johnm wrote:NigelC wrote:Sorry that is incorrect, before vaccination very much greater numbers may have died but herd immunity was eventually achieved for those that survived.
Episodes include Influenza's introduction to North America and the 1918/19 influenza epidemic.
Diseases such as measles mumps polio and chickenpox were endemic before vaccines helped achieve a level of herd immunity, the degree of immunity is affected by resistance, the degree of transmissibility and other factors. Flu is still around merrily mutating which is why the annual vaccination is recommended and the vaccines are constantly re-engineered, Covid will be similar but is more of a problem because of its highly varied impact and high levels of transmissibility.
That's too simplistic. Herd immunity works more like this:
1. For measles, mumps and chickenpox before vaccines, the adult population had herd immunity from natural infection. But children kept arriving with no immunity, and so they got infected (particularly as schools clump kids together - I remember chickenpox sweeping through the local schools when I was a kid, didn't half itch!). These viruses don't mutate much, and immunity is generally life-long, so herd immunity for adults was maintained by natural infection and these were largely childhood ailments. Smallpox was much the same.
2. Flu mutates all the time, which is why we never achieved adult herd immunity. Even with vaccines we're never going to get to herd immunity for flu.
3. Covid seems to work like flu but mutates more slowly. Probably (but not certainly) natural infection on its own could never achieve herd immunity, or if it did it would be at the cost of a lot of deaths. By the looks of things we are close to (or perhaps have already) achieved herd immunity for (all or some of) the current Covid variants because our vaccine programme has reached so many people, and natural immunity through infection has supplemented that.
4. Maintaining herd immunity for Covid has two uncertainties: (a) how long do natural and vaccination immunity last (too early to tell yet)? and (b) will it mutate to variants which evade immunity (and if so, how fast/often, and how serious will the consequences of infection be)?
Because we can't yet answer these last two questions, but only make predictions, some would like to maintain restrictions while others, more optimistic, think it's an acceptable risk to relax now. It's important to note that delay (unless it's for a year or so) won't answer those questions, but it would answer whether we have reached herd immunity yet and whether the latest Covid variants evade immunity and have serious consequences. And delay won't answer these questions for any new variants which turn up.