Explained: What do serosurvey results tell us about neutralising antibodies and ‘herd immunity’?

So, what are these?

Neutralising antibodies, like other antibodies that are created to fight the disease, are nothing but proteins. These are a small subset of the disease-specific antibodies that are generated once an infection has occurred. The neutralising antibodies become special because they have the ability to thwart the entry of the same virus inside human bodies in the future. The other antibodies help in fighting off the virus once the infection has already happened.

Serosurveys, like the ones that have been conducted in Delhi, Mumbai, Pune and other cities, only look for the presence of antibodies in people. Their purpose is to find out whether a person has been infected with the virus or not, and through random testing of people, estimate the extent of spread of the disease, or prevalence, in a population group. Detection of disease-specific antibodies confirms that the person had been infected with the virus in the past.

But a further test needs to be carried out to detect the presence of neutralising antibodies. This is what has happened in the case of Pune. So far, this is the only study that has looked for neutralising antibodies.

The serosurvey was conducted on about 1,650 individuals in five prabhags (each consisting of three to four municipal wards) of Pune. (Express Photo)

The study shows that while a majority of infected people do develop immunity, a significant proportion (15% in this case) who do not become immune even after getting infected. That would mean that they are at risk of reinfection. Although very few cases of reinfection have been documented in the current epidemic, it cannot be ruled out. That is why people who have recovered from the disease are also urged to continue to take protective measures such as practice physical distancing and wearing a mask.

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Is immunity permanent?

It can be against some infections, but in the case of the novel coronavirus, it is not yet known whether the immunity acquired through natural infection lasts for weeks, months or years. The longevity of the immunity against a disease depends on a variety of factors, including the quantity of neutralising antibodies generated by a patient.

In the Pune study, for example, about half the people with neutralising antibodies had generated high levels of these antibodies, while the others had relatively low levels. Those with a high quantity of neutralising antibodies (or specific protein molecules) could be expected to remain immune to the disease for a longer time.

“There are many important caveats to this, and the amount of neutralising antibodies would be just one factor, but in general one can say that higher quantities of neutralising antibodies would be associated with longer-lasting immunity against the disease,” said Vineeta Bal, an immunologist who is a co-author of this study. 📣 Express Explained is now on Telegram

This is because neutralising antibodies, as also other antibodies, decrease with time. The rate of decline varies in different diseases and individuals. So, in the same time that people with low neutralising antibodies see their immunity becoming ineffective, those with higher levels could still have enough to continue to fend off the virus.

In general, those with higher amounts of neutralising antibodies are also better protected. What is difficult to ascertain, however, especially in the case of a new virus like the one behind the present epidemic, is the optimum level above which a person would be definitely immune to the disease.

Also, scientists do not fully understand why certain people do not generate neutralising antibodies. “Their immune system seems to be working fine, but for whatever reason they do not create those specific proteins. No biology that we know of can explain this as of now,” Bal said.

Rapid Antigen test in progress at Baiganwadi, Mumbai. (Express Photo: Amit Chakravarty)

What about vaccine-induced immunity?

As in the case of naturally-acquired immunity, we do not know, as of now, how long the immunity provided by a vaccine would last against the novel coronavirus. Dr Aarti Nagarkar, one of the investigators for the Pune study, said the level of immunity generated by a vaccine is usually expected to be longer and better. That is because the vaccine is designed to trigger a strong immune response with a high fraction of neutralising antibodies.

But since the vaccines currently under production have been under trial for only a few months, the duration of immunity they provide is not known. If the vaccines that are eventually approved for use do not provide long-lasting immunity, they are unlikely to have high acceptance among the general public.

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Is Pune approaching ‘herd immunity’?

That is something that scientists are careful not to assert. The serosurvey was conducted on about 1,650 individuals in five prabhags (each consisting of three to four municipal wards) of the city. About 51% of those who were tested, or about 850, were found to have antibodies. Of those with antibodies, now 85% have been found to have developed immunity.

Disease prevalence in Pune, like any other city, is not uniform. But the findings of the study could be applied well in the small population groups within the prabhags where the serosurvey was conducted. The findings do suggest that the disease prevalence within these population groups had reached such levels that the concept of herd immunity could be playing out.

Health workers collect blood samples and take details during the sero survey at a dispensary in Majnu Ka Tila in New Delhi. (Express Photo: Abhinav Saha)

This is also supported by the fact that in Lohianagar, the prabhag with the highest disease prevalence detected in the serosurvey, the incidence rate had fallen sharply in the last three months.

“I don’t think we can still say that Pune has achieved, or is reaching, herd immunity. But the study is important because it shows that wherever there was high seropositivity, incidence rate has fallen subsequently,” said Dr Gagandeep Kang, a professor at Christian Medical College, Vellore, and one of the co-authors of the study.

This article first appeared in the print edition on November 21, 2020 under the title ‘Which antibodies give immunity?’