Dr Yashoradha Raje
After significant decline in COVID 19 cases from the start of this year, India is again witnessing an uptick in COVID 19 infections. People should have been more learned and responsible by the time now but are showing carelessness and the results can be seen. Some people thought of one important prevailing concept of Herd Immunity for decline in cases. Was that the reason ? Can Herd Immunity be really achieved or it’s just a hope and Mirage. Amuch talked Sero survey results in the national capital Delhi in Jan -Feb indicated that in at least some of its parts, the disease prevalence had gone beyond 50%.The results had found lot of interest and many experts had been saying for sometime then, that the prevalence of COVID-19 in the population has reached a level where dynamics of ‘Herd Immunity’ can be said to start playing a role.The results were thoughtful but before making any presumptions, I think some important points must be taken into consideration.
First of all, what actually is Herd Immunity is? “Herd Immunity” is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection. Percentage of population which should be immune to achieve Herd Immunity varies with diseasee.g. for measles it was 95% and for polio it was 80%. In case of COVID-19 infection this threshold is still not clear. Also, keeping in mind the highly infectious and fast transmissible nature of the virus with emerging mutations,it will take great toll on lives of people to achieve Herd Immunity without vaccination.
The second thing is the immune response of COVID-19 infection.The protective effects of COVID-19 specific antibodies is not yet known. So how long does the immunity gained through natural infection or through vaccination lasts remains a question.
Third is the serology tests and the pitfalls associated with them. The sero survey tests are ELISA (Enzyme-Linked Immunosorbent Assay) based tests and are designed to minimize cross reactivity, but this cannot be ruled out completely.This depends on specificity and sensitivity of tests which according to Centre for Disease Control and Prevention is 99% and 96% respectively. As per the details of earlier rounds of serosurvey tests in the country, the tests were done using COVID KAVACH ELISA (Enzyme-Linked Immunosorbent Assay) testkits approved by Indian Council for Medical Research.This test kit has 97.9% specificity and 92.3% sensitivity.So, there may be some false positive or false negative results. Also, one more thing is the proportion of antibodies that are neutralizing antibodies which is something one should exactly look for before coming to an answer. Sometimes the serosurveys underestimate building of Herd Immunity. Different studies use different sampling methods and different time frames. This creates a wedge in findings unless properly adjusted when making predictions. Antibodies prevalence data from serosurveys must be interpreted cautiously and correctly.
Last is the homogeneity of the sero prevalence. A last conducted national sero survey in the country has shown less than 10% sero prevalence which is far from achieving Herd Immunity. These results were not consistent with that of Delhi. Higher seroprevalence in one area doesn’t mean that the area is closer to Herd Immunity. Whetherthe figuresthat come are homogeneous or of a particular area needs to be segregated. The results at that time may be pointing to the reason for decline in positive cases in last months but this may also be a false sense of security in the population where vaccine hesitancy and again resurgence in COVID infections is seen across the country. Natural infection does not elicit as strong immune response as vaccination does. Thus, it does not take away the need for vaccination even if the figures are suggestive of natural immunity. In India where the trajectory of epidemic had been different and disease was not as vicious as in other countries, but with resurgence in cases it is high time to curb the virus spread with aggressive vaccination before it grips the nation again.
The results of sero surveys may sometimes seem reassuring but that did not infer that Herd Immunity is achieved. The spectacular decline in the cases across India will not be understood properly without carrying out more serologic surveys at regular intervals. This will clear the ground reality and help us understand the exact situations. The virus is notgone and there is no room for complacence. In the resurging infection, letting our guards down assuming that Herd Immunity will be here shouldn’t be done as it may result in worsening of situation. Yes, maximizing the vaccination uptake and following COVID protocols and precautions can surely help in preventing infection.
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