In the previous chapter I mentioned that the contagious capacity of a virus is measured by the value of the R₀ number, or basic reproduction number. Current SARS-CoV-2 has an estimated R₀ of three, while its predecessor SARS-CoV-1, or the flu, has an R₀ of one to two. A virus with an R₀ less than one tends to disappear. A virus with R₀ greater than one tends to spread.

The fact that SARS-CoV-2 is so contagious is due to several factors, but one that seems to be very important is the ability of this virus to anchor itself to the ACE2 receptor protein, which exists on the surface of some human cells. The SARS-CoV-2 virus has different proteins in its envelope and one of them, the S protein —spike protein in English— is the one that has the perfect shape to stick to ACE2, which is the gateway to human cells. If we did not have ACE2 on the surface of our cells, SARS-CoV-2 could not enter them and would not affect us. It happens that we need the ACE2 protein to regulate some processes of our physiology. ACE2 is present in human cells at different parts of our body, such as the lungs, nose, or throat, which are the main routes of entry of the virus ¹⁴.

The spike protein of the virus is the striker of the opposing team, and ACE2 is our goal. If the virus gets its RNA (genetic material) into us, it infects us and scores a goal.

Peter Crouch, a former player for Liverpool and the England national team, looks like a spike. He’s 2.01 metres tall. Crouch was very good with the ball on the ground despite his height. He scored some great goals, as a scissor kick in the Champions League against Galatasaray. Crouch was a fine player who, throughout his career, between 2000 and 2019, played for several teams scoring 145 goals in 599 games. So, the slender Crouch guaranteed, on average, one goal every four games. If we think of Peter Crouch as the spiky protein of a coronavirus, we could say that one out of four times that it approaches the cell membrane, it remains attached to the ACE2 receptor and scores a goal.

Peter Crouch was quite effective, but not as effective as Nikola Žigić, a 2.02 meter-tall Serbian footballer who played for Racing Santander, Valencia and Birmingham, among others. Žigić was paired at Racing with Pedro Munitis, who is 1.70m, and they were a funny couple of strikers because of their different sizes.

Žigić had less class than Peter Crouch and a different body structure. He was portlier than Crouch. Žigić set aside defenders like someone who push flies away. Between 1998 and 2015, Žigić scored 208 goals in 413 matches. These stats tell us that, on average, one in two times that “Žigić spike protein” approached the goal, hooked on ACE2 and scored a goal. Nikola Žigić was twice as effective as Peter Crouch.

For the matter at hand, Peter Crouch would be the spike protein of the past SARS-CoV-1, and Nikola Žigić the spike protein of the current SARS-CoV-2. The greater efficiency of binding of the SARS-CoV-2 spike protein to the ACE2 receptor has been noted by the researchers crystallizing the protein, which is a way of seeing its detailed structure in three dimensions ^{15, 16}.

The high affinity of the SARS-CoV-2 protein S for the ACE2 receptor in our cells is one of the causes of the high R₀ number of this coronavirus, but it is not the only one. SARS-CoV-2 has longer incubation periods than other viruses —several days— so it can be contagious for longer without symptoms appearing, and even these symptoms do not appear in some cases. It is as if Žigić, in addition to being twice as efficient as Crouch, played many more games, and for more years. This would increase their total number of goals.

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