Why did Covid-19 spread so quickly and take more lives than SARS?

SARS-CoV-2 is not the first virus to spread rapidly from country to country, and Covid-19 is not the first disease to be declared a pandemic, but in just a short time, tens of thousands of people have lost their lives and millions have lost their jobs.

There are similarities between Covid-19 and SARS, the first pandemic of the 21st century, which broke out in early 2003. Both were caused by coronaviruses, which are believed to have originated in Chinese wet markets. Before being contained, SARS spread to about 30 countries around the world, infecting more than 8,000 people and killing 447. The WHO declared it a pandemic in mid-2003. Those numbers are huge, but they pale in comparison to Covid-19, which has so far infected more than 1.6 million people and killed more than 95,700.

SARS killed 14% to 15% of those infected, while the WHO estimates the death rate from Covid-19 is now between 3% and 4%.

Why does Covid-19 kill more people than SARS?

Part of the problem, says Peter Collignon, an infectious disease expert at the Australian National University Medical School, is the extent to which Covid-19 affects humans. The problem, he says, is that the virus can go undetected because the majority of infected people have mild symptoms.

“On the other hand, SARS, despite its relatively high mortality rate, spread less quickly because people could detect it quickly. Like Ebola, which caused a large number of people to get sick and die, we would know about it before it spread,” Collignon stressed.

Viruses like SARS-CoV-2 thrive by letting infected people survive, says Sanjaya Senanayake, associate professor of medicine at the Australian National University.

“In fact, to be a potent virus, it has to keep the host alive so it can continue to infect,” said Sanjaya Senanayake.

Asked why Covid-19 has prompted such a strong response from governments around the world, Senanayake said: "The disease is highly contagious and none of us have immunity."

Another scary problem, according to Professor Stuart Tangye of the Garvan Institute of Medical Research, is that many people are silent carriers, not even knowing they have the disease until it is too late.

“Basically, there are still a lot of healthy people who carry the virus early on, and they don't know that they are unknowingly spreading the virus to those around them,” noted Stuart Tangye.

“For example, there are a few people on the boat who have Covid-19 but are asymptomatic. When everyone gets off the boat, suddenly you realize that hundreds of people are positive for the virus. These people carry the virus, travel everywhere and to other parts of the country and the world,” explained expert Stuart Tangye.

Meanwhile, for SARS and MERS, people usually have typical symptoms when they get sick. “The symptoms appear not long after you get infected with the virus, so you are quickly isolated to avoid spreading it to the community.”

Compared to influenza, Covid-19 is also more complicated. Influenza is estimated to cause about 646,000 deaths each year. But Professor Tangye said there are three things that separate influenza from Covid-19: community immunity, preventive vaccines, and treatment drugs. Antiviral drugs can shorten the duration of influenza. But with Covid-19, humans have not achieved any of these three things, so people are not protected from infection by the virus. Moreover, the mortality rate of Covid-19 is also said to be higher than most strains of influenza.

More evolved than the SARS virus

SCMP cited a recent study saying that the SARS-CoV-2 virus can multiply rapidly in the human throat, making it more easily transmitted from person to person than SARS. This study was published in the journal Nature on April 1 by a team of scientists from Germany and the UK, based on the clinical treatment of a group of 9 Covid-19 patients. These 9 patients all had mild symptoms, ranging in age from young to middle-aged, and were being treated at a hospital in Munich.

Accordingly, throat swabs taken from patients in the first week of Covid-19 symptoms tested positive for the virus. However, throat swabs taken from SARS patients only tested positive in less than 40% of cases. “In addition, the amount of virus also differed significantly between Covid-19 patients and SARS patients. In the current study, the peak density of SARS-CoV-2 appeared before day 5 and was even more than 1,000 times higher than the density of the SARS virus at its peak. This suggests that the Covid-19 virus has multiplied itself in the cells of the upper respiratory tract.”

Like the SARS coronavirus, SARS-CoV-2 has protein spikes that help it fuse with a receptor on human cells called ACE2, allowing the virus to enter cells. These receptors are more common in the lower respiratory tract, which is why patients are more susceptible to lung damage when they have Covid-19.

However, the spikes of SARS-CoV-2 have unique properties that make it better able to bind to human cells. German researchers note that this allows the virus to appear in high concentrations in the upper respiratory tract, even though this part has fewer ACE2 receptors than the lower respiratory tract.

According to research, the virus that causes Covid-19 is similar to the virus that causes SARS in its ability to replicate itself in the lungs and digestive tract. However, SARS-CoV-2 is more contagious because it can multiply in large numbers in the patient's throat and then spread through the patient's droplets./.