CMAAO CORONA FACTS and MYTH BUSTER 88 HERD Immunity
Dr K K Aggarwal
President Confederation of
Medical Associations of Asia and Oceania, HCFI, Past National President IMA,
Chief Editor Medtalks
With inputs from Dr Monica Vasudeva
823: Herd Immunity
Some authors use it to describe the proportion immune
among individuals in a population. Others use it with reference to a particular
threshold proportion of immune individuals that should lead to a decline in
incidence of infection. Still others use it to refer to a pattern of immunity
that should protect a population from invasion of a new infection.
824: Herd effect
That
the risk of infection among susceptible individuals in a population is reduced
by the presence and proximity of immune individuals (this is sometimes referred
to as “indirect protection” or a “herd effect”).
825: Smith in 1970 and
Dietz in 1975 threshold theorem HERD IMMUNITY THRESHHOLD (HIT)
Incidence
of the infection would decline if the proportion immune exceeded (R0 − 1)/R0.
R0 functions as a measure of contagiousness, so low R0 values are associated with lower HITs, whereas higher R0s result in higher HITs.
For
example, the HIT for a disease with an R0 of 2 is theoretically only 50%, whereas with disease
with an R0 of 10 the theoretical HIT is 90%.
826:
Definitions
Basic
reproduction number
|
R0
|
Number of secondary cases generated by a typical
infectious individual when the rest of the population is susceptible (ie, at
the start of a novel outbreak)
|
Critical vaccination level
|
Vc
|
Proportion of the population that must be
vaccinated to achieve herd immunity threshold, assuming that vaccination
takes place at random
|
Vaccine effectiveness against transmission
|
E
|
Reduction in transmission of infection to and
from vaccinated compared with control individuals in the same population
(analogous to conventional vaccine efficacy but measuring protection against
transmission rather than protection against disease).
|
If vaccination does not confer solid immunity against infection to all
recipients, the threshold level of vaccination required to protect a population
increases. If vaccination protects only a proportion E among those vaccinated (E standing for effectiveness against infection
transmission, in the field), then the critical vaccination coverage level
should be V c= (R0− 1/R0)/E.
827: If E is <(1− 1/R0) it would be impossible to eliminate an
infection even by vaccinating the whole population.
Waning vaccine-induced immunity demands higher levels of coverage or
regular booster vaccination. Important among illustrations of this principle
are the shifts to multiple doses (up to 20) and to monovalent vaccines in the
effort to eliminate polio in India, where the standard trivalent oral polio
vaccines and regimens produce low levels of protection.
Disease
|
Transmission
|
R0
|
HIT
|
Measles
|
Airborne
|
12–18
|
92–95%
|
Pertussis
|
Airborne droplet
|
12–17
|
92–94%
|
Diphtheria
|
Saliva
|
6–7
|
83–86%
|
Rubella
|
Airborne droplet
|
||
Smallpox
|
5–7
|
80–86%
|
|
Polio
|
Fecal-oral route
|
||
Mumps
|
Airborne droplet
|
4–7
|
75–86%
|
SARS
(2002–2004 SARS outbreak) |
2–5
|
50–80%
|
|
1.4–3.9
|
29–74%
|
||
Bodily fluids
|
1.5–2.5
|
33–60%
|
|
Airborne droplet
|
1.5–1.8
|
33–44%
|
828: Should we allow all below 40 to get the infection as a part of
herd immunity plan
No. The concept is based on the assumption that if 50-60
% of people ( all less than 40 years with least mortality) are infected one
will be able to control the epidemic and also prevent a second wave next year. But,
vulnerable people should not be exposed to Covid-19 right now in the service of
a hypothetical future.
The hypothesis is to achieve “herd immunity” in order to
manage the outbreak and prevent a catastrophic “second wave” next winter. The
argument is generating immunity in younger people is a way of protecting the
population as a whole.
We talk about vaccines generating herd immunity but, this
is not a vaccine. This is an actual pandemic that will make a very large number
of people sick, and some of them will die. Even though the mortality rate is
likely quite low, a small fraction of a very large number is still a
large number.
At the peak of the outbreak the numbers requiring
critical care would be greater than the number of beds available. This is made
worse by the fact that people who are badly ill tend to remain so for a long
time, which increases the burden.
Second waves are real things, and we have seen them in
flu pandemics. This is not a flu pandemic. Flu rules do not apply. But
vulnerable people should not be exposed to a virus right now in the service of
a hypothetical future.
Transmission occurs before symptoms develop. You should
instead look to the example of South Korea, which, through a combination of
intense surveillance and social distancing, appears to have gained some
semblance of control over the virus. We can learn from South Korea, Singapore,
Hong Kong and Taiwan, all of which have so far done a good job mitigating the
worst outcomes despite having reported cases early in the pandemic, and in the
case of South Korea, suffering a substantial outbreak.
Policy should be directed at slowing the outbreak to a
(more) manageable rate. What this looks like is strong social-physical distancing.
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