Covid herd immunity

The two mega-cities of Mumbai and New Delhi (NCR) have been bearing the brunt of the COVID-19 pandemic in India. However, in the last few weeks, it has become easier to find empty hospital beds and unused ventilators. This has led to euphoria and the hope that we might be seeing light at the end of the tunnel. Unfortunately, our country continues to document about 60,000 new cases daily. So, when are we going to win this battle?

We all know that to control any infectious and contagious disease pandemic, there are only three solutions:

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1 Eliminate the last trace of the infective organism (like in small pox).

2 Invent an effective vaccine and use it as long as required (like polio).

3 Generate sufficient herd immunity in the general population so as to attenuate community transmission to a trickle.

The first one is extremely unlikely at present since globally COVID cases have now crossed the 20 million mark and daily new cases are not showing any signs of falling. All expectations that the COVID-19 virus will not survive extremes of temperatures, hot summers and torrid rains have faded away. It continues to continue marching around the globe, without faltering in its strides.

One of the places considered safe from the COVID-19 pandemic was New Zealand. Till recently there was no evidence of community spread and no new local cases identified between April 30th and August 10th. Even here, the virus has raised its ugly head and COVID alert level 3 lockdown has been implemented in Auckland area since midday of 12th August because of 9 new cases.

Vaccines look quite promising with at least three inventions in phase III trials and India is also studying two potential candidates (study of first vaccine involves 1150 subjects across 8 investigation sites and the second vaccine is being evaluated in 1000 subjects from 5 centers. However they are still several months away, perhaps will be available for mass production in the second half of 2021.

That leaves us with one hope – herd immunity. However we find a lot of conflicting evidence and expert opinions. So what is the truth?

How real is the COVID-19 herd immunity?

Here are two examples of communities showing exposure to COVID-19 in sufficiently large numbers to make herd immunity a possibility. In the slums of Mumbai (remember Slumdog Millionaire), population density is about 60,000 people per square kilometer (e.g. Andheri East and Dharavi). Often more than a handful of people are living in a single room of size 8 x 10 feet and use a common toilet. Physical distancing is out of the question. The Government of Maharashtra survey shows that as many as 57% of the people are COVID-19 positive. A similar figure of 59% positivity was documented on a cruise ship from Argentina going to the Antartica carrying 217 passengers.

Sweden attempted to allow development of protective herd immunity by deliberately avoiding lockdown and permitting their people to mingle without any restrictions. Unfortunately COVID-19 did not respect their wishes. Today, Sweden has one of the highest COVID-19 mortality in Europe (57 deaths per 100,000 population) and is even higher than USA (50 deaths per 100,000 population). Compare this to Sweden’s neighbours who sensibly followed lockdown and physical distancing measures (COVID-19 mortality being 5/100,000 people in Norway and 11/100,000 in Denmark).

While the rationale was logical, Sweden could not develop any herd immunity because only 7% of their sparse population developed antibodies to the virus. Sweden has therefore learned the bitter lesson that protection by herd immunity in their country as a result of community infection is neither realistically achievable nor ethical.

Another bad news is the development of second waves in some countries and regions. Just when the authorities were patting themselves about having “contained” the pandemic in their backyard, any relaxation in the lockdown strictness led to a resurgence. Let us take the example of Spain. Like Italy and France, it was also hit severely since March 2020. By mid May new cases were down and till mid July they seemed to be well on their way to petering out (few hundreds daily). As it turned out, they are now facing a second wave of resurgence with new cases rising 10-fold to about 2500 every day.

The final example i would like to cite is the story of one prison (they like to call it correctional facility) in USA – the San Quentin in California. In this prison, over two months, a disastrous situation has been unfolding over the last two months. More than 2,200 inmates have become COVID-19 positive and 25 have died. Considering that the prison has a total strength of 3,260 persons, 68% have become infected. Surely this would be sufficient for the protective effect of herd immunity, especially with inmates densely packed like in the slums of Mumbai. Unfortunately, daily new cases continue unabated. In fact, the situation in San Quentin is equivalent to death rate of 767 per 100,000 people. Factors that contribute include overcrowding, restricted prolonged stay in close proximity of each other and lack of ventilation.

So is the situation all bad?

Fortunately no. There is some evidence that herd immunity can be achieved with lower number of people having been infected. Some investigators fear that just because a person is sero-positive, may not automatically mean that he is protected from the virus. Nor is it clear whether the antibody titres (even if protective) will be long lasting. Fortunately, our body’s immune system also includes T lymphocytes and memory cells might be able to mount sufficient immune response even if the antibody titre is not protective.

In conclusion, we are still confused whether herd immunity against COVID-19 is real or not. The good news for India is that COVID-19 recovery rate in April was 7.85% and now (August) it’s 64.4%, with more than one million Indians having recovered. Perhaps Indians already have some degree of immunity against COVID-19. [1]

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