fatality rates, symptoms, infectious periods and a nuance in their reproduction number. Consider these features in turn, for they explain why covid-19’s virus has been so destructive—and how we can best fight it.
… the case fatality rate, which is the probability a person will die if they come to medical attention. SARS’s case fatality rate was around 11%. Covid-19’s is estimated to be in the range of 0.5-1.2%, making it one-tenth as deadly as SARS. This makes it harder to control because there are more walking wounded. SARS did not spread as far because it was, paradoxically, too deadly.
The period between becoming infected with a pathogen and showing symptoms is called the “incubation period.” This ranges from 2 to 14 days for covid-19 (hence the recommended 14-day quarantine period) and is typically 6-7 days. For SARS, the incubation period is 2-7 days.
But the crucial difference between the two pathogens is a related metric called the “latent period”. This is the time between becoming infected and being able to spread the disease to others. The incubation and latent periods are not always the same, a difference known as the “mismatch period”.
When the incubation period is longer than the latent period, asymptomatic carriers abound, as with HIV: an infected person is unaware of it without a blood test. When the latent period is longer, as with smallpox, a person shows symptoms before (or at the same time as) they are infectious: the illness is clearly visible to all. Because the incubation period of covid-19’s virus is generally longer than the latent period, it has been far more devastating than the virus behind SARS. Covid-19 patients take about seven days from infection to show symptoms, but they can spread the disease for 2-4 days before they are symptomatic. In fact, the 1-2 days before symptoms may be when they are most contagious.”
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