Herd Immunity – Finding the right time to propose and implement the concept in the current COVID 19 pandemic

Dr. Krishna Prasad G, DNB (EM), MRCEM, MNAMS

10^th April 2020

This statement was one of the novel ideas brought forward to tackle the COVID-19 outbreak. On the day when the Chief Scientific Advisor gave this statement, there had been only 11 deaths in the UK. Also, it was on this day that the WHO director general, Tedros Adhanom Ghebreyesus told that the situation in Europe is now so severe that the continent must be considered the centre of the pandemic [1].

But why did the Chief Scientific Advisor of the United Kingdom put out this concept, when in most other parts of the world were following a similar policy which included lock down of workplaces, schools, and public gatherings? Was he completely incorrect?

To understand this, the best way is to ponder back into the history of mankind.

What is herd immunity?

Herd immunity is the indirect protection from a contagious infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection.

This means that even people who aren’t vaccinated, or in whom the vaccine doesn’t trigger immunity, are protected because people around them who are immune can act as buffers between them and an infected person.

Once herd immunity has been established for a while, and the ability of the disease to spread is hindered, the disease can eventually be eliminated [2].

Why is herd immunity important?

The more infectious a disease, the greater the population immunity needed to ensure herd immunity. COVID-19 is said to be less infectious than measles or mumps [2]. But to assume that COVID -19 was just like flu or other viruses was a terrible assumption.

Lessons from history

Though coined almost a century ago, the term ‘‘herd immunity’’ was not widely used until recent decades, its use stimulated by the increasing use of vaccines, discussions of disease eradication, and analyses of the costs and benefits of vaccination programs.

If herd immunity has been established and maintained in a population for a sufficient time, the disease is inevitably eliminated and no more endemic transmissions occur.

To date, two diseases have been eradicated using herd immunity and vaccination: Rinderpest and Smallpox.

As per medical literature, herd immunity concept was an outcome of an observation analysis and not an interventional one. In the 1930s, when A.W. Hedrich, an Associate in Biostatistics at the School of Hygeine and Public Health, John Hopkins University, Baltimore while studying the epidemiology of measles in Baltimore, he observed that after many children had become immune to measles, the number of new infections temporarily decreased, including among susceptible children.

It was also postulated as an external marker of high levels of immunity, thus making it an inclusion while calculating cost-benefit analysis of various vaccination programs.

Only by vaccination, herd immunity can be effectively implemented into a society. This was the success story behind smallpox. By mass vaccination (thus implementing herd immunity), we have been able to control the spread of other diseases such as diphtheria, measles, mumps, pertussis, polio and rubella also.

Even though, we have seen many positive results supporting the concept of herd immunity, most of the ideologies are based on assumptions. In recent decades, it has been recognized that the dominant strain of a microorganism in circulation may change due to herd immunity, either because of herd immunity acting as an evolutionary pressure or because herd immunity against one strain allowed another already-existing strain to spread [3].

Change of strategy

The SARS-CoV-2 virus is novel in all means, having a morality rate from 4% in China to 12% in UK (as of 10^th April 2020).

Initial data (mostly from China) suggested that this novel virus affects only the elderly population and that too with multiple comorbidities. This prompted many world leaders to take this outbreak not very seriously. The numbers which were making rounds was that the mortality rate was less than 2 percent. This eventually turned out to be wrong with increasing rates in certain European countries and now in UK too.

The game changer in UK was the Imperial College Mathematical Prediction Model. Even though the model was made based on numbers from Wuhan and Italy, the graphs had far reaching impacts.

A new strategy had come following the publication of a government-commissioned report from Imperial College London’s COVID-19 Response Team, which models different strategies and outcomes, focusing in particular on the UK and the US [4].

The team, led by epidemiology professor Neil Ferguson (an expert in pandemic response) projects that had the government continued pursuing a mitigation approach, that could have led to an estimated quarter of a million deaths in the UK alone (and 1.1-1.2m in the US), with demand for intensive care beds outstripping supply by a factor of eight to one. The suppression strategy, they predict, will lead to “a few thousands or tens of thousands” of deaths [4].

The whole study can be best summarized from the following graphs:

The suppression strategy which would constitute of a combination of case isolation (i.e. isolate if you’re symptomatic), social distancing of the entire population, and either household quarantine or school and university closure over a 5 month period would result in fewer deaths. These models are all predictive and have not taken into account many factors. But this study was a game change in both the UK and US. They quickly changed their existing methods of tackling the virus and advised quarantine measures [4].

Why was herd immunity concept wrong in the initial phase?

There are multiple reasons:

1. The virus has not been studied completely. We do not have sufficient data to even explain the mortality rate.
2. Herd immunity has been an outcome of an observational analysis. We do not have any past experience of introducing a disease into a society to achieve herd immunity.
3. Herd immunity is essential to combat infectious or highly contagious diseases. Having said so, diseases like COVID-19 has got high mortality rate, which warrants us to think about controlling the disease and finding a cure (by either drug/vaccine/other means) rather than creating herd immunity.
4. There are more than one mutated versions of the SARS-CoV-2 virus which would delay development of herd immunity.
5. Human population move a lot in this modern world. Therefore, even if we do develop herd immunity, it is not going to be sustainable. Mutated virus strains will be hard to combat.

Lock down

Starting 23 January 2020, the Chinese government had locked down Hubei Province, including Wuhan, the city of 11 million where the outbreak started. They halted transportation in and out and barred tens of millions of people from working or going to school and closed all shops except those selling food or medicine. In some areas, residents were even forced to limit trips to the store, or order supplies for delivery.

This unprecedented lockdown of tens of millions of people was considered a “vast experiment” – but it may have worked. Following the lockdown, cases began to slow. On 19 March, China’s National Health Commission reported no new confirmed infections in Hubei.

Italy and Spain have been under similarly intense nationwide lockdowns, from 9 March and 15 March, respectively, with citizens required to stay in their homes except for work, food shopping or medical appointments.

In parts of Italy where lockdowns started earlier, however, we're already seeing a "flattening of the curve". Lodi, for example, locked down on 23 February, but Bergamo did not lock down until 8 March. Now, cases seem to be leveling off in Lodi [5].

Can herd immunity be postulated as an exit strategy from lock down?

We know that herd immunity implementing strategy had failed in the initial phase of the outbreak. Having seen many positive results from the lock down having worked in China and Italy, time has come to exit the lock down. In certain countries like UK and Spain, the curve is still yet to flatten. To implement a successful exit strategy, a thorough knowledge of the mortality rate is required. Let’s take an example of 3 countries: China, Italy and UK. These numbers may not be accurate but approximate [6, 7].

The UK death toll has been taken from the ICNARC study and may not represent the complete figures.The age group will give a possible solution on how to develop an effective exit strategy.

Here are some postulates:

1. In countries like UK, the mortality rate peaks up for age group above 50. Therefore people above 50 (high risk) should continue in isolation till there are zero cases reported. 
2. People less than 50 can start going back to work but they should keep a safe distance through social distancing from people aged more than 50.
3. Schools can be reopened and children can be sent to school. Children and younger groups will be the ones in the front to develop herd immunity. Basic hygiene such as hand washing and proper use of face masks should still be encouraged. But still, they should be kept away from people above 50 years till no new cases are reported.
4. Until zero cases are reported, the government should invest more money in social care and community support. This would give us some more time until an effective vaccine is developed.
5. A staggered exit strategy keeping an eye on the age group will be the key to this problem. If implemented along with elaborate testing, this would turn out to be cheaper in the long run.

Final message – Herd immunity is a very beautiful concept, but shouldn’t be instituted early. It can be an effective tool later. But the essential need for the hour is a vaccine for which we need to invest more into research.

References:

1. Online - https://www.nationalgeographic.co.uk/science-and-technology/2020/03/uk-backed-herd-immunity-beat-covid-19-well-ultimately-need-it
2. Online - https://www.gavi.org/vaccineswork/what-herd-immunity
3. Online - https://en.wikipedia.org/wiki/Herd_immunity
4. Online - https://ftalphaville.ft.com/2020/03/17/1584439125000/That-Imperial-coronavirus-report--in-detail-/
5. Online - https://www.weforum.org/agenda/2020/03/why-lockdowns-work-epidemics-coronavirus-covid19/
6. Online - https://www.vox.com/2020/4/1/21203198/coronavirus-deaths-us-italy-china-south-korea
7. Online - https://www.icnarc.org/DataServices/Attachments/Download/41bbc4bb-2c7b-ea11-9124-00505601089b