[Bored chemist]

it does look like herd immunity has been almost achieved.

It is far too early to tell.
Come back in 3 years.

[Petrochemicals (OP)]

It will always infect the vulnerable. In fact it will infect anyone who isn't vaccinated (and a few who are), but only kills the most vulnerable. It just takes longer to infect a rural population.

I would not call Austria rural, plus it seems to be spreading amongst the vaccinated.

I begin to wonder whether the astra zenica is better than the Pfizer in longevity of protection.

[evan_au]

herd immunity looks like a dead duck

If natural immunity is lifetime, there is a good chance that a vaccine will also give lifetime immunity, eg measles.
If natural immunity is almost non-existent, a vaccine will be a real struggle, eg HIV/AIDS.
One could conclude that if natural immunity lasts a only few years, the vaccine might only last a few years, too, eg influenza or coronaviruses (COVID is a member of this family).

In the shorter term, a form of herd immunity might be achievable with high vaccination rates. The following is a back-of-the-envelope calculation:
- Delta strain is thought to have an R₀ around 6-8 (let's call it 7)
- That means, in the absence of other measures (like masks, or social distancing), one infected person will infect about 7 others, on average.
- The infection will die down if each infected person comes in contact with less than 1 susceptible person.
- If the vaccine were 100% effective, that means you would need a vaccination rate of > 6/7 or >86%
- We know that the current vaccines only give about 50% protection against Delta, after 6 months.
- That means you need a vaccination rate of >13/14, or >93%
- Some countries are approaching this level of vaccination, like parts of Australia
- Infection by the virus itself also provides immunity, so you need a (vaccination+infection) rate of >13/14, or >93%
- Wearing masks and social distancing also reduce transmission significantly
- So there are signs that Delta could be contained for now, in some countries
- Other variants are a lottery!
 
There are differences in the type of immunity produced by vaccination & natural infection:
- The vaccines just target part of the spike protein, while natural infection will generate antibodies to many parts of the virus
- The vaccine-induced antibodies tend to neutralize the virus so it can't infect cells; not all natural infections produce neutralizing antibodies
- If the spike protein mutates, the vaccines may lose much of their effectiveness, while natural infection will still have antibodies that recognize other, non-mutated parts of the virus, so it is a broader immunity.

Apparently, being vaccinated halves your chances of catching COVID (a breakthrough infection), and if you are infected, halves your chance of getting Long COVID. So that's a bonus for risk of disability!
See: https://theconversation.com/long-covid-double-vaccination-halves-risk-of-developing-long-lasting-symptoms-165270