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CMAAO Coronavirus Facts and Myth Buster 91

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Dr KK Aggarwal    10 May 2020

(With inputs from Dr Monica Vasudeva)

 

823: Herd Immunity

Some experts use the term to describe the proportion immune among individuals in a population. Others may use it with regard to a particular threshold proportion of immune individuals that should lead to a fall in incidence of infection. Yet another group of experts may use it to refer to a pattern of immunity that should provide protection to a population from a new infection. [Clin Infect Dis.]

824: Herd effect

The risk of infection among susceptible individuals in a population is diminished by the presence and proximity of immune individuals. This is sometimes termed as “indirect protection” or a “herd effect”. [Clin Infect Dis.]

825: Smith in 1970 and Dietz in 1975 recognized the threshold theorem - Herd Immunity Threshold (HIT)

Incidence of the infection would decline if the proportion immune exceeded (R0 − 1)/R0.

R0 is a measure of contagiousness. Therefore, low R0 values are associated with lower HITs, while higher R0s lead to higher HITs.

For example, the HIT for a disease with an R0 of 2 is only 50%; however, with a disease with an R0 of 10 the theoretical HIT is 90%.

826: Definitions

Basic reproduction number 

R0 

Number of secondary cases generated by an infectious individual when the remaining population is susceptible (for instance, at the start of a novel outbreak) 

Critical vaccination level 

Vc 

Proportion of the population that has to be vaccinated to attain 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 provide solid immunity against infection to all the recipients, the threshold level of vaccination needed to protect a population increases. If vaccination confers protection to only a proportion E among those who are vaccinated (E means effectiveness against infection transmission, in the field), then the critical vaccination coverage level should be c(R0− 1/R0)/E. [Clin Infect Dis.]

827: If E is <(1− 1/R0) it would be impossible to eliminate an infection even by vaccinating the whole population.

Decreasing vaccine-induced immunity calls for 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 order to eliminate polio in India, where the standard trivalent oral polio vaccines and regimens yield low levels of protection.

828: Estimated R0 and HITs (herd immunity threshold) of well-known infectious diseases

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%

COVID-19(COVID-19 pandemic)

1.4–3.9

29–74%

Ebola(Ebola virus epidemic in West Africa)

Bodily fluids

1.5–2.5

33–60%

Influenza(influenza pandemics)

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 below 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 often talk about vaccines generating herd immunity but, this is not a vaccine. This is a pandemic that is going to make a large number of people sick, and some of them are going to die. Despite the fact that 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 number of people requiring critical care would be greater than the number of beds available. This is further worsened by the fact that people who are badly ill remain so for a long time, thus adding to the burden.

Second waves are real, and have been seen in flu pandemics. This is not a flu pandemic, so flu rules do not apply here. But susceptible people should not be exposed to a virus right now in the service of a hypothetical future.

Transmission occurs before symptoms develop. Look to the example of South Korea, which, by means of a combination of intense surveillance and social distancing, seems to have attained some semblance of control over the virus. We can learn from South Korea, Singapore, Hong Kong and Taiwan. All of them have done a good job curbing 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.

 

Dr KK Aggarwal

President CMAAO, HCFI, Past National President IMA, Chief Editor Medtalks

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