Explained: Are we approaching the stage of herd immunity?
Herd immunity is a stage of an epidemic in which some members of a population group remain protected from infection because a majority of those around them have already developed immunity, either through vaccination or because they have been infected earlier.
A blood sample is collected during the serosurvey in Delhi. (Express photo: Abhinav Saha/File photo)
A recent serological survey in Delhi found the presence of coronavirus-specific antibodies in about 23% of the samples tested. The results of that survey are being interpreted to suggest that about 46 lakh people in Delhi could so far have been infected with the novel coronavirus, and that “herd immunity” could be approaching. Even Chief Minister Arvind Kejriwal made the suggestion on Friday.
Scientists, however, caution against drawing such broad conclusions. The serological surveys were carried out to answer a very specific question. Any talk of “herd immunity” at this stage is not only premature but also misplaced.
What was the serological survey about?
The serological survey was meant to detect whether the person being tested had developed antibodies against the coronavirus. The antibodies are proteins produced by the immune system to fight external organisms like viruses that try to enter the body. These are produced only after the infection has happened, and are specific to the attacking virus or bacterium. The presence of antibodies, therefore, is an indication that an infection by that particular virus or bacterium has already occurred. Subsequent attempts to infect the body can be thwarted by these antibodies.
Vaccines work in a similar manner. They inject harmless doses of a virus or a bacterium inside the human body to trigger the production of antibodies by the immune system. These antibodies can then fight off an actual attack by those viruses or bacteria.
In the context of current pandemic, the serological survey was carried out to assess how widespread Covid-19 could have become. Since it is not possible to test everyone, it is not clear how many people in the population are infected, especially since most of the patients do not show any symptoms of the disease. Detecting antibodies in random sets of people is an indirect way of estimating the extent of disease spread in a community.
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A recent serological survey in Delhi found the presence of coronavirus-specific antibodies in about 23% of the samples. (Express Photo: Abhinav Saha)
What were the results?
The survey found coronavirus-specific antibodies in about 23% of the roughly 21,000 people who were tested. This means that these many people had, at some point or the other, had been infected. Since random people were tested, it indicated that the spread of the disease was much wider than what diagnostic tests suggest. In Delhi, about 14% of those who have been tested for the virus have turned out positive.
Information about the extent of spread is very important for authorities to make decisions and plan containment measures.
Scientists say this serological survey, and similar ones in other parts of the country, are quite useful for this limited purpose. But they warn against any interpretation further than this. Suggestions that all those with antibodies are now protected against the disease, and that “herd immunity” stage was nearby, are particularly problematic, they insist.
Do antibodies ensure Immunity?
The mere presence of antibodies does not mean that the person is protected against the disease. These are two different things, scientists say. What is also important is the amount of antibodies present, and whether it also includes what are known as “neutralising antibodies”. These are the ones that actually fight the disease. Serological surveys are not designed to assess either the quantity of antibodies or detect the presence of neutralising antibodies.
“The serological tests ask a very basic question that has a Yes/No answer. Are there antibodies present? That’s it. Now, this is useful to know whether the person had been infected with the disease or not. The presence of antibodies tells us that. Protection against the disease, however, is a very different question. The serological tests do not provide us enough information on that,” said Satyajit Rath, a pathologist and immunologist now associated with Indian Institute of Science Education and Research (IISER) in Pune.
While there is no documented evidence of re-infection in the current pandemic, Shahid Jameel, a virologist, said there were studies that indicated the “neutralising antibodies” could lose their effect after four months. “It is very well possible that a person has antibodies against Covid-19 disease but no protection. These two are not the same thing, and therefore we need to be cautious when we interpret scientific findings in this manner,” Jameel said.
L S Shashidhara, a biologist, also at IISER Pune, said the coronavirus was still evolving and could undergo several mutations, and therefore it was too early to talk about permanent protection, and certainly not from the results from an antibody test.
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Blood samples collected for the serosurvey in New Delhi on June 30, 2020. (Express Photo: Abhinav Saha)
What, then is herd immunity?
The suggestion that herd immunity could be achieved in the near future was even more problematic, the scientists said.
Herd immunity is a stage of an epidemic in which some members of a population group remain protected from infection because a majority of those around them have already developed immunity, either through vaccination or because they have been infected earlier. So, everyone in the population group does not need to get infected before the epidemic is over. Once a certain proportion of population gets infected, and thus builds immunity against the disease, the epidemic begins to slow down and eventually stop.
What should that proportion be?
The problem is no one clearly knows what percentage of the population needs to be infected before herd immunity kicks in. It is different for different diseases, and different population groups. In general, herd immunity is unlikely to happen before at least half the population is infected.
In the case of measles, for example, herd immunity is reached only when 85% to 90% of the population attains immunity. In some other diseases, the threshold could be lower. For Covid-19, different studies have suggested that between 55-70% of the population would need to be infected before herd immunity would develop.
“Herd immunity is mentioned in very loose terms these days. And it is also badly misunderstood,” Jameel said. “This concept can be applied in very specific situations only. For example, herd immunity would apply only in closed population groups, those that are cut off from neighbouring societies. Thus, talk of herd immunity in Delhi is pointless if there is a free movement of people in and out of the city. Also, there is no uniform infection level in the population over which herd immunity would come in. Even in a city like Delhi, different localities and neighbourhoods would reach herd immunity at different levels of infection, if they isolate themselves from others.”
Besides, it is extremely difficult to determine the level of disease spread necessary for herd immunity when the epidemic is still raging. “Every parameter in this calculation is dynamic and evolving right now. Only after the epidemic is over, can we reliably estimate at what point did herd immunity take over,” Jameel said.
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