CMAAO CORONA FACTS and MYTH BUSTER 88

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)/R_0.

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.

828: Estimated R0 and HITs (herd immunity threshold) of well-known infectious diseases[50]
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 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.