It was terrible, killing millions.
Where has it gone? Is it still around?
In: Biology
It’s become endemic, meaning it circulates through the population and pops up especially during winter. It has also mutated to become less dangerous.
This is the typical process for viruses that jump from animals to humans. Initially, the virus is more dangerous. See, viruses mostly evolve to keep their target hosts alive. If a virus is very deadly then it doesn’t spread well. So deadly viruses die out while less-deadly ones stick around and multiply. But when a virus jumps to a different host, e.g. to humans, all bets are off. The virus hasn’t adapted to thrive in that host and so it may end up being more deadly.
Next, one of two things happens. Either the virus doesn’t fare well in humans (because it’s just too deadly or doesn’t spread easily) and dies out, or it sticks with us. In the latter case, it has the potential to cause a pandemic, because it’s a new virus that no one in the world has any immunity to. This is what happened with the Spanish flu a century ago, and what happened with SARS-CoV-2 recently.
Because no one has any immunity, the virus can initially afford to be a little “sloppy” as far as adaptation to humans is concerned. It can afford to be a little more deadly than your typical ‘flu or cold, for instance. Over time, though, people will build up immunity to the new virus. At the same time, the large reservoir of infected people will lead to new mutations popping up. The selection pressure on these mutations will favor those that make the virus spread more among humans. This can include adaptations that just help the virus enter our cells, for instance, but also adaptations that make the virus less severe, because a less sick person is more likely to transmit the virus to others* (esp. compared to a dead person). As more and more people become immune, an additional selection pressure starts to build towards simply changing the virus to become less recognizable to immune systems that have already faced it before.
(*Edit: just wanted to clarify that some symptoms of course make the virus spread more, such as coughing or sneezing. But those aren’t *serious* symptoms (normally). What you want to avoid, as a virus, is making people so ill that they don’t leave the house and reduce their contact with others. Ideally you want people walking around pretty happily whilst simultaneously coughing, sneezing, blowing their noses, etc.)
So, generally speaking, the process of a virus becoming endemic is accompanied by that virus becoming less dangerous to us. This is also what is likely to happen to SARS-CoV-2. It is true that people are predicting that we will be needing booster shots of the vaccine for a while, but that doesn’t mean we’ll need those forever – at least not the whole population. In a sense, we are also still giving people yearly shots for the “descendants” of the Spanish Flu virus – just not everybody but only those who are at risk of death or serious illness if they get infected. This is a likely outcome for SARS-CoV-2 too. But there may be an intermediate period where the level of immunity among the population, as well as the seriousness of the current virus strain makes it a good idea to keep vaccinating a large part of the population, until we’re sure that the virus has reached a level where this is no longer necessary.
(Disclaimer: the above is what I understand to be the likely outcome given an informed layperson’s understanding of immunology and epidemiology. It is not an expert’s opinion, and even if it were I don’t think an expert would be certain either. But this (AFAIK) is what has always happened with new viruses so far, and so it seems plausible that something similar will happen this time.)
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