Updated: November 21, 2020 12:04:47 pm
Health worker collecting a sample during the Covid 19 test serosurvey at Darya Ganj Ramjas school on Thursday in New Delhi. Express Photo by Amit Mehra August 06, 2020
A new study in Pune has revealed that nearly 85% of people found to be infected with the new coronavirus in a serosurvey, conducted previously, had developed neutralizing antibodies. In other words, these people had developed immunity against the disease.
It is sometimes thought that everyone who is infected with a virus that causes a disease and recovers becomes immune to the disease, because they develop antibodies against it. But that is not the case. Although the creation of antibodies is necessary for the recovery process, it does not guarantee immunity against a future attack from the same virus. Immunity comes from what are known as “neutralizing” or “protective” antibodies.
So what are these?
Neutralizing antibodies, like other antibodies that are created to fight disease, are nothing more than proteins. It is a small subset of disease-specific antibodies that are generated once an infection occurs. Neutralizing antibodies become special because they have the ability to thwart the entry of the same virus into human bodies in the future. The other antibodies help fight the virus after infection has already occurred.
Serological surveys, such as those conducted in Delhi, Mumbai, Pune, and other cities, only look for the presence of antibodies in people. Its purpose is to find out if a person has been infected with the virus or not, and through random tests of people, to estimate the degree of spread of the disease, or the prevalence, in a population group. Detection of disease-specific antibodies confirms that the person had been infected with the virus in the past.
But an additional test is necessary to detect the presence of neutralizing antibodies. This is what has happened in the case of Pune. So far, this is the only study that has looked for neutralizing antibodies.
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The serosurvey was carried out on approximately 1,650 people in five prabhags (each consisting of three or four municipal districts) in Pune. (Quick)
The study shows that while the majority of infected people develop immunity, a significant proportion (15% in this case) do not become immune even after becoming infected. That would mean 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 taking protective measures such as practicing physical distancing and wearing a mask.
Is the immunity permanent?
It may be against some infections, but in the case of the new coronavirus, it is not yet known whether the immunity acquired by natural infection lasts for weeks, months or years. The longevity of immunity against a disease depends on a variety of factors, including the amount of neutralizing antibodies generated by a patient.
In the Pune study, for example, about half of the people with neutralizing antibodies had generated high levels of these antibodies, while the others had relatively low levels. Those with a large amount of neutralizing antibodies (or specific protein molecules) could be expected to remain immune to disease for longer.
“There are many important caveats to this, and the amount of neutralizing antibodies would be just one factor, but in general it can be said that higher amounts of neutralizing antibodies would be associated with a longer-lasting immunity against the disease,” Vineeta Bal said. an immunologist who is a co-author of this study. 📣 Express Explained is now on Telegram
This is because neutralizing antibodies, as well as other antibodies, decrease over time. The rate of decline varies in different diseases and individuals. So, at the same time that people with low neutralizing antibodies see their immunity become ineffective, those with higher levels may still have enough to continue defending against the virus.
In general, those with higher amounts of neutralizing antibodies are also better protected. However, what is difficult to determine, especially in the case of a new virus like the one behind the current epidemic, is the optimal level above which a person would be definitely immune to the disease.
Also, scientists don’t fully understand why certain people don’t make neutralizing antibodies. “Their immune system seems to be working fine, but for some reason they don’t create those specific proteins. No biology that we know of can explain this as of now, ”said Bal.
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Rapid antigen test underway in Baiganwadi, Mumbai (Express photo by Amit Chakravarty)
What about vaccine-induced immunity?
As in the case of naturally acquired immunity, we do not know, until now, how long the immunity provided by a vaccine against the new coronavirus would last. Dr Aarti Nagarkar, one of the researchers on the Pune study, said that the level of immunity generated by a vaccine is generally expected to be longer and better. This is because the vaccine is designed to trigger a strong immune response with a high fraction of neutralizing antibodies.
But since the vaccines currently being produced have been in testing for only a few months, the duration of the immunity they provide is unknown. If the vaccines that are ultimately approved for use do not provide long-lasting immunity, they are unlikely to be widely accepted by the general public.
Is Pune close to “herd immunity”?
That is something that scientists are careful not to state. The serosurvey was carried out on approximately 1,650 people in five prabhags (each consisting of three to four municipal districts) of the city. About 51% of those who were tested, or about 850, had antibodies. Of those with antibodies, it has now been found that 85% have developed immunity.
The prevalence of diseases in Pune, as in any other city, is not uniform. But the study’s findings might apply well to small population groups within the prabhags where the serosurvey was conducted. The findings suggest that the prevalence of the disease within these population groups had reached levels such that the concept of herd immunity might be working.
Health workers collect blood samples and take details during serosurvey at a dispensary in Majnu Ka Tila in New Delhi (Express photo by Abhinav Saha)
This is also supported by the fact that in Lohianagar, the prabhag with the highest prevalence of disease detected in the serosurvey, the incidence rate had dropped dramatically in the past three months.
“I don’t think we can say yet that Pune has achieved, or is achieving, herd immunity. But the study is important because it shows that wherever there was high seropositivity, the incidence rate has subsequently decreased, ”said Dr. Gagandeep Kang, professor at Christian Medical College, Vellore, and one of the study’s co-authors.
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