MESSAGE
| DATE | 2020-10-04 |
| FROM | Ruben Safir
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| SUBJECT | Subject: [Hangout - NYLXS] consensus on herd immunity - there is none..
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sciencemediacentre.org
expert comments about herd immunity
23-29 minutes
March 13, 2020
There have been questions from journalists regarding 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.
“Finally, it is true that ‘Herd immunity makes it harder for a disease
to spread due to people becoming immune after already catching it or
getting vaccinated’, and this is why vaccinations have been the
cornerstone of the prevention of infectious diseases. Letting a
potentially lethal infectious disease burn through a population is
another way to reach herd immunity, but it comes with considerable risks
and downsides (see above). This is why we still need to all work
together to try to slow down and minimise the spread of Covid-19 by
practicing hand hygiene and social distancing. This will hopefully
reduce the pressure on the NHS, allowing for more people to be treated
and thus reducing the number of people dying of this infection.
“While Covid-19 is not as bad as the Spanish flu, which had a
considerably higher case fatality rate, it is probably going to be
comparable to that outbreak in terms of the societal impact and how it
will affect our daily lives for the foreseeable future. Communities
will need to come together to provide ‘social herd immunity’ by helping
those out that are in quarantine (for example, by checking in on them
via social media or direct messaging apps and doing some shopping for
them). We are currently living through an unprecedented health crisis
but we can handle this if we act collectively.”
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.
“But it is not as simple as this as individuals are always moving into
and out of a population, largely through birth and death. But in this
case future years will see a reduction in the average age of infection
and so risk of more serious disease will be lower.
“This does not mean that achieving herd immunity through natural
infection should be an objective as this does imply that a lot of people
will suffer the infection and risk severe illness and even death. But
if our most vulnerable people are protected during this stage then it
may well be possible to ultimately reduce the death rates as infection
rates in subsequent years will be substantially reduced and a vaccine
may become available that would protect our most vulnerable citizens
even more.”
Dr Bharat Pankhania, Senior Clinical Lecturer with the University of
Exeter Medical School, said:
“The concept of herd immunity is to try to create an extremely large
group of people who have immunity against an infectious agent. This
means either vaccinating, or allowing people to get infected and
recover, so they have developed memory against the infecting agent and
for example can produce antibodies when encountering the infectious
agent again. This constitutes the concept of immunity.
“To create herd immunity, via an infection route, people get infected
and recover, and hope that once they recover, they are hopefully also
immune.
“With herd immunity, when a virus circulates, there’s the hope that it
will invariably encounter people who are already immune.
“In general for a good, successful herd immunity, we need more than 90
per cent of the population to be immune in order to afford protection to
the remaining 10 per cent who are not immune and are therefore
vulnerable from a potential infection.
“Where the concept of heard immunity works, it can be very successful –
this is how we’ve eliminated small pox virus and almost wiped out polio too.
“The concept of creating herd immunity by infection is similar to
creating it by vaccination. The difference is that when you vaccinate,
you are using tried, tested and extremely safe vaccinations.
“Trying to create herd immunity through Covid-19 brings in questions of
safety. You can’t control infection spread to “high risk” people.
Therefore, some people who become infected will develop very severe
illnesses, and some of those would die.
“There is some herd immunity against flu, which is mediated by
vaccination – but it’s not excellent as the majority of the population
are not vaccinated, nor is the Influenza vaccine highly immunogenic.”
Prof Martin Hibberd, Professor of Emerging Infectious Disease, London
School of Hygiene & Tropical Medicine, said:
“The current aim of the social distancing put in place by the UK
government seems to me to be to delay and ‘flatten’ the peak so that
those people who suffer the more severe forms of the disease are able to
be cared for properly; whereas if they all occurred at the same time, in
a very tall and sudden peak, the healthcare facilities would be
overwhelmed and not able to provide proper care. This might mean that
by winter, when the disease might be expected to get worse, those who
had not had the disease would be in the minority and would be protected
by herd immunity.
“I think that the delay techniques promoted by the government are likely
to achieve this aim. We are not sure how the UK policies will work
compared to other European policies but I suspect they will be similar.
I feel that no European country is putting in place the full measures
that China was able to use to curtail the outbreak and in any case
conditions are more difficult now as the disease has become a pandemic.
“The alternative, ‘contain’ approach, that has been implemented
successfully in countries such as Singapore (and recently dropped in the
UK), may already be too late for many countries.
“The evidence is increasingly convincing that infection with SARS-CoV-2
leads to an antibody response that is protective. Most likely this
protection is for life, although we need more evidence to be sure of
this, people who have recovered are unlikely to be infected with
SARS-CoV-2 again.
“As more and more people become infected, there will be more people
recovered who are then immune to future infection. As these numbers
build up, it will be harder for an infected person to transmit to other
people, because some of the people that meet the infected person will be
resistant (because they had the infection previously and are now
immune). When about 70% of the population have been infected and
recovered, the chances of outbreaks of the disease become much less
because most people are immune (resistant to infection), this is called
herd immunity.
“In a good scenario, the 70% infected, recovered and immune would be
people who were expected to have mild disease and the 30% who were
vulnerable to severe disease would be protected by this herd immunity.
“The Government plan assumes that herd immunity will eventually happen,
and from my reading hopes that this occurs before the winter season when
the disease might be expected to become more prevalent.
“However, I do worry that making plans that assume such a large
proportion of the population will become infected (and hopefully
recovered and immune) may not be the very best that we can do. Another
strategy might be to try to contain longer and perhaps long enough for a
therapy to emerge that might allow some kind of treatment. This seems
to be the strategy of countries such as Singapore. While this
containment approach is clearly difficult (and may be impossible for
many countries), it does seem a worthy goal; and those countries that
can should aim to do.”
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.
“But it doesn’t have to be – and it won’t be – this way. By reducing
the number of people that one person infects, on average, then we lower
the point at which herd immunity kicks in. If we reduce it to 1.3,
COVID19 becomes more like flu, and herd immunity kicks in when about one
quarter of the population has had the disease and is now immune. So,
from an epidemiological point of view, the trick is to reduce the number
of people we are in contact with (by staying more at home), and reduce
the chance of transmission to those we are in contact with (by frequent
hand washing) so that we can drive down the number of contacts we
infect, and herd immunity starts earlier. The sweet spot comes at the
point where one infected person infects one, or less than one, person on
average. But, importantly, we will need to sustain this until we have a
vaccine: only at that point can we return to normal behaviour patterns,
with herd immunity now achieved by vaccination, not disease.”
Professor Peter Openshaw, Past President of the British Society for
Immunology and Professor of Experimental Medicine at Imperial College
London, said:
“Herd immunity occurs when a large percentage of the population is
protected against a particular disease, stopping the ability of that
disease to spread within communities. This protection can either be
gained through methods such as vaccination (which induces the body to
produce antibodies which protect you against catching the disease) or
through enough people in the population having been infected and
generating antibodies by their body fighting the pathogen directly.
Modelling studies show that, over time, we can expect 60-80% of the
population to be infected with SARS-CoV-2. Generating herd immunity in
the population, and particularly in younger individuals who are less
likely to experience serious disease, is one way to stop the disease
spreading and provide indirect protection to older, more vulnerable groups.
“SARS-CoV-2 is a novel virus in humans and there is still much that we
need to learn about how it affects the human immune system. Because it
is so new, we do not yet know how long any protection generated through
infection will last. Some other viruses in the Coronavirus family, such
as those that cause common colds, tend to induce immunity that is
relatively short lived, at around three months. However, these viruses
have co-evolved with the human immune system over thousands of years
meaning they may well have developed methods to manipulate our immune
responses. With the novel SARS-CoV-2, the situation may be very
different but we urgently need more research looking at the immune
responses of people who have recovered from infection to be sure.”
Dr Erica Bickerton, The Pirbright Institute, said:
“When a large proportion of a population become immune to an infectious
agent, such as a virus, there can be “herd immunity” or “community
immunity” which provides some protection to others from catching the
same virus. It is more difficult for a virus to spread throughout a
population if there are not many people who can be infected. Herd
immunity is usually achieved through vaccination rather than natural
infection, although natural infection can contribute to it. However,
herd immunity does depend on how long individual people remain immune
and there is no guarantee that herd immunity will protect people who are
not immune from being infected.
“An example can be found in seasonal influenza. A level of herd
immunity against seasonal influenza virus is achieved by annual
vaccination, which is required because the circulating strains of
influenza virus vary each year. Vaccination of a high proportion of the
population against seasonal influenza virus helps to protect more
vulnerable members of the population, who cannot receive the annual
vaccine, from being infected.
“Immunity to SARS-CoV-2 is not yet well understood and we do not know
how protective the antibody response to this new virus will be in the
long-term. This is a new coronavirus and there is a lot of work going
on to understand immunity to this virus. It is too early to say how
long immunity lasts or how the virus will adapt to escape immunity.
There is still much to be learned.”
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.”
Prof Rowland Kao, Sir Timothy O’Shea Professor of Veterinary
Epidemiology and Data Science, University of Edinburgh, said:
“Herd immunity is a potentially confusing term because it really has
nothing directly to do with the immune system. When everyone in a group
(i.e. a ‘herd’) is susceptible to a disease, and able to transmit it
once infected, this means that once anyone in the group becomes
infected, then everyone else is at risk. And so the disease has a good
chance to propagate. However, if some of the group are protected, for
example by vaccination, then this means that at least some of the time,
a contact that would have been infectious, isn’t infectious, because the
contact was with someone who couldn’t get infected. Because the number
of contacts over the lifetime of an infection is limited, this therefore
means that the disease’s ability to reproduce is impaired. If there are
enough individuals protected so that, on average, the disease when
introduced can infect less than one other, this means that the disease
will infect maybe a few, but won’t spread broadly through the
population. The important point is that not everyone needs to be
protected in order for the group as a whole to have little chance of
getting infected. The concept is therefore called ‘herd immunity’
because it means that, at the group or herd level, there will be
relatively few infections in the group, even if there is at least one
infection introduced.”
Prof Christl Donnelly, Professor of Applied Statistics at University of
Oxford, and Professor of Statistical Epidemiology and Deputy Director of
the WHO Collaborating Centre for Infectious Disease Modelling at
Imperial College London, said:
“The basic reproduction number (denoted R-nought, R0) is the average
number of new infections a single infected person causes in a fully
susceptible population. If the proportion (1 – 1/R0) of people are
immune to infection (whether through vaccination or through natural
infection), then the average number of new infections per infectious
person is 1, which would cause the daily incidence of new infections to
plateau. Once the proportion is greater than (1 – 1/R0), then the
average number of new infections per infectious person will be less than
1, which will cause the daily incidence of new infections to decline.
This is how the immunity of populations (herd immunity) reduces
transmission and can contribute to control.”
Dr Thomas House, Reader in Mathematical Statistics, University of
Manchester, said:
“The R0 of a disease is equal to the number of infections that a typical
case will cause before they recover early in the epidemic. If it is
bigger than 1, the epidemic grows, and if less than 1, it shrinks.
Suppose R0 is 3, so the epidemic initially grows, but that at a later
time over two thirds of people have become immune. Then the average
case will make three infectious contacts, but we expect that two of
these will not lead to new cases due to immunity and so the epidemic
will no longer grow. This is called herd immunity and can be achieved
two ways. Ideally it happens through vaccination, which does not
involve illness. It can also happen due to infection leading to disease
and later recovery. Social distancing measures do not lead to herd
immunity, so when they are lifted the epidemic may grow again. Whether
we aim for it or not, herd immunity will happen at some point in the
future since neither a growing epidemic nor social distancing measures
can continue forever, and the aim of policy should be for this to happen
with the minimum human cost possible.”
All our previous output on this subject can be seen at this weblink:
http://www.sciencemediacentre.org/tag/covid-19
The SMC also produced a Factsheet on COVID-19 which is available here:
https://www.sciencemediacentre.org/smc-novel-coronavirus-factsheet/
--
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