When your body encounters a virus, any virus, the virus enters your body and starts to multiply. It multiplies fast. It does this by taking over the genetic systems of the cells, and so the cells can’t maintain themselves and die. When this happens the cells burst open and release many copies of the virus from each cell into your body. Each copy of the virus finds another cell to invade. This applies to all viral infections, not just Covid-19.
Let’s say that each cell releases 100 copies of the virus, a number which is far smaller than the real number. The real number is much higher. So in the first step one virus becomes 100 viruses. Each copy infects another cell, and each produces 100 viruses. So, at the second step, 10,000 virus copies are released. At the third step, 1,000,000 copies are released. At the fourth step, 100,000,000 copies of the virus are released. By the time that the replication has gone through 10 steps, an astronomical number of copies of the virus are floating around your system. After 20 steps… Remember that replication factor is enormously more than 100, too. This is termed exponential growth.
Of course, the virus doesn’t have things all its own way. Your body has an immune system. It detects the virus and starts to fight it. If the virus has not yet managed to reduce your lungs to a bag of slime, or dismantled your nervous system, or whatever, your immune system starts to fight the virus. It starts killing infected cells thus preventing them from creating copies of the virus.
It recognises infected cells by proteins on the surface of the cell. These proteins are different for different viruses, and the immune system ‘remembers’ the signature of an infected cell, and eventually, if you are lucky, the immune system succeeds in killing off all virus infected cells and the infection is over.
(Please note that specialists in this field would probably find the above hysterically funny, but I don’t think that it is too far off the mark.)
So that’s what happens when a person encounters a virus for the first time. The outwards signs of the battle between the virus and the immune system are what we consider to be the disease. That is, raised temperature, headache, spots, cough, sneezing, and so on. Maybe more life-threatening symptoms. The virus does not directly cause any symptoms itself.
When a person who has already encountered the virus encounters it again, the immune system already knows about the virus and kicks in immediately. It doesn’t have to work out, firstly, that there is a virus and secondly, how to fight it. Your temperature might peak and you might have a headache, but any symptoms will likely be much reduced this time around, and virus will be killed off much faster.
But while your body is fighting the virus, there is a short time interval when the virus is in your body and you may be mildly infectious. Since your body is already fighting the infection, the infectious period will be short, and you won’t be ‘shedding’ as much virus to infect others.
The vaccine, any vaccine, is designed to fool the immune system and provide it with the necessary information to fight the virus without actually inflicting the virus on the body. It does this either by supplying the body with the dead virus or with a very much weakened virus, or with the information necessary to detect virus infected cells.
To do this, it needs to trigger some parts of the immune system into action, but doesn’t need to do more than that, so any side effects will be minor. A sore arm, for example.
A side effect indicates that your body is configuring your immune system to handle the virus. If you had previously encountered the virus in the wild, you would, at that time, possibly have had a severe infection. Side effects of the vaccines are rare, but if there is one, it signifies that your body is doing its job, and preparing a defence against the virus.
Of course, it is theoretically possible that you might have a reaction to some component of the vaccine. It’s theoretically possible that you will be killed by a piece of falling space junk such as a falling space toilet, of course. Or you could win the lotto. All these things would be of the same order of possibility. They are theoretically possible but very unlikely.
Worrying about the contents of the vaccine seems to be silly. If you end up in hospital after an accident, or you are hospitalised for something like pneumonia, or something worse, you would be pumped full of all sorts of things. Full blood. Blood plasma. Antibiotics. Many other things that the doctors and nurses would put into your body to save your life. They might even shoot X-rays through you, or irradiate you to kill cancer cells. It is ludicrous to worry about the contents of a simple vaccine, most ingredients of which are present only in microscopic amounts.
If you tell people that there is the possibility of a side effect on taking a medicine, then a number of people will experience that side effect, even when they are not given the medicine, but are instead given a placebo. A chalk pill or an injection of plain saline. If they are worried or concerned, the likelihood of the side effect will be higher. This is called the nocebo affect, and been blamed for up to two thirds of the ‘adverse reactions’ to the Covid-19 vaccine.
When someone who hasn’t been in contact with virus catches it, it takes a relatively long time for the body to learn how to fight the virus, which means that the body is shedding virus during the period that the body is learning how to fight the virus, and while the body is destroying the virus.
On the other hand, if the person has already had the virus or has been vaccinated against it, the body doesn’t have to learn how to fight the virus. That means that the shedding period is much shorter and the level of the virus in the body is lower, so that the total amount of virus that is shed is very much lower.
In summary, a vaccinated person sheds less of the virus for a shorter period and doesn’t usually get sick. There is less chance, therefore that they will infect someone else.
People often ask if you can catch the Covid-19 virus twice. Of course you can. The protection that the vaccine gives you wanes over time, which is why we need booster shots. This is also true for other viruses, like the flu virus. If you have had the flu, then you are protected for a while by your immune system, but the immunity fades over time. That, and variants of the virus, are why we have to have a flu booster shot every year.
If you don’t come in contact with the virus, then vaccination has no effect. If you do encounter it, it doesn’t stop the virus from entering your body. No vaccine does. It just allows your body to deal with the virus quickly and efficiently. Sometimes, in spite of you catching the virus before or being vaccinated, your body doesn’t properly remember that you have had it or been vaccinated, because the protection has waned.
In which case you will get the infection again, but it is likely that your body will at least partially remember the previous infection, so if you catch the virus twice the second time will likely be less severe.
Can you infect others if you are vaccinated? Yes, you can, if your body is fighting off the infection at the time, but the chances of you doing so are very much reduced. The virus will be in your body for only a short period, and you will not have time to shed much of the virus in that time.
I have tried to explain what happens when you are infected by a virus, such as Covid-19. It is always a good idea to be vaccinated. If you decide not to get vaccinated because of the very small chance of having a usually mild side effect, that is like being in a crashing plane and refusing a parachute because ‘you know, those parachute things have been known to get tangled and people have been killed by tangled parachutes’.