Wow! I came back to see what had happened and didn't realise that the thread had descended into comedy.
[wipes away tears of laughter]
'Bathing in a pool of mild childhood illness' And which illnesses would they be exactly? Measles with a one in 5,000 case fatality in the UK? Pertussis where you struggle for breath and has even worse case fatality? Or maybe diptheria which can easily be imported from abroad? What a lovely bath to expose your children to in an erroneous belief that their immune systems are somehow better by not being vaccinated! Yes, they probably do have slightly better immunity now (all immunity wanes over time - it's the lack of natural boosters though that's the main reason for less immunity in vaccinated populations but our children are being exposed to the same diseases and my children are getting boosters of some diseases thanks to yours suffering the disease) - but they've also had the disease and its potentially life threatening complications.
You see, some of you have said about how you agonised over not vaxing and that you're not really anti-vax but then when you say crap like that, you realise that it's all a show because you wish your children had bathed in childhood illnesses like measles, whooping cough, diptheria and polio 
By the way, if you have more than one child, often secondary cases in the same household have a worse case of the infectious disease as they have often had more sustained exposure to the pathogen than their sibling leading to a worse case of the disease. I'm sure that will lead to great family dynamics.
Herd immunity is a 'myth'! 
That's the funniest thing I've heard in ages. And from a neurologist who may have had one lesson in epidemiology in med school 40 years ago. Like it or not, herd immunity is nuts and bolts epi and it's an empirically observed mathematical model.
OK, fudamentals of epi again (we learnt earlier that a study encompasses many types of investigation and also why a screening programme can't work - if you're going to talk about epi, let's get things right).
I can't believe that RPO the lovely gooseberry actually had this idea that an individual is somehow conferred magical 'herd immunity' powers and it stops their immune system being overpowered. And then the lightbulb moment when she realised that it's all about exposure. Well, DUH!!! Of course it is! It's all about the probability of exposure. Did you just figure that one out? Or did your mis well informed friends fill you in?
How do you think smallpox was eradicated? Was 100% of the population immunised? Was 100% of the population naturally immune? No and no. We reached a high enough percentage of vaccination to prevent any sustained transmission. As there was no animal reservoir, the disease was eradicated globally. How could that be if we didn't vaccinate 100%? Herd immunity of course!!
The threshold for herd immunity which means that virus transmission cannot be sustained in the community varies by disease. It is higher for measles than many other diseases as measles is very contagious. What you need to do is ensure that the basic reproductive rate (the number of secondary cases generated from a single infective case introduced into a susceptible population) is below 1. In other words, one infected person can infect on average less than one other person (natually, measles R is about 7 or 8). Infection persists if R => 1 and there is a steady influx (births) of susceptibles (whenever maternal antibodies are no longer protective). By immunising, you are reducing the number of susceptibles in the population. When you have a low enough number, R is below 1 and the transmission cannot be sustained.
That's why natural immunity can't result in herd immunity (that was one of the funniest things I've ever heard - like the virus cares!) unless no-one has any more children (and no migration) following an epidemic. Sure, people are now immune (and possibly with better immunity than the vaccine but they've had and even died or permanently damaged from a nasty disease) but as new susceptibles enter, R will go up. And anyway, if you had full herd immunity with no children being born, natural immunity would also wane over time in the absence of exposure to the virus. Eventually, when R exceeds 1, you'll get an epidemic. That's one of the reasons why epidemics occur cyclically. What part of this is troubling you exactly?
It's true that the determination of R is more complicated. For example, it depends on how well the population mixes and the contact rate. So R is lower among the elderly and higher among school children who mix with one another more. So you need higher rates of immunisation among children to lower R and prevent a disease becoming endemic (when a disease is endemic it is being transmitted but not epidemic and means that R is about 1 - that's the case for measles in London).
The threshold is also not a magic one. If you go under the threshold for edemicity, you may still be able to prevent epidemics as you can keep R at around 1. The more susceptibles you have in a population, the greater R and the greater the likelihood of secondary transmission. It's very simple really. It's not an all or nothing situation. Less suscpetibles means less transmission and less severe epidemics. But there is a threshold to prevent sustained transmission whcih varies by disease and population parameters. Where's the myth exactly?
The only part you could possibly argue with is to say that a lower proportion of the population are vaccine immune than otherwise said. Possibly. But that could equally be an argument for a booster rather than arguing against the whole concept of herd immunity. At least make sense for gods sake, surely you could pay find a sympathetic epidemiologist rather than turning to a neurologist for epidemiological explanations when he clearly doesn't have a clue!
Thanks guys for cheering me up!