Herd Immunity

Group or herd immunity is still a difficult but important scientific concept. No, it does not mean there are no infections, also not that a virus will disappear. And no, no "definitions" have been changed, although definitions are often loosely used.

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In simple terms; if you are not infected: you cannot pass a pathogen on. If you are infected, and deal with it very quickly; you reduce the time, the chance and the load of passing a pathogen on. If enough people have this immunity, the impact of the pathogen will be curtailed. Group immunity does not mean "sterilising immunity": does not mean SARS-CoV-2 disappears, does not mean we will not have infections, does not mean COVID-19 will not happen. Biology is much more nuanced: different viruses can not be compared 1-to-1.

Many people think of measles and equate this to group immunity. But:
1) we do get infected with measles. This has been pointed out many times.
2) measles virus has a different infection "strategy": it infects mainly the non-immune. This means children in many counties: unless you are living on a island then it will be hit in generation cycles, adults and children.
3) Measles has this "strategy" not by choice, it has a high resistance to antigenic evolution. It cannot change much, and hence avoiding neutralizing antibodies is not an option.
4) But, antibodies wane! Indeed they do, and that is why Measles virus can infect. But, it has a long incubation time. By the time the virus gets going (~8 days) your immune system has frequently cleared it and you are none the wiser you were infected.

But some people do get measles; and that has bad consequences. The frequency is low: yes, bc of group immunity, achieved due to childhood vaccination. Transmission risk is very low, so cases are very low! 
SARS-CoV-2 is a respiratory virus that can mutate well. The combination of new variant selection, overcoming neutralisation antibodies, and antibody waning means we will get frequently infected. But, we do acquire immunity that reduce waves and impact.
We will collectively (individual differences) keep waves at bay for a good ~8 months. But (!), it takes time to achieve this. Time to synchronise people´s responses to a collective: that will force a more seasonal infection pattern. And no, we are not there yet, but we are close. The impact of a new wave is very noticeable less than the early waves. No/limited testing does hide the current waves, but wastewater testing do highlight them.

A relatively modest first wave with high casualties: a second wave (alpha and Xmas). Then vaccinations began: large cases episode (Omicron) with relative much fewer casualties, providing (hybrid)-immunity. *limited testing from second half 2022. Wastewater monitoring, such as here in NL; shows the distinct waves no longer seen due to absence of large population-based screening; but underpinning the strength of immune protection against disease that holds.

Note that wastewater monitoring for SARS-CoV-2 has been unprecedented and other viruses are also much more prominently present than based on screening cases alone.

Then new waves will keep coming; we will not be infected every year, but the chances are ~3 years: not dissimilar than other respiratory viruses. That will provide us with a boost in immunity, and we will continue to contribute to group immunity; reducing viral spread (when not infected), reducing the time (immune response is quicker) in which it happens, reducing the viral load (immune response is quicker, infected cells killed) and the infectiousness (opsonization and neutralisation of released virions). 

Bron  Seasonality of Common Human Coronaviruses, US, 2014–2021

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