California Herd Immunity?
I know a lot of people who had the worst “flu” of their lives this past late fall and early winter. And many of the Asian kids wearing masks to school.
https://www.ksbw.com/article/new-study-investigates-californias-possible-herd-immunity-to-covid-19/32073873
= new reply since forum marked as read
on the 
(reproductive ratio) An of 2.5 would mean 60% would need to be exposed. An of 2.0 would mean 50% would be the number. An would mean 66% is needed.
Some estimates
https://www.sciencemediacentre.org/expert-comments-about-herd-immunity/
Prof Willem van Schaik, Professor of Microbiology and Infection, University of Birmingham, said:
“Herd immunity describes the phenomenon that at-risk individuals are protected from infection because they are surrounded by immune individuals. The spread of the virus is thus minimised. Currently, we talk mostly about herd immunity in the context of vaccines. If a sufficiently high number of individuals in a population are vaccinated, they will provide herd immunity to the small number of people that are not vaccinated (e.g. for medical or religious reasons). We have recently seen cases of measles outbreaks where herd immunity was not sufficiently high because children were not vaccinated out of completely unfounded fears against vaccination.
“Herd immunity exists for flu. If large proportions of the population would get a flu vaccine that could protect non-immunised individuals. The problem with flu is that it is difficult to know which strains of flu (think of strains as variations on a theme: it is flu but just a little bit different) will be causing infections at any given point in time and so that is why the flu vaccine is not always 100% effective. The major problem with coronavirus is that this is a novel virus that has never spread before, which means that everyone is at risk for infection. Herd immunity can only be reached by widespread vaccination (but there is currently no vaccine, and it may take a long time before an effective vaccine becomes available) or by individuals falling ill and recovering thereby developing natural immunity against the virus.
“Unfortunately, a very rough estimate suggests that we will only reach herd immunity to Covid-19 when approximately 60% of the population is immune (and remember that immunity is currently only reached by getting the infection as we have no vaccine!). The major downside is that this will mean that in the UK alone at least 36 million people will need to be infected and recover. It is almost impossible to predict what that will mean in terms of human costs but we are conservatively looking at 10,000s deaths, and possibly at 100,000s of death. The only way to make this work would be to spread out these millions of cases over a relatively long period of time so that the NHS does not get overwhelmed. Social distancing might contribute to this. Clearly the government believes that this process is manageable and building up herd immunity is the most effective way to stem Covid-19. I note that the UK is the only country in Europe that is following this strategy. Other countries also use scientific advice to guide their research and it is unclear to me why the UK is alone in their laissez-faire attitude to the virus. Perhaps the government has access to modelling data that suggests that the numbers I quoted above in numbers of cases and deaths are unavoidable in any scenario (e.g. even with prolonged social distancing strategies) but unfortunately these data, if they exist, have not been made available to the wider academic community so it is difficult to comment. However, last night’s U-turn banning large events suggests that the government’s policy is still very much subject to change. This change was perhaps influenced by the unprecedented outcry of scientists on the lax containment policy of the government.
snip
Prof Paul Hunter, Professor in Medicine, UEA, said:
“Immunity is when an individual has acquired resistance to infection with a particular pathogen (a virus, bacterium or parasite that causes disease) because they have already had an infection with that pathogen and recovered or have been immunised. Herd immunity occurs when a large enough proportion of a population are immune that an infection does not spread so easily or it can actually die out.
“In determining what level of herd immunity is necessary to stop the spread of infection we need to know the R0 (or reproductive ratio) this is the number of people that are likely to be infected by a single case when a new pathogen appears in a community with no prior immunity.
“So assume a pathogen with an R0=2, this means that after the first case there will be 2, then 4, then 8, etc. But by the time half the population is immune, on average half the people exposed from a single case will be immune and therefore only one person per infected person gets the infection.
“So the sequence 1, 2, 4, 8, 16, 32, 64, 128, 256 ….. becomes 1, 1, 1, 1, 1, 1, 1, 1, 1 and the disease will actually die out fairly quickly. Estimates of the R0 for COVID-19 vary somewhat but are in the order 2.0 to 3.0. Assuming that R0=3 then after about 66% of the population becomes infected then the virus will die out in the population.
Prof Matthew Baylis, Institute of Infection, Veterinary and Ecological Sciences, University of Liverpool, said:
“What is herd immunity? In a nutshell, everyone in a population is protected from infection before all of them are immune. The reason is that at a certain level of immunity (i.e., a proportion of people are immune, from having had the disease or having been vaccinated), the point comes when – on average – one infected person does not manage to contact and pass the infection on to one other person. Most of their contacts are already immune. The occasional contact is still susceptible, and the odd transmission event happens, but not often enough to sustain the disease. Transmission grinds to a halt, even though some or even many people have still not had the disease. This is herd immunity. It is one of the reasons boys are vaccinated against rubella: by vaccinating boys, boys are less likely to transmit to girls (an effect of immunity), and by vaccinating boys, girls are less likely to transmit to girls (an effect of herd immunity). For herd immunity, it does not matter whether the immunity comes from vaccination, or people having had the disease; people just need to be immune.
“A key question is how much immunity is needed before we get herd immunity? It varies per disease, depending on how transmissible it is. For a highly transmissible disease, like measles, on average one person might infect up 20 others, and herd immunity kicks in at 95% immune – and so, the target coverage for MMR vaccine is 95%. For flu, on average one person infects just 1.3 others; in this case herd immunity kicks in at about 25% immune or less; and so the target coverage for flu vaccine is much less than it is for measles (three quarters of over 65s).
“So what about COVID-19? Estimates are that one person may infect as many as 2-3 others, on average, meaning herd immunity should kick in at 50 – 67% of the population immune. And so in the absence of a vaccine, there would appear to be nothing to stop the spread of the virus until 50-67% of us have had it; and at that point herd immunity will kicks in and transmission will decline or stop. This is where the 60% of the population statistic has come from. And this is deeply concerning – taking the low fatality rate estimate of 1%, even 50% of the UK population infected by COVID-19 is an unthinkable level of mortality.
Dr Simon Gubbins, The Pirbright Institute, said:
“For a viral disease “herd immunity” refers to the indirect protection an uninfected individual receives if a proportion of the population is immune to infection. This could be achieved due to previous infection with the virus or more likely due to vaccination. The protection comes about because in a partially immune population infected individuals are less likely to encounter uninfected ones and so transmit the virus to them. Consequently, infection chains are interrupted and spread is stopped or slowed.
“The proportion of the population that needs to be immune for the number of new cases to decline depends on the basic reproductive ratio of the virus, known as R0. This is the average number of secondary cases that arise from each primary case when a virus is spreading in a wholly susceptible population.
“For SARS-CoV-2 estimates for R0 are around 2.5, so the proportion of the population that needs to be immune to achieve herd immunity is around 60%.
“Herd immunity acts as an evolutionary pressure for a virus to adapt so that it can escape immunity and can spread more easily. Influenza viruses are very good at this and frequently mutate to produce new strains to which people are not immune. This is the reason the seasonal flu vaccine needs to be updated annually. There is no information to show whether something similar will happen with SARS-CoV-2.”
Dr Ed Wright, Senior Lecturer in Microbiology, University of Sussex, said:
“Herd immunity is the required proportion of a population that needs to be immune to a pathogen to stop it from spreading within that same community. This immunity can be stimulated by vaccination or recovery following infection. The level of herd immunity required depends on how transmissible the pathogen is.
“This can be gauged from its basic reproduction number (R0) – the average number of people a positive case will go onto infect. For instance, to stop measles virus spreading within a population requires upwards of 90% of people to have immunity because the R0 for the measles virus is high (12-18) – it’s an airborne virus. The latest R0 for SARS-CoV-2 puts it between 2 and 3 so estimates suggest around 60% of the population would need immunity to stop the virus from taking hold in a community.”
snip
