People with coronavirus disease (Covid-19) are the most infectious during the first week of symptoms, according to a recent study, which explains why this disease is spreading faster than other coronavirus diseases, like the Severe Acute Respiratory Syndrome (Sars) outbreak caused by Sars-CoV-1.
Sars-CoV-2, the virus that causes Covid-19, replicates in the upper respiratory tract before moving down to infect the lungs, which leads to people shedding huge amounts of the virus through coughing in the first week of symptoms (peak at 7.11 × 108 RNA copies per throat swab), according to a study published in the journal Nature on April 1.
Researchers analysed data from nine patients in Munich with comparatively mild symptoms to understand how infectious they were and when, and how much virus they shed, or excrete.
The active replication of Sars-CoV-2 begins in the upper respiratory tract of the patients, from the larynx up where it stays and infects people before infecting the lungs and causing severe respiratory symptoms, the study found.
Sars-CoV-2 is genetically very similar to Sars-CoV-1, but subtle genetic differences lead to significant differences in how it spreads and causes severe illness. Both Sars-Cov-1 and Sars-CoV-2 infect the throat and lungs by using their external spike proteins (which give the virus family its name, Corona, Latin for crown) to enter the humans’ cells by attaching to a cell protein called ACE2, which is also present in the cells in the throat and lungs. Once inside, they hijack the human cell machinery to replicate and cause infection.
“Sars-CoV-2 takes up residence in the throat cells first, which doesn’t cause significant symptoms. The person can remain asymptomatic or might not think they have anything worse than a cold. And from that person’s throat… it can readily spread to others. Over the course of a week, in some patients, it will move into the lung neighbourhood and replicate just as Sars-1 would, causing severe symptoms, by which point the person is quarantined, but no matter since it had successfully spread,” said Peter Kolchinsky, virologist and author of The Great American Drug Deal, in a tweet.
“These are two different viruses (Sars-CoV-2 and Sars -CoV) so their behaviour will be different. The host-virus interaction properties are different. In Sars, there was only lung infection and one could spread it when the infected person coughed, but in case of Sars-CoV-2, the infection begins in the upper respiratory tract so you can infect even if you talk. The virus is in the nasopharyngeal fluid which mixes with saliva so you can infect when you sing, speak or shout. Silent transmission is common in this disease. So screening at airports for temperature was appropriate in case of Sars but not for Sars-CoV-2,” said Dr Jacob John, veteran virologist and professor emeritus and former head of virology at Christian Medical College.
On the other hand, Sars-Cov-1 directly enters the lungs and causes serious respiratory symptoms very rapidly, leading to the person being hospitalised and isolated very quickly after getting infected. This limits the chances of the virus replicating in the throat and spreading to others through coughing.
“So Sars-1 was a comparatively dumb virus. It went straight for the lungs, announced itself before it could spread to others, and so got social distanced into extinction. But Sars-CoV-2, the one plaguing us now, is stealthier, spreading first before revealing itself (and causing harm),” said Kolchinsky.
“Basically, Sars-COv-2 is much more infectious (than other coronaviruses). Patients are seen to be shedding a very high amount of virus. Now there are studies showing that the infection can spread not only through droplets but also aerosols or microdroplets. Otherwise, it has the same pathology as Sars-like cytokine storm, the involvement of the lung and GI tract. The involvement of the upper respiratory tract in SARS-CoV-2 makes it more infectious,” said Dr Shobha Broor, former head of the department of virology, All India Institute of Medical Sciences, New Delhi.
Sars-CoV-2 is also more severe than seasonal flu in part because it is more efficient at preventing cells under attack from mounting an immune system. For example, Sars-CoV-2 blocks the cells from making interferon, the alarm-signalling protein, by snipping off protein markers that serve as distress signals. This makes the transmission rate of Sars-CoV-2 higher than that of the 2009 H1N1 influenza pandemic, which was also a less deadly disease.
The study also found the virus remained detectable in the sputum after symptoms stopped, with two of the patients with some early signs of pneumonia, continuing to shed high levels of the virus in the sputum until day 10 or 11
“(The takeaway is) …that beating this virus means social distancing and wearing masks even if we think we aren’t infected. Because we might be. The virus might be replicating in our throats without us knowing (that’s its evil plan!), so put up a roadblock,” tweeted Kolchinsky.
