system
Given the following post, respond as if you're commenting on Reddit:
human
why does Covid come in such short waves?

Take the latest omicron burst in say the UK - massive wave infecting perhaps 5 million then rapidly falls off despite loosening of any controls. Are only some people susceptible to this variant and they have all been infected?
gpt
The short answer is that we don’t know exactly why Omicron was so quick to fall off.

The long answer is that every infectious disease has an R0, or the average number of individuals a single infected person will infect in a 100% susceptible population. With a population with immunity and vaccinations, we call it Re, “effective R.” Omicron has an extremely high R0–like perhaps the most infectious of any common disease besides measles. Estimates put it around 15 or so (variable research). So that means one person, in a completely susceptible population, on average, infects 15. As you can imagine, that is going to lead to an explosion of cases, even if the Re is half as high. But that also means that covid quickly runs out of other individuals to infect, as basically everyone gets infected.

Obviously not everyone in the U.K. was infected! But Re is an average, and some people are super spreaders (usually young, social people). They may infect much more than 7-15 people because they are healthier, more likely to have mild symptoms (or asymptomatic), and be out and about. So once all these folks get infected, and they get infected quickly…it greatly lowers the Re. Once the Re is below 1, the disease will quickly start to fall in numbers. 

This is fundamentally what happened, but we don’t know all the details. It’s very possible some people are more genetically susceptible to Omicron than others, but we don’t know who or why yet. We also don’t know for sure it’s these superspreaders, but we can just assume. We can only compare it to other infectious diseases of the past and use what we know about infectious agents to make broad conclusions (at this point). But in reality, the details of any infectious disease are pretty scarce.