Serological survey in Delhi: How far is herd immunity from coronavirus? All you need to know

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July 27, 2020 3:06 PM

Testing for the presence of antibodies in random sets of people is taken to be an indirect method of determining how widespread the virus is in the community.

Antibodies are produced only when the infection has occurred.Antibodies are produced only when the infection has occurred.

Coronavirus immunity: As the number of cases in India rapidly increase, the health authorities are conducting serological surveys among certain people across the country. According to a recent serological survey conducted in Delhi, coronavirus-specific antibodies were found to be present in 23% of the samples that were tested, a report in the IE said. An interpretation of these results is being taken to suggest that so far in Delhi, around 46 lakh people could have been infected by the novel coronavirus, as a result of which they developed antibodies against it, and that the country was approaching a stage of herd immunity. Such a suggestion was also put forth by Chief Minister Arvind Kejriwal on Friday, the report added.

However, the report said that scientists have warned against such broad conclusions, since the sero surveys had been conducted for the purpose of answering some very specific questions. Talking about herd immunity at this stage is premature and misplaced, the report added.

Serological surveys: Their purpose

The report stated that the serological surveys were conducted to detect whether someone had developed the antibodies against coronavirus. Antibodies are proteins that the immune system produces in order to fight viruses and other external organisms that try to enter the human body. Antibodies are produced only when the infection has occurred, and they remain specific to the virus or the bacterium they are fighting. Hence, antibodies being present in the body indicates that the infection by that particular organism has already taken place. The report stated that these antibodies could thwart further attempts of these viruses or bacteria to infect the body.

The report stated that vaccines also work in a similar way, injecting harmless doses of the virus into the body in order to trigger the body’s immune response of producing antibodies. These antibodies could then fight against the virus during an actual attack.

As far as the current pandemic is concerned, in order to assess the spread of COVID-19, a serological survey was conducted. However, since it is not possible to test everyone in the country, the report states that so far it is unclear how many people of the population were actually infected, especially since a lot of them are asymptomatic. Testing for the presence of antibodies in random sets of people is taken to be an indirect method of determining how widespread the virus is in the community.

Results of the serological survey

It was found that in the about 21,000 people who were tested, antibodies specific to coronavirus were present in around 23%. This indicates that at some point, these many people had been infected by COVID-19. Moreover, since the tested people were chosen randomly, the spread of the disease seems to be much wider than as suggested by the COVID-19 diagnostic tests. Delhi saw about a 14% positivity rate among those who have been diagnosed for the infection, the report added.

It is important for the authorities to understand how widespread the infection is in order to take appropriate decisions and devise proper containment measures.

The report cited scientists as saying that for this limited purpose, serological surveys conducted in Delhi and in other parts of India are useful, but they should not be interpreted further than this. They said that suggesting that people who had antibodies were protected against the disease and that the stage of herd immunity was near was problematic.

Is immunity ensured by the presence of antibodies

The report stated that only antibodies being present does not necessarily translate to the person having protection against the disease. According to scientists, these are two different things. It is also important to understand the amount of antibodies present and whether “neutralising antibodies” are included in them or not, since these are the antibodies that actually fight the disease. Serological surveys, the report stated, neither determine the amount of antibodies, nor do they detect whether neutralising antibodies are present or not.

A pathologist and immunologist associated with the Pune-based Indian Institute of Science Education and Research (IISER) Satyajit Rath was quoted by the report as saying that the serological tests only ask a very basic question about the presence of antibodies and it only has a yes or no answer. He added that while they inform whether a person had been infected or not, they do not provide enough information to determine whether a previously infected person is protected against the disease in the future.

Virologist Shahid Jameel was quoted as saying that studies had indicated that neutralising antibodies could become ineffective after four months, even as there was no documented evidence of re-infection of COVID-19. He said that it was possible for a person to have antibodies but no protection, since the two are not the same thing. He added that there was a need to remain cautious when scientific findings were interpreted in this manner.

Biologist at IISER Pune L S Shashidhara said that the novel coronavirus was an evolving one and it could undergo mutations. This means that it was too early to discuss about there being a permanent protection, especially from the results of a serological survey.

Herd immunity explained

Scientists said that what was even more problematic was the suggestion that herd immunity was possible to be achieved in the near future, the report said.

When during an epidemic, there comes a stage where some members of the population are protected against the infection because most of the people around them are immune against it either by vaccine or by an earlier infection, it is known as herd immunity. Therefore, before the epidemic ends, not everyone in the population group needs to be infected by the disease, since once a part of the population catches the infection and subsequently builds immunity against the disease, it leads to the slowing down and an eventual end of the epidemic.

When can herd immunity be developed?

The problem with herd immunity is, there is no clear knowledge about the percentage of the population that needs to be infected before that stage can be reached. Herd immunity varies according to diseases and the population groups. Generally, the report stated, it is unlikely for herd immunity to kick in before at least 50% of the population is infected.

As an example, herd immunity against measles kicks in only once 85 to 90% of the population becomes immune to it. This threshold could be lower for other diseases. According to different studies, herd immunity against COVID-19 would kick in once 55-70% of the population is infected.

The report quoted Jameel as saying that herd immunity is taken very loosely these days and it is also misunderstood. He added the concept could only be applied in specific situations only. Herd immunity, he said, would be applicable in closed population groups only which are cut off from their neighbouring societies. Therefore, talking of herd immunity in Delhi would be pointless since the city has free movement of people in and outside. Apart from that, there is also no uniform level of infection after which the population would attain herd immunity. In Delhi itself, he said, different neighbourhoods and localities would reach herd immunity at different stages if they isolate from each other. Moreover, while the epidemic is still raging, determining the level of disease spread which can lead to herd immunity is very difficult, since every parameter is dynamic and evolving. He said that it would only be after the epidemic ends that scientists would be able to reliably estimate the point at which herd immunity took over.

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