Is herd immunity even a viable strategy for COVID-19?

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In the absence of a vaccine or drug, and without a clear understanding of the disease pathology, it would be dangerous to seek to achieve herd immunity through infection.

Inherent infectiousness varies based on population density, population structure, differences in contact rates across demographic groups, and intervention measures such as vaccination, social distancing, lockdowns, and quarantines.

The year 2020 started with news about a disease caused by a “novel virus”. Suddenly, there was new information thrown at us about animals — particularly bats — being a breeding ground for several viruses, with potentially new diseases being “discovered” every day; many felt that mutations might make COVID-19 deadlier. Even now, the way forward looks really blurry — with each country trying to find the balance between resuming economic activity and containing spread. Some countries, such as the UK and Sweden, chose different mitigation strategies altogether — choosing to live life “normally”, allowing the virus to spread (the UK abandoned this later) and hoping that “herd immunity” would prevail. With this in mind, what are some of the factors affecting the decisions that will be taken in the near future?

The basics of disease transmission

Before we get to herd immunity and the decisions, let us first understand why people don’t always drop dead in large numbers every time a new disease hits. Thanks to the human body’s immune system, we are equipped to deal with diseases reactively. The human body uses different kinds of ammunition or lines of immune defence when challenged with an infectious agent, which essentially rescues us either right away or over a period after being infected, depending on how deadly it is. In the case of a novel (nothing similar previously seen by the body) disease-causing organism (pathogen), the body has to adapt to this new change, learn about this new enemy first, and then develop immunity. The exposure could occur through infection or through vaccination (most commonly, a weakened form/part of the pathogen is proactively injected into the body to spur immunity). But what are the factors that govern the deadliness of a pathogen/ disease?

 

The deadliness of a pathogen/disease depends on many factors such as:

● The inherent infectiousness of the disease (how many people does one person infect on average?) — termed as Reproduction number

● The mode of transmission (is it through common modes such as air and water, or through relatively rare modes such as bodily fluids?)

● Mortality (the proportion of people dying due to the infection)

● How long it takes for one person to infect another (serial interval)

 

There are mathematical relationships and models (beyond the scope of this article) that have been developed to understand the interplay of these disease-specific parameters, which have been listed below.

Disease

Reproduction number

Mortality

Mode of transmission

Serial interval

Herd immunity threshold

Measles

12-18

0.2%*

Contact, air

11.7 days

90-95%

Smallpox

5-7

30%

Contact, air

17.7 days

80-86%

Ebola

3-20

50%

  Bodily fluids

14-15 days

67-95%

Cholera

1.1-2.7

1-50%

Water

3-5 days

9-66%

Dengue

2

1-20%**

Mosquito

14-17 days

50%

Influenza

1.5-2.5

0.1-10%

Droplets

2.2-2.8 days

33-44%

SARS

2-5

15%

Contact, air, surfaces

2-7 days

50-80%

COVID-19

1.5-3.5***

3-4%***

Contact, air, surfaces

4-5 days

29-80%***

*This is typically low as a vaccine is available

**Varies heavily based on availability of medical care

*** Not conclusive information due to preliminary data

For example, a measure of how deadly smallpox was can be gauged from the above table — on average, each infected person spread the disease to 5-7 more people, and this happened at some point during an 18-day period, which led to 30% deaths on average. Taking this to its extreme, 1 “typical” infection causes 7 in the first stage, and then escalates to 1.17 lakh in merely 6 unfettered iterations. Many readers would have seen such exponential models being used to project the growth of COVID-19 infections. And since COVID-19 is new, many of these parameters are yet to be reliably estimated due to different factors.

But when does this end? Can this continue indefinitely? It can’t, because the actual population of humans would act as the upper limit. So as the disease spreads, the pool of “infectible humans” reduces, and the pathogen “runs out” of uninfected humans as it keeps “bumping into” people who have already developed immunity to the disease, thus reducing its spread. This is the point at which individual immunity acts synergistically at the societal level — when herd immunity is achieved, it means that the pathogen can no longer infect in large numbers. And how can this endgame be achieved? The same way humans individually achieve immunity — through infection or vaccination. For instance, for smallpox, this number is ~86% of the population, which was achieved through worldwide vaccination.

 

However, biological parameters are often interlaced with underlying socio-cultural and socio-economic aspects, and the patterns get more complex and harder to determine as they tend to be very region-specific. For example, inherent infectiousness varies based on population density, population structure, differences in contact rates across demographic groups, and intervention measures such as vaccination, social distancing, lockdowns, and quarantines.

Herd immunity in COVID-19

Over the months, scientists have seen that COVID-19’s reproduction number has varied from 1.5 to 3.5 in different parts of the world. And since large-scale surveillance programs have not been done, and since the disease has not run its full course anywhere, experts can only approximate mortality rates from symptomatic patients and not from all those who actually have been infected by the virus (many of them don’t display any symptoms). So, after many months, we only have a reasonable understanding of how long the person is infectious and the possible modes of transmission. Some debates about sexual and other possible modes of transmissions, and other disease parameters are still ongoing in the scientific community.

But beyond all these, there are big questions still plaguing the scientific community — the longevity of the immunity and how quickly the virus transforms itself through mutations (rendering the earlier antibodies powerless). Since it is still early days, it is not only difficult to determine but would also yield an inaccurate estimate of these two factors. A clear understanding of these aspects is necessary to gear up the systems and take appropriate policy decisions. It may so happen that after several mutations, COVID-19 could behave like the influenza virus that warrants the need for a yearly flu shot, if/when a vaccine becomes available. It must be noted that in case of the yearly flu shot, even though some people have less risk, they are urged to take the shot to achieve and maintain herd immunity so that the risk of spread is minimised.

In the absence of a vaccine or drug, and without a clear understanding of the disease pathology, seeking to achieve herd immunity through infection is a dangerous strategy. Allow the disease to spread too quickly, it overwhelms the health system and causes many people to die “unnecessarily”; do it too slowly, and it takes that much longer for life to come back to “normal”. Therefore, for almost all countries, at this juncture, it is a cruel choice between saving lives and saving livelihoods.

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