Herd Immunity explained for Dummies

Well I don't know about you , but herd immunity is a bit like, we all know what it is , 'cos we've been told - So it's easy like ????? its about 90% innit ?

So far, so good. A standard definition is the amount that would stop an infection getting out of control, in that for every person infected, they, in turn , would infect [ on average] no more than one other person .

For this reason epidemiologists often quote the reinfection rate for clarity. Coronavirus is 2.4 , ie for every person getting it, on average 2.4 people get it from that person [ assuming no other situations affect this] . Let's take a reinfection of 1.00 for simplicity,ie for every person catching it, they reinfect another person. So for a family of 4 , one person gets it- 1-2 family members catch it ..... Wrong.... this is about statistics [ long word, so just call it odds if you know a little about betting] . If one family member gets it, it becomes likely that the entire family may well succumb to the bug . On the other hand, another family will likely not be touched at all, so we still get that 1 gets it- and gives it to one other.

However, at this point , one or more people in the population have got the bug or have recovered fromit, so can no longer catch it, so although the reinfection rate is still 1, sometimes that person who would have been infected by chance is actually immune or has it already, so the number of people becoming part of the next round is slightly less , and as the disease progresses, the number becomes fewer and fewer as more and more people become immune.

This is herd immunity, basically it means that as the disease spreads, it hits on more and more immune chaps/esses and reinfects fewer and fewer peeps......... Obviously the more people who cnat get the bug, the better and the epidemic slows down faster and faster . 

If youve  been with it so far, you'll have noticed that this rate actually gives you the cut off point for the  start of this immunity. For stopping an epidemic quickly, you need a lot more than the peak rate, but it gives a 'limit' . Coronavirus has an R of 2.4 . This gives the simple equation of [2.4-1]/2.4 or 58.3% , so at 60%, the pandemic will start to die down by itself [ slowly] . Epidemiologists have been stating that 80% of us will catch it , so they are expecting a 20% overshoot from that limit, which is to be expected as 60% just starts to give us a slowdown in the infection rate, and there is the incubation period and so on which whacks it upwards ....

This reinfection rate is an average , so if we were looking at sparcely populated areas with a smaller interaction [say], then the local rate would be less than the national rate. Also if a member of a household catches it, the other household members would be much more likely to get hit than the average, due to their closer proximity  . 

What governments across the world are trying to do is reduce interactions and hence slow down the rate of infection . Once these restrictions are lifted , this rate of infection will automatically increase again , unless something stops it, ie the immunity is above this 60% figure , either naturally or by a vaccine .

    Once lifted [ and it will be soon or the world becomes bankrupt] , different nations will show different properties. Ignoring China as their figures are suspect, we have at one extreme USA. This country has gone for a sweeping epidemic. Human costs aside, they will become immune to he virus and hence will get back to full production uickest. At the other extremes , say Korea. When this country lifts some of its restrictions, the rate of infection will quicken and they will require another lockdown [ repeat as necessary].  History will prove which was the best tactic ...