Why does herd immunity not exist with these vaccines?

[Swankiss]

So with other viruses people are persuaded to take them because it increases herd immunity, examples are measles and polio.

But it doesn’t seem like herd immunity is a thing this time around? Even if you’ve had the vaccine, you can still catch it and spread it too.

Just wondering if this is unusual? And I wonder why herd immunity isn’t kicking in this time.

Because you can catch it more than once.

The herd immunity required I reckon is over 90percent
There's a formula somewhere.

Because not all vaccines are identical (other than the particular disease they are targeted against). Measles and Polio vaccines are “sterilising” which in this sphere means they actually kill all the virus that enters your body so you can’t possibly pass it on. The covid vaccines are more like a prophylactic treatment - they are designed to give an antibody response, in particular when the virus is systemic aka it’s in your bloodstream, rather than when it’s in the membranes of your respiratory system. This is why 1. It only reduces the severity of symptoms in people who may have been at risk of getting severely ill. 2. It doesn’t stop you catching or transmitting it. 3. It was only ever supposed to be given to people who needed protection from severe illness.

I think herd immunity with vaccines has been massively oversold.

The point of the vaccines is to prevent severe illness and death. The other four common cold coronaviruses still mutate and circulate and we don't worry about it. I'd expect Covid-19 to join these and just become a cold.