The Role of Herd Immunity in Control of Contagious Diseases
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International Journal of Research and Review
Vol.7; Issue: 7; July 2020
Website: www.ijrrjournal.com
Review Article E-ISSN: 2349-9788; P-ISSN: 2454-2237
The Role of Herd Immunity in Control of
Contagious Diseases
Janki Vyas, Apeksha Kadam, Rajshree Mashru
Dept. of Pharmacy, The M.S. University of Baroda, Vadodara, Gujarat, India
Corresponding Author: Rajshree Mashru
ABSTRACT
Keywords: Herd Immunity; Herd immunity
Herd Immunity is a form which occurs when a threshold; Basic Reproductive number,
large percentage of a population has Infectious disease; Vaccination
become immune to an infection, whether
through vaccination or previous infections so as INTRODUCTION
to provide indirect protection against an When the immunity of a large
infectious disease and can measure protection proportion of the members in a susceptible
for individuals who are not immune to the population is developed against any
infection. The Basic Reproduction number (R0)
infectious disease, spread of the
is an important term which helps calculation to
calculate the capability of an individual to infect transmission is ceased of and there is
an individual further and enables to discover the consequent lessening of the likelihood of an
minimum amount of population coverage affected individual coming into contact with
immune to develop herd immunity of that a susceptible individual. This is termed as
specific disease. When a critical proportion of Herd Immunity. [1] It consists of two words
the population becomes immune, called the herd mainly Herd meaning a group or community
immunity threshold (HIT), the infectious disease and Immunity which has to be interpreted.
may no longer persist in the population, ceasing The term immunity means is a state of
to be endemic at the end. Social networks resistance of an organism to invade biotic
however undergo constant growth, and it may and abiotic pathogens and their harmful
be argued that network growth may change the
effects which prevents the spread of
level of herd immunity present in social
networks. Vaccination helps prevent circulation infection. [2] Herd Immunity is also called as
of the infectious agents in susceptible Community Immunity; Population
population. High uptake and more effective the Immunity or Social Immunity. The
vaccine is more are the chances of success to Immunity can be tested for, as the presence
eradicate infectious diseases like in the case of of antibody, or as skin test or lymphocyte
smallpox and polio. Herd immunity's inclusion response to stimulation, against the chosen
in vaccination programmes makes it more antigen. Herd immunity is quantifiable by
favorable in cost-effectiveness or cost–benefit testing a sample of the population forthe
ratios and an increase in the number of disease presence of the selected immune parameter.
cases avoided by due to vaccination. Herd or It is not dependent on the ease or difficulty
also known as Population Immunity is highly
of circulation of neither an infectious agent
debatable subject and had different views by
researchers regarding achievement of herd nor its elimination. It may be due to natural
immunity in any disease control approach. In infection, or immunization, or a
[1]
this article we have tried to enlighten the combination of both. We can develop
concepts regarding herd immunity; the diseases resistance naturally. When our body is
which can be accounted with it; its link with exposed to a virus or bacteria, it makes
vaccination strategies and part of the disease antibodies to fight off the infection. When we
eradication.
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Vol.7; Issue: 7; July 2020
Janki Vyas et.al. The role of herd immunity in control of contagious diseases
recover, body keeps these antibodies. The individuals do not develop immunity, but
body will defend against another infection and enjoy the protection because of break in
that was the reason which stopped the Zika spread of infection. Thus, the herd
virus outbreak in Brazil. Two years after protection is the major beneficial
the outbreak began, 63% of the population component of immunization for an
had had exposure to the virus. Researchers unimmunized population in an infectious
think the community reached the right level disease. [4]
for herd immunity. Similarly vaccines can
also build up resistance. They make your
body think a virus or bacteria have infected it.
You don’t get sick, but your immune
system still makes protective antibodies. The
next time your body meets that bacteria or
virus, it’s ready to fight it off. This is what
stopped polio in the United States. A
community reaches herd immunity on the
basis of calculation of reproduction number
(R0). It tells you the average number of people
that a single person with the virus can infect if
those people aren’t already immune. The
higher the R0, the more people need to be
resistant to reach herd immunity. [3] Herd Figure 1 [33]: It states that Green: immune, Red: infected, Blue:
protection: Protection to the unimmunized not immune but protected due to herd immunity
individual without inducing immunity,
virtually by breaking the transmission of the It is proposed that herd effect be
infection or lessening the chances of defined as the alteration of the
susceptible coming in contact with infective epidemiological frequency parameters (of
individual. In a case when majority of a infection or disease as the case may be) in
given population generally 80-90 % is the unimmunized segment of a population
immune to a contagious; or maybe as a result of immunizing a proportion of the
infectious disease (spreads by any bacteria population. The alteration is usually a
or virus) but are recovered through innate decline of incidence of infection(hence
immunity by producing antibodies or can be lower incidence of disease). However, in the
achieved by vaccination. It is applicable age group immunized there will always be a
only to the infections which spread from reduction of incidence of the infection if
one person to another person .When any there was herd effect. In most, if not all
infectious disease in a large population other cases, herd effect of high herd
liable to be immune spreads by the immunity induced by immunization is
susceptible individual from person to person beneficial in reducing the burden of disease;
then there are less chances of the infection its extreme benefit is the interruption of
to be transmitted effectively as healthy transmission itself.
immune individuals would act as an
immunological barrier against the HISTORY
transmission in the community. Thus this In 1923 W C Topsley and G S
immunized person gives back through Wilson published a paper briefing series of
namely through herd protection. experiments on populations of susceptible
In clinical practice, herd immunity and immunized mice which was first to
does not play significant role, while herd articulate a fundamental problems in
protection plays a major role, though to a infectious disease research and control came
limited extent, because unimmunized to a conclusion by firstly coining the term
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Janki Vyas et.al. The role of herd immunity in control of contagious diseases
‘Herd Immunity’. [5] Moreover A W Hedrich an individual’s system. For example very
recognized it as a naturally driven young children and elderly as well as very
phenomenon in his research on frail adults due to weak immune system
epidemiology of measles in Baltimore while might be more susceptible to the infections.
it came into effect in 1930’s ; where the
Research mainly emphasizes on fact that the
number of new infections eventually
decreased including susceptible population ,
when more number of the children were
immune to measles. [6] Moreover a simple
threshold theorem which used to calculate
any particular disease’s herd immunity
threshold was recognized by Smith in 1970
& Dietz in 1975. [7] Herd Immunity came
into consideration by the small pox
eradication campaign in 1970’s by the
running practice of ‘Ring Vaccination’
Figure 2: Phase II of Herd Immunity [9]
where every person in the ring should be
immune around the infected individual in
order to prevent spreading of the infection. When only small percentage of
[8]
Then Herd Immunity had gained broader population is vaccinated the risk of the
after their connection with Mass vaccination outbreak of the disease is much greater than
programmes, cost–benefit analyses of one in which most of them are vaccinated.
vaccination, discussion topics on their The unvaccinated members of the
integral role in disease eradication. [7] population are not directly protected and
Thereafter since the adoption of mass and each of the community member have higher
ring vaccination, complexities and risk of being infected.
challenges to herd immunity were been
discovered. [7,8]
Phases of Herd Immunity
Figure 3: Phase III of Herd Immunity [9]
When a large percentage of
Figure 1: Phase I of Herd Immunity [9] population is vaccinated, the spread of
disease is limited. This indirectly protects
When a contagious disease enters an the unimmunized individuals including
unvaccinated group, many members are those who cannot be vaccinated and those
affected as they lack immunity against the for whom vaccination was not successful.
disease; an important element of it can be When more of the population is vaccinated,
determined by the maturity and strength of the spread of infection is also less. If the
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Vol.7; Issue: 7; July 2020
Janki Vyas et.al. The role of herd immunity in control of contagious diseases
number of immunized exceeds 80-95%; We consider an “idealized” population in
most likely the herd immunity’s effect also which during the period of infectiousness an
increases. [10] infectious person has contact with 4 other
BASIC REPRODUCTION NUMBER persons and with 2 of the 4 the contact is
Basic Reproduction number (R0): It is a “close” enough for the infection to be
fundamental statistic in epidemiology for transmitted. Here when the infectious
the purpose of studying infectious disease person is introduced into a totally
dynamics to summarize a complex set of susceptible population, he or she infects 2
factors affecting the rate of transmission in a others (first generation) and then these 2
population. It is mainly number of others each go on to infect a further 2 others
secondary cases generated by a typical of their 4 contacts (second generation) In
infectious individual when the rest of the this situation the basic reproductive number
population is susceptible (at the start of a (R0) is 2. Now consider the same situation
novel outbreak). [11] Basic reproductive after half of the population has been
number varies differently from various rendered immune to infection by
types of infectious agents depending on vaccination, so that (on average) 2 out each
factors such as: its life in through the person’s 4 contacts will be not susceptible
environment, dose necessary for the to infection.
infection, duration of infectiousness on host. Initially when any infectious person
R0 may also vary from population to is introduced into the population only one
population depending on factors such as infection results (first generation) as one of
population density, which may affect the the contacts who previously would have
number of effective contacts a person has been infected has been protected by
while he/she is infectious. It may also vary vaccination. Similarly, the one newly
with season for some infections, as the infected person only goes on to infect one
ambient conditions may affect the survival person (second generation) because half of
of the agent in the environment and the the contacts have been directly protected by
extent to which people have close contact vaccination. In this situation the effective
with each other may be different in warmer reproductive number (sometimes designated
and colder periods. The indirect protection by “Re”) is thus 1. Note the person circled
that may be provided to unvaccinated in Figure 6. This person was infected in the
persons if the level of herd immunity is scenario considered in Figure 1 but was not
increased by vaccination is illustrated infected in the scenario considered in Figure
schematically in Figure 5. 6, even though the person had not been
vaccinated in Figure 6.
This is because the person was indirectly
protected, as the person who would have
infected them was themselves protected by
vaccination and therefore did not pass the
infection on. This is an illustration of a
herd-protective effect of vaccination, where
increasing the level of vaccination (in this
case from zero to 50%) has reduced the risk
of infection among the unvaccinated.
Figure 5: Scenario before Vaccination [13]
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Janki Vyas et.al. The role of herd immunity in control of contagious diseases
sustainable within the population and the
infection will be eliminated. In general, the
effective reproductive number (Re) will be
lower than the basic reproductive number
(R0), depending on the proportion (P) of the
population who are immune to infection.
Such immunity may be induced either by a
previous infection with the agent (if such
infection produced immunity) and/or by
immunization with an effective vaccine.
Simply, R = (1-P) * R0. Therefore,
for infection elimination or eradication – i.e.
Figure 4: Scenario after Vaccination [13]
to reduce R below 1, then P must be equal
to at least (1-1/R0). So, for example, if R0 =
Thus, in Figure 5 the attack rate 5 then P must be at least (1-1/5) = 0.8. That
among susceptible in the second generation is, 80% of the population must be immune,
was 25% whereas in Fig2 it is 12.5% .When either through previous infection or
an infection which is transmitted from vaccination.
person to person, or for which humans are The value of P that reduces R to at
the principal reservoir, to be maintained in a most 1 is commonly called the “herd
population, each case of infection must give immunity threshold” – the level of
rise to at least one other case – i.e. the population immunity that is necessary for
Effective Reproduction number(Re) must be the infection to be no longer self-sustaining
above 1.R0 is an indication of the in the population. The table mentioned
transmissibility of a virus, representing the below shows estimated values of R0 for
average number of new infections generated some common infections and the derived
by an infectious person in a susceptible values for the herd immunity thresholds
population. using the formula given above. The higher
For R0 >1, the number infected is the value of R0 the higher the level of
likely to increase, and for R0
Janki Vyas et.al. The role of herd immunity in control of contagious diseases
Measles In 2018, measles caused an
Measles is a highly infectious and estimated 10 million cases. WHO
one of the most contagious viral diseases recommends 95% coverage using two doses
caused by the paramyxovirus family. It of measles vaccine to prevent the outbreaks.
usually spreads through direct contact (close In 2018, only 86% 0f children globally
personal contact with infected nasal or received the first dose through routine
throat secretions) or through airborne immunization while in the second case, the
transmission (coughing and sneezing) which coverage globally is just 69%. At 2.3
causes febrile illness typically in young million, India is at second highest number of
children. [14,15] It is a human disease and is children who are not vaccinated against
not known to occur in animals. It remains measles which is the report published in
active and contagious in air up to2 hours; Morbidity and Mortality Weekly Report
gets transmitted by an infected person from (MMWR). There were nearly 70,000 cases
4 days prior to the onset of the rash to 4 of measles in India in 2018, the third highest
days after the rash erupts. [14] Each person in the world. In 2019,over 29,000 confirmed
with measles may go on to infect 9 to 18 cases have been reported to the WHO. The
others in a susceptible population according WHO recommends 95% coverage using two
to the basic reproductive number calculated. doses of measles vaccine to prevent the
[15]
outbreaks. In Mass Immunization
Measles outbreaks can result in Campaigns are effective strategy for
epidemics that cause many deaths, delivering vaccination to children who have
especially among young, malnourished otherwise been missed by routine services.
[17]
children. More than 1,40,000 people died
from measles in 2018 – mostly children Herd immunity, in the conventional
under the age of 5 years, despite the sense of total protection of susceptible
availability of a safe and effective vaccine individuals by immunity in persons around
according to the report in MMWR. Global them, is very difficult to establish against
measles deaths have decreased by 73% from measles, because the virus is extremely
an estimated 5, 36,000 in 2000* to 1, 42,000 contagious and a very high level of
in 2018. In countries where measles has population immunity is required. However,
been largely eliminated, cases imported population immunity affects the frequency
from other countries remain an important of virus transmission and hence the age at
source of infection. All children diagnosed infection. Immunity to measles derives from
with measles should receive two doses of two sources: active immunization resulting
vitamin A supplements, given 24 hours from infection with wild or attenuated
apart as it restores low vitamin A levels measles virus, and passive immunization by
during measles that occur even in well- transplacentally acquired antibody. Active
nourished children and can help prevent eye immunization makes the more important
damage and blindness which have shown to contribution to herd protection, but the
reduce the number of measles deaths. [14] duration of passive protection is quite
The basic reproduction number R0 is variable, and it is important in providing
estimated to be 12-18. protection during the most vulnerable age.
[18]
Measles is preventable by
immunization but requires a very high
vaccine uptake to maintain herd immunity. INFLUENZA VACINE
The need for maintenance of this high Flu is a contagious respiratory
coverage to stop transmission requires a illness caused by influenza viruses that
robust vaccination programmes to be infect the nose, throat, and sometimes the
followed. [16] lung and symptoms include chills/cough,
sore throat, runny or stuffy nose ,muscle or
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Janki Vyas et.al. The role of herd immunity in control of contagious diseases
body aches, headaches and causes about
34,200 deaths according to CDC(Centre for
Disease control and Prevention) USA. [19]
The best way to prevent flu is by getting a
flu vaccine each year. Flu vaccines are
available every year before the flu season
begins in the fall and everyone aged 6
months and older is recommended to get the
vaccine. [20] Population including older
people, young children, and people
with certain health conditions like
HIV/AIDS or Asthma or severe heart
conditions or Cancer are at high risk of Figure 7 [34]
serious flu complications.
Influenza is a vaccine preventable The basic reproductive number R0
disease. A trivalent inactivated influenza enables the minimum percentage of
vaccine is most widely used and a cold population (Y) required to be immune for
adapted live attenuated influenza vaccine providing the herd immunity for the entire
(LAIV) is also licensed in some countries. population. [7, 24]
The efficacy of influenza vaccines against Y= (R0– 1)/R0X 100
laboratory confirmed influenza is variable, As per given data, R0= 2-3 as per recent
most probably because of frequent drift of reports.
the virus. A systematic review confirmed If R0= 2, then Y=[(2-1)/2]X100=50 %
overall ‘moderate protection’ of the vaccine. Similarly, when R0=3,
Some studies have also provided evidence Then Y=[(3-1)/3*100=66.66
of herd protection for the same. [21] %.Therefore, for R0= 2-3, nearly 50 to
66.66 percent (threshold) of the populations
HERD IMMUNITY AND COVID-19 required to be immune against COVID-19
The initial cases of novel corona for the protection of susceptible individuals
virus now known as COVID -19 were found in a given population through herd
in Wuhan, Hubei province, China since 29 immunity. [24]
December 2019 initially identified on the Since the onset of SARS-COV-2
surveillance mechanism of “pneumonia with spread, various studies have estimated the
unknown etiology” which was discovered in basic reproductive number (R0) of the virus
the [SARS] Severe Acute Respiratory to be in the range of 2 to 6. From an initial
Syndrome outbreak in the year 2003 with cohort of 425 confirmed cases in Wuhan,
the aim for the identification of novel China, an R0 of approximately 2.2 was
pathogen. [22,23] It has spread worldwide estimated, meaning that, on average, each
triggering into pandemic. The basic infected individual gives rise to 2.2 other
reproduction number is a central concept in infections. More recent estimates place the
infectious disease epidemiology, indicating R0 higher at 5.7, although many estimates
the risk of an infectious agent with respect fall within this range. This variation reflects
to epidemic spread. [12] Covid-19’s basic the difficulty of obtaining accurate R0
reproductive number has been derived to be estimates in an ongoing pandemic, and the
median of 2.79. current estimatedSARS-CoV2 R0 values
likely do not indicate a complete picture of
the transmission dynamics across all
countries.
Assuming an R0 estimate of 2 for
SARS-CoV-2, the herd immunity threshold
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Vol.7; Issue: 7; July 2020Janki Vyas et.al. The role of herd immunity in control of contagious diseases
is approximately 66.67%. This means that More research is needed on it .As vaccine is
the incidence of infection will start to yet to be discovered, a large number of
decline once the proportion of individuals population needs to be exposed and recover
with acquired immunity to SARS-CoV-2 in before we can have herd immunity .The
the population exceeds 0.66. As discussed only selective pressure on SARS-CoV-2 is
above, this model relies on simplifying transmission—stop transmission and you
assumptions, such as homogeneous stop the virus .The linchpin for a strategy to
population mixing and uniform sterilizing move out of lockdown seemingly rests on
immunity in recovered individuals across increased testing and contact tracing,
demographic groups, which are unlikely to possible return-to-work permits based on
hold true. Nevertheless, this basic model can immune status, repurposed or new
give us a rough idea of the number of therapeutics, and, finally, vaccination.
individuals that would need to be infected to Current discussion, addresses the notion that
achieve herd immunity in the absence of a scaled up antibody testing will determine
vaccine given an approximate herd who is immune, thus giving an indication of
immunity threshold and a country’s the extent of herd immunity and confirming
population. [25] The possibility of a mutation who could re-enter the workforce. Currently
and emergence of a new strain of virus we should see the evidence-based
cannot be ruled out and it can make herd assumptions on herd immunity, rather than
immunity ineffective although there is a optimistic guesses. [26] There is an urgent
chance of emergence of vaccination for the need for reliable antibody detection
effective herd immunity. [24] ELISA can be approaches to support diagnosis, vaccine
calibrated to be specific for detecting and development, safe release of individuals
quantifying SARS-CoV-2 IgM and IgG and from quarantine, and population lock-down
is highly sensitive for IgG from 10 days exit strategies. [27]
following first symptoms.
Numerous clinical trials to evaluate EBOLA VIRUS
novel vaccine candidates and drug Given that the recent 2014 Ebola
repurposing strategies for the prevention epidemic had a lower R 0 values hence less
and treatment of SARS COV-2 infection are vaccine coverage than previous epidemics
currently ongoing. However, it is unknown would be needed in order to prevent
whether these trials will produce effective transmission. For future plans Ebola virus
interventions, and it is unclear how long transmission can be blocked by vaccinating
these studies will take to establish efficacy susceptible populations. It is easiest to
and safety, although an optimistic estimate prevent an outbreak when a virus has a low
for any vaccine trial is at least 12–18 R0value and high vaccine efficacy. [31] A
months. In the absence of a vaccine, Study suggest that Ebola viruses with R0≥ 3
building up SARS-CoV-2 herd immunity and vaccine effectiveness of 70%, (higher
through natural infection is theoretically level) nearly all of the population would
possible. However, there is no need to be vaccinated in order to establish
straightforward, ethical path to reach this herd immunity
goal, as the societal consequences of It is important to note that there are
achieving it is devastating that virus causes. several limitations to this study. The study
[25]
utilizes a simple threshold theorem which
The main obstacle to achieving makes several assumptions: (1) random
COVID -19Herd Immunity is that virus that vaccination within the population, (2)
causes the disease is “novel” namely it homogenous mixing of persons within the
hasn’t infected humans before and everyone population, (3) homogeneous distribution of
is at risk of infection and there is no inbuilt vaccine-induced protected and infected
immunity existing against the virus. Yet persons within the population, and (4) fully
International Journal of Research and Review (ijrrjournal.com) 115
Vol.7; Issue: 7; July 2020Janki Vyas et.al. The role of herd immunity in control of contagious diseases
susceptible population. Currently, there are number of individuals is not always possible
known contraindications to Ebola within desired time range to control
vaccination for specific age ranges. The transmission. Thus, it is important that
phase 1 trial by sarwar was limited to adults future Ebola out-break protocols include
aged 18–60 years. Due to the restrictions in intensive measures for containment to
age, a larger percentage of the adult prevent transmission. If supply is a potential
population needs to be vaccinated in order issue it would be wise to distribute vaccine
to protect those who cannot be vaccinated. supply in levels of priority. Similar to
If children and older adults were also vaccination protocols within hospitals an
eligible to be vaccinated, it would be easier important target population to vaccinate
to reach the herd immunity threshold by would be healthcare workers. The Guinea
being able to vaccinate more individuals. trial demonstrates the value of ring
The most recent phase 3 trial of Ebola vaccination in Ebola and the importance of
vaccination in Guinea involved ring vaccinating individuals who have been or
vaccination in a cluster-randomized style. will be in close contact with the virus. This
Ring vaccination involves administering can include contacts of sick individuals such
vaccination only to a cluster of high risk as family and friends, caregivers, and
individuals who are in close contact with a contacts of contacts. As clinical trials are
confirmed isolated infected person. Eligible still underway for development of Ebola
individuals in the clusters were given either vaccines, the consequences in future are
immediate or delayed Ebola vaccination. Of awaited for the novel vaccine efficacy data
the immediately vaccinated individuals the to determine vaccine coverage needed to
vaccine was 100% effective as determined prevent outbreaks. [28]
by no symptoms of ebolavirus disease 10
days after vaccination. Of all eligible VACCINATION AND VACCINE
individuals who received immediate or HESITANCY
delayed vaccination (21 days after Vaccination include partial or
randomization) the vaccine was 75.1% complete protection against symptomatic
effective and 76.3% effective spending on illness, improved quality of life and
cluster. Because this was a cluster trial of productivity, and prevention of death as
individuals in close contact with isolated well as overall benefits to society including
infected person(s) the R0valueof the virus is creation and maintenance of herd immunity
likely higher than in populations where against communicable diseases, prevention
individuals are not in close proximity. of disease outbreaks, and reduction in
Therefore, it is possible that critical health-care--related costs. [14]
vaccination coverage values may be lower A British Physician Andrew
in other communities. Importantly, the basic Wakefield published a study which claimed
reproductive value of the outbreak in that there might be a link between the
Guinea was not reported so it is difficult to vaccine for measles, mumps, and rubella
conclude exactly how the critical (MMR) and autism which is a
vaccination coverage would be affected. In developmental disorder in the year 1998.
future outbreaks with a 100% efficacious There is no such evidence proved; only
vaccine, as reported by the Guinea ring small group researchers entertained this
vaccination trial, 42–63% of the population theory about autism and there were almost
would have to be vaccinated to prevent rare conflicts among the scientists regarding
transmission. There are several stressors that the debate. Although the trends were only
could affect the ability to vaccinate coincidental, Wakefield’s paper helped run
individuals against Ebola in the future. a debate in the media for about 15 years The
When outbreak occurs, access to adequate paper got a growing concern amongst the
supply of vaccine to vaccinate sufficient parents in the US and UK about a possible
International Journal of Research and Review (ijrrjournal.com) 116
Vol.7; Issue: 7; July 2020Janki Vyas et.al. The role of herd immunity in control of contagious diseases
connection between the rising number of protected by achieving Herd Immunity.
childhood vaccinations and the rising rate of Herd Immunity generally applies only to
autism among kids due to which there was a diseases that are contagious; generally does
fear among the population stoked by the not apply to diseases like tetanus, food born
media coverage which caused delay or infections, intoxications etc.
decline in the vaccination of their children Herd immunity also assumes; if a
which led to increase in the measles person is infected it will cause infection to
infection gradually. [29] Education about others making them immune towards the
herd immunity and local vaccination disease. In India the polio has been
coverage could be a useful tool in increasing eradicated in children because of herd
vaccination rates and benefiting immunity due to its effective integral mass
communities. Vaccination protects vaccinations strategy. But in case of covid -
individuals directly and communities 19 herd immunity can only be achieved
indirectly by reducing transmission. [20] when 60-90 % of the population gets
If effective control of measles is to infected, then they tend to get immune and
be attained in developing countries, a stop being the carriers of the virus and the
program must be designed to build on, and remaining population which didn’t came
expand, herd immunity. [18] Vaccination into contact were indirectly been protected
risks range from common, minor, and local from virus . But in India it is highly
adverse effects to rare, severe, and life- dangerous due to overpopulation as we need
threatening conditions. [14] To be to have a robust health care system with
biologically interpretable and to be robust to appropriate number of beds, ventilators,
different transmission conditions and oxygen cylinders for those who are being to
indirect effects, the parameters must take get infected. UK tried to adopt herd
account of the type and amount of exposure immunity for covid -19 but the cases went
to infection either by study design or by too far to control the virus. Recently
mathematical modeling. Otherwise, Sweden also approached herd immunity
equivalent populations with the same thinking of having a strong healthcare
transmission conditions could yield different system to approach. Earlier measles and
efficacy estimates because of the indirect polio were able to develop herd immunity
effects of vaccination. [30] Thus awareness by the help of vaccination and not just
regarding vaccination is needed amongst naturally. Vaccines were invented and
population to utilize its benefits to the vaccination camps were been introduced to
fullest immunize the children and help in vision to
eradicate the disease. According to WHO
DISCUSSION only about 5 to 10% of the world’s
When most of the populations in a population has been exposed to corona virus
community are immune to a particular until now and experts say that India could
infection that is spread from person to develop herd immunity in 12 to 36 month
person, the natural transmission of the time period. Still question arises that
infection is effectively inhibited. Thus herd immunity developed against corona virus;
immunity indirectly protects people who will it persist for whole life period or few
have inadequate access to immunization and months or years we don’t know. So ideally
to people who remain unvaccinated by it would be much better if we develop herd
choice. People with multiple diseases at a immunity with the help of vaccines. in many
time or those who are hospitalized could be other cases there have been too many
benefited with Herd Immunity. Those group variable factors for herd immunity to be a
of people undergoing chemotherapy success.
sessions or having disease like HIV which No matter how large the proportion
weakens the immune system are also been of immunes in the total population, if some
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Vol.7; Issue: 7; July 2020Janki Vyas et.al. The role of herd immunity in control of contagious diseases
pockets of the community, such as low 10. History of Vaccines. available from
economic neighborhoods, contain a large https://www.historyofvaccines.org/index.ph
enough number of susceptible among whom p/content/herd-immunity-0.
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