Herd immunity

[HopefullyAnonymous]

Am I completely misunderstanding?

It’s a long time since I studied this at school but my understanding is that it’s the percentage of people with immunity needed to prevent the spread of a particular infection (either through vaccination or immunity from having had the illness).

The level of herd immunity required for measles for example is 93+% so how are we expecting herd immunity to play any part in overcoming Coronavirus? The level of immunity needed would be worse than the “worst case” predictions of 80%

Am I missing something?

Nope. You're not.

Actually, I saw it modelled that for a virus with a lower r0 than measles, 80% would be about right. I think that's pretty much where that figure comes from.

Thanks. Relieved I’m not losing my mind but not exactly reassured by their approach.

Where have you come across people talking about herd immunity?

When dealing with a novel pathogen, the entire herd is vulnerable.

Where have you come across people talking about herd immunity?

It was mentioned in Boris’s press conference, either by him or one of his experts.

Yes, no one is currently immune to this virus so herd immunity is irrelevant, frankly. Carriers may be prone to reinfection and there is no vaccine.

Once 80% of people have had it, it's most likely that it will not be able to circulate.
80% is an estimate based on the type of virus.

If those 80% get it over a long period of time, then the death rate will be minimised.

How would the death rate be minimised?

The death rate can be min8mised by

1. keeping the virus out of the way of the most vulnerable until the virus essentially wears itself out ie ( can't transmit because too many potential hosts have already had it and now are mostly immune

2. ensuring that we avoid everyone getting ill at the same time. If we have say 5000 respirators, then we min8msie the death rate by having fewer than 5000 people needing respirators at any one time.

3. stall the virus as much as possible as its likely that some treatment will become available before a vaccine is developed which will reduce the severity in the vulnerable

we have to accept most people will catch the virus at some point in time .

I think herd immunity = people who get it in the next few months being regarded as collateral damage. I am not reassured.

@HopefullyAnonymous As you seem to understand herd immunity, I won't explain in detail and anyone who wants to know more can google it...

In brief, the R0, the average number of people an infectious person passes the disease to, for coronavirus is estimated at 2-3. The R0 for measles is 12-18.

The herd immunity threshold (how many people need to be immune for the disease to no longer be able to spread) depends on the R0 for a disease. The formula is (R0 -1)/R0. For an R0 of 2 that would be 50%, for an R0 of 18, it would be 94%.

Why is the government suggesting 80% of people may get it in the worst case scenario if the R0 for coronavirus is 2-3? Presumably because they are being cautious and think the R0 could be higher, sadly, a percentage of people will die rather than recover so eg for 50% of the population to be immune, more than 50% would have to catch it, not everyone will become immune after infection and immunity may wane with time.

@goingoverground thanks for that...I wasn’t aware that’s how it was calculated so the measles reference was more for example than comparison.

The R0 seems low but then I suppose I’m basing that on the original “super-spreader” (Brighton?!) and I suppose there will always be anomalies. I guess until we know more it’s still a case of those in the know crunching the numbers and working on best estimates.

For what it’s worth, I’m not necessarily for strict lockdown at this stage, but I’m concerned at the rather relaxed way in which Boris warned us that many people will “die before time” and what this might mean in terms of the government having a figure in mind before they act more decisively.

R0 is not easy to estimate and we don't have much data to go on, furthermore it is a theoretical number and an average based on the assumption of a homogenous population so there will be anomalies. In real life, some people are in contact with hundreds of different people every day (eg they commute on the tube, work in a large office or have close physical contact), whereas others may only have contact with the same one or two people.

I think part of the idea of suppressing the huge expected peak of infection, is to have some people who have had the virus and who are now immune, come out the other side and able to carry on normal life. Unlikely to have a very large number of immune people though for many months.