A serosurvey template for the whole of India

The Ahmedabad case study is ideal to help track the threshold for herd immunity and aid evidence-based policymaking

Updated - July 24, 2021 02:39 pm IST

Published - July 24, 2021 12:02 am IST

New Delhi: Medics collect blood sample of residents for serological survey to analyse the spread of COVID-19, at a Mohalla Clinic at Geeta Colony in east Delhi, Tuesday, June 30, 2020. The serological test or sero-surveillance study is a part of the new Covid-19 response plan prepared by the Centre for Delhi. (PTI Photo)(PTI30-06-2020_000060B)

New Delhi: Medics collect blood sample of residents for serological survey to analyse the spread of COVID-19, at a Mohalla Clinic at Geeta Colony in east Delhi, Tuesday, June 30, 2020. The serological test or sero-surveillance study is a part of the new Covid-19 response plan prepared by the Centre for Delhi. (PTI Photo)(PTI30-06-2020_000060B)

The Ahmedabad Municipal Corporation (AMC) needs to be complimented for conducting seroprevalence studies on its own. The latest seroprevalence done in June 2021, for which the data was released a few days ago, has shown that the city population has COVID-19 antibodies in 81% of the population sampled. This is one of the highest rates seen anywhere. The study only measured antibodies, as there are no easy methods to check cell-mediated immunity or neutralising antibodies. Some studies in other countries have shown that besides the antibodies, there is a substantial proportion of infected people who will not show antibodies but will have cell-mediated immunity.

Studies in Ahmedabad in earlier rounds of sero-surveillance showed that up to 30% of the population with past infection, which is not recent, do not show antibodies in such surveys. It is possible that the waning of antibodies and cell-mediated immunity together can indicate that an additional 10-15% of persons might be protected. Therefore, the proportion of people with some immunity protection at the population level in Ahmedabad may be close to 90-95%.


On herd immunity

The Ahmedabad Municipal Corporation is one of the most progressive municipal corporations to use serosurveys evidence to plan and decide the course of action. As part of the COVID-19 control work, besides the routine testing, tracing and isolation work — that most cities or districts have done — the Ahmedabad Municipal Corporation also invested in conducting periodic sero-surveillance of its own. It is time to use the efforts of Ahmedabad city as a case study to revisit the discussion on the threshold for herd immunity.

At the national level, the National Institute of Epidemiology (NIE) of the Indian Council of Medical Research (ICMR) has conducted four rounds of serosurvey; the Council of Scientific & Industrial Research (CSIR) too conducted a nationwide serosurvey. The States, including Karnataka, Kerala, Haryana, West Bengal, Odisha, Maharashtra, Gujarat, Delhi, Tamil Nadu and Punjab, have done citywide population-based serosurveys. The sero-surveillance from cities show the percent of the population infected with the virus and has antibodies in a specific time period. The threshold for herd or population-level immunity is a local geographical phenomenon, wherein the population or herd is intermixing with each other and spreading the disease within itself. Hence, it is reliable only to estimate the level of immunity at the city or town level, given that there are no sizeable amounts of migration occurring, changing the population dynamics of transmission.

For example, in Gujarat, two nearby cities,Ahmedabad and Baroda, behave as two separate communities or herds as there is not much travel and intermixing between the people of the two cities compared to mixing within the same city. So, both cities may have very different immunity levels — say 80% in one and 20% in another. So, the disease might infect many people, as a greater proportion of the susceptible are present in the city with 20% immunity but not spread greatly in the city with higher levels of protection, say 80% immunity.


Some serosurveys which combine smaller numbers from multiple studies might be misleading. For example, suppose we do a common study between two cities. In that case, we may estimate 55% of persons having immunity — inferring that both cities have a high degree of vulnerability. While a limited inference will mean that the disease will spread equally in both cities, it is not completely true as one city will have a lower burden in the future, with 80% of people having some form of immune response. Each of the major cities or towns must conduct its own sero-surveillance to estimate the level of protection and estimate the proportion of the susceptible population in that city.

Other surveys

A nationally representative sample of serosurveys is useful to provide a bird’s-eye view of the situation. The NIE should be complimented for its efforts and for conducting four rounds of serosurveys over the last 15 months. The efforts led by the Ministry of Health and Family Welfare, Government of India and the World Health Organization’s tuberculosis programme in the field and technical partnership led by the NIE are praiseworthy for any public health agency; comparable to the best in class in a rigorous process, and peer-reviewed publications in high impact journals.

Opinion | Serosurveys underestimate building of herd immunity

The results from these surveys showed that antibody levels increased from 0.7% in April 2020 to 67% in June 2021. There is also not much of a difference between urban-rural, and male-female groups. A higher prevalence is seen in people above the age of 45, in vaccinated people and health-care workers. But one must understand that all these are average numbers from a study of a total sample of about 29,000 people. As the sample is taken scientifically, we have to assume that it represents the true situation in the country. While the aggregated data at the national level inspires the good work done by the NIE-ICMR scientists and other researchers in the collaboration, it is not a reason to celebrate in enthusing unreasonable optimism. We should be careful and not jump the gun in declaring that we are nearing the threshold of population immunity at the national level.

We can still have major outbreaks in specific geographical regions in the future, as seen in the United Kingdom, Israel, and others. There will also be regional and State-wise differences. Urban areas, as sampled by the ICMR study, showed a prevalence of 69% — slightly higher than in the rural areas, which had a figure of 65%. The rural areas have a greater spread of the virus similar to urban areas, despite rural areas being less congested with lower social interactions. There is not much crowding in the rural areas as seen in urban areas. Compared to the Indian urban sample of the ICMR, the Ahmedabad city seropositivity is higher at 81%. The time ahead will be a marker to know how many cases will be detected in Ahmedabad and how transmission dynamics and seroprevalence will change in the future. Hence, the surveys in this city provide a good case study for the country to review and plan city-specific actions.

Ahmedabad experienced a very bad second wave. A similar second wave was also seen in Delhi, Mumbai, Bengaluru and other big cities. If sero-surveillance is done in these cities, we may see very high levels of antibodies. Urban local bodies and State governments should launch rapid and successive rounds of serosurveys.


Aiding policy making

A more efficient way is to set up sentinel surveillance sites in all public hospitals and estimate the trend in overall seroprevalence. One such effort was done by Karnataka in utilising the strengths of the National Aids Control Organization’s field team along with technical supervision by multiple academic institutions in Bengaluru, including the Public Health Foundation of India, the Indian Institute of Science, the Indian Statistical Institute, etc. These efforts can guide and inform decisions on the extent to which the city’s business, educational institutions, and markets can be opened up. Wider and faster vaccination coverage is an additional and absolute necessity.

The distribution of vaccines, stepping up the hospital response, and severity in future waves can be understood and addressed by the periodic antibody prevalence from serosurveys. Based on sero-surveillance studies, such an evidence-based approach will be very useful in the process of decision making while unlocking cities and increasing economic activities. Till supply constraints are completely resolved, it will also help deploy scarce vaccine resources to the places in most need of them. Also, sero-surveillance does not cost much — at a price of ₹500 per test, it will cost ₹25 lakh to test 5,000 people. This is not a major cost for any major city. We constantly face one problem: cities do not have mechanisms to use evidence generation and analysis expertise from public health professionals including epidemiologists.


In summary, every major city with a population of more than 10 lakh should do a rapid sero-surveillance survey and set up sentinel surveillance to confirm the protection levels in the existing population and plan. It is time to use the information on existing levels of antibodies in the population to guide evidence-based policymaking. These efforts will help understand and mitigate the risk of opening up the economy and society, and deciding the priority for vaccination. Over 40 cities have a million population, and another 300 cities have a population between one lakh to 10 lakh. There is an urgent need to take up sero-surveillance studies to help guide the COVID-19 control strategy throughout rural and urban India.

Dr. Dileep Mavalankar is Director, Indian Institute of Public Health, Gandhinagar. Dr. Giridhara R. Babu is a Professor, Head-Lifecourse Epidemiology at the Public Health Foundation of India

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