One in every four or five Covid-19 patients do not develop effective anti-bodies to provide long-term immunity from the novel coronavirus, says Dr SK Sarin, Director of the Institute of Liver and Biliary Sciences, who also heads the five-member committee formed to assist the Delhi chief minister to tackle the pandemic.

In this exclusive interview to Sources, Dr Sarin explains why allowing community to develop �herd immunity� through virus transmission is not the best way to deal with the pandemic.

With nearly 24% of Delhi testing positive for Covind-19 in the last serological survey, the new survey is expected to throw up higher percentages of people. Is the capital headed towards herd immunity?

We have almost lived the past four months in the Covid-19 era, but I just want people to understand two things to make them fear less. Like you have HIV as an infection and AIDS as a disease, so when you say that I have the virus, it does not mean that you have the disease. That means you have an infection of coronavirus or Sars-2 coronavirus.

And if you get the disease, that is your lungs are infected and you have fever, then you have Covid-19. So Covid-19 is a disease. The full form is coronavirus disease. That is how it is Covid and it was born in 2019, date of birth is around December. That is how you have Covid-19.

Now, the major issue is everybody who gets infection is detected by a test but what about the remaining people, and is the herd immunity always a group of people who never had the symptoms but had the disease? How does the virus and the disease help?

So, the recent survey from Delhi and maybe from the whole country, they just gave some data. In the beginning, there were about 1 % of the people from the first ICMR study. But the Delhi study showed that above 23% of the people had antibodies. What does it mean? When an infection rate is about 2.6 % in Delhi, out of 2 crore people, just 1.5 lakh had the virus. That means 0.6 % carried the virus, but 23 % of the people had antibodies.

There are merely two meanings to this. One, we missed out on so many, if you had tested one-fourth of Delhi - 50 lakh - impossible in a way, then you would have found this. So one is you missed out on those.

Second, these people got infected without getting symptoms. So they were just exposed. Now those people, those 23%... if you do it after a month, it can go up to 30% or higher. What does it mean?

So, in the first studies which came in Germany, Spain, people said they will issue an immunity certificate/immunity passport, which means you can go from one country in Europe to another country because you have anti-bodies. So are these 23% people in Delhi safe, or stronger than the remaining people to move around?

First you have to understand how the virus is cleared from the body. Somebody gets a virus and it takes five to seven days for the virus to replicate in the throat and other places. During this period, the body's white cells, macrophages, these are the antigen processing cells, dendritic cells. They try and contain the virus. By that time, body senses that yes, there is an intruder in my body, and they start making what is an 'adaptive immunity'.

First comes your 'natural immunity', and then you adapt to it. That 'adaptive immunity' is also of two parts -- one is making antibodies. As everybody knows, you have an antigen, you make an antibody. But certain cells in the body also have the virus, so you make some white cells which go and kill the virus-containing cells.

So adaptive immunity is of two parts -- one antibody which everyone makes and one is to kill these cells which contain this virus. And that is the basis of the new vaccine also. In the Oxford vaccine, the beauty is that it not only makes the antibodies against coronavirus, but also stimulates your T-cells, your white cells to find out where it is.

Now this 23% may say, look we have antibodies. So we have to understand five to six things about these antibodies.

All antibodies are not 'neutralising' antibodies. 'Neutralising' means the virus comes, you neutralise it. So these people can actually get the infection.

Second, these are on small payrolls. After sometime, these antibodies decline. There is a recent paper in the New England Journal that maybe after 90 days these antibodies start declining.

Third, and very important enough, these antibodies are not produced in everybody.

Now, the question arises on 'herd immunity', which means everybody has silently got exposed to the virus, and now the virus cannot infect. But this is a high price. This is not the natural way we will get it because nobody has got exposed.

That Spanish study which came in Lancet said 5% of Spain�s population had coronavirus, 95% is vulnerable. Now, if you wait for the virus to go and infect to get herd immunity, you will pay a high price, deaths will occur. I would say it is very, very risky to allow others to develop 'herd immunity'.

Firstly, we don�t have it and secondly, it will be very costly on the human life that we allow other people to get infected.

Those who got infected are not truly protected. We do not know their antibodies. There is no science behind it, and this is usually used for a vaccine. When you give a vaccine like Polio or you give a vaccine for other infectious diseases, then you expect that 70-80% of the population is already infected and 10-20% of the population may need a vaccine protection. Here, it is reversed.

In corona, nobody was ever exposed. So to get herd immunity, you generally require 80-90 % of the population already infected, and then the vaccines are effective.

To protect people, we have to wait for a very perfect vaccine and for that. Although, and I hope I am wrong, the feeling is that vaccines may not be truly protective in the beginning. What type of antibodies will they make? How much will the level of antibodies be and will these antibodies be protective in all age groups? Whether you need a higher dose in some people who are obese, smokers, alcoholic, diabetic?

I don't think that we will have answers at least a year-and-a-half till vaccine trails are conducted in large populations. In the absence of a very effective vaccine, allowing people to get infected is a very risky proposal.

What is the critical percentage of people in Delhi who must be infected for the rest to get immunity?

There are two ways to get immunity. One is to suffer, and one is to get a vaccine. Which one would you like? To get 'herd immunity', as I said you need almost 80% of the population -- one crore sixty lakh people of Delhi -- to get corona.

We do not want our people to get infected to get immunity which may be transitory, may not be lasting, may bring you more deaths. I don�t think the concept of allowing 'herd immunity' is the best way. Yes, you may develop it silently like the people who got it without suffering. For protection, the vaccine remains our only hope. Till the vaccine comes, don't think that 'herd immunity' is an alternative. It is not.

Is it irresponsible for people to be taking about it?

Let's say Delhi had survey of 21,400 people. Now, those who were 23% positive should be checked every month for let's say six months -- whether their antibodies persist or are they decreasing? Now, the question is 77% who were not infected, are they getting infected? What is the incidence of new infection in a given area? So, say 21,400 people get re-tested, that is one way. The second way is you take new people and then find out every month for a period of three-to-six months what the incidence of new infections is. Having done that, we will have some data, and I hope the Delhi government has initiated some very interesting studies.

This data will give us the backbone for future strategies.

Why is the immunity developed not a static state like it is in some other diseases?

That is a basic question in science. We have memory cells. The body has a physiological memory and then it has a memory of intruders. Now certain viruses also change the memory cells, not allowing things to register. The body has something called 'T cell' (thymus cells) , that is 'T lymphocyte' which fight and 'B lymphocyte' (bone marrow cells) which make antibodies.

Sometimes, the message to 'B lymphocyte' is also abrogated. It is stopped. So 'B cells' don't make antibodies. So, this virus is smart. It changes, it mutates. As of now, we know it makes antibodies. Antibodies may be good for few months � six months -- because the virus is young. So, don't take it for granted.

And some people do not make antibodies. I can say, one in four or five do not make good anti-bodies. When we are doing a plasma (bank), we have a large number of plasma donors who come. When we screen them, out of 100, say 20 or 30 don't have good neutralising antibodies. So, we can't use that plasma to protect somebody. So all plasma is also not protected.

As of now, all these tests are new. So the antigen test has fallacy rate of about 15%, RTPCR sample is not collected properly so you may have 70 to 80% positivity. Antibody tests are new and developing so we have very limited knowledge about their life.

So, till the science is on very sound footing, and it is re-produced in every country, we need to be cautious.

If you have got Covid-19 once, it is possible that you get it again?

You can. Don't take it for granted. Once God has been kind and blessed you to get out of it, don't think you are hero or Hercules, and step out without a mask. In a Korea night club, people got re-infected. There are many such instances where those who recovered got re-infected. Although the infection was mild I must say. And these people are better off than those who never got infected.

Somebody who got infected, recovered and had antibodies, at least has some kind of currency in the body. So the 23% in Delhi are definitely better off than others but that does not mean you can go into open markets and buy anything like immunity.

What is the consolation then?

Consolation for those 23% is that they never suffered. They were blessed. They got the virus silently. They were able to get rid of that virus. They certainly have some immunity in them.

Why is the prevalence so high when there was strict lockdown?

Let me tell you, to get antibody you must have got infected a month ago. So your survey of June 26-July 10 means the guys got it in the month of May. May be even earlier. So, these were times when people were moving, there were migrations. Although there was strict norm on masks, people were not so careful. I guess now they are more educated, more disciplined. So I think the reason they got it was this. But now every month, we must have a survey.

I had a feeling that it was there because I had a feeling by now that x number of people would have got infected. This was my gut feeling because the reproduction rate was quite high. It was 1 to 2.5 to 3. So, if Delhi has so many, at least there would be 20 to 30% who would be infected. Asymptomatic, but never got tested. It is impossible to test everybody. Now, I feel the positivity rate may go up to one-third in a month or you may see 30% or so.

Interestingly in the hospitals, the antibody rate is 8% or so only, not 23%. That is because the doctors and staff used extremely strict measures. So we are going to do it every month to see if it is increasing or not. It means that the staff are not following the orders and rules. The same should be done for citizens. Red zone areas should have repeated screening. Do not think that it is the responsibility of the police. It is the citizen�s responsibility. How can police give you health? How can law give you health? You are responsible for your health. Everyone has to understand this.

Why are positives disproportionately low when you compare it to the high percentage of people exposed?

The reasons I guess is partly because the populations we are doing are all probably asymptomatic. So, I think if you segregate those who come to hospitals versus those who are tested outside, and you will find the difference. Earlier, mainly our positives were related to hospitals, those coming to hospitals or clinics. That may be one reason why we are not picking up people who are positive, but that does not mean that the infection is not spreading. I think that the capacities are increasing. Antibody test will be a better way now for community spread, if at all, than antigen test.