The Ahmedabad case study is ideal for helping track the herd immunity threshold and assisting in evidence-based policy making
The Ahmedabad Municipal Society (AMC) is to be commended for carrying out seroprevalence studies itself. The last seroprevalence carried out in June 2021, for which the data was published a few days ago, showed that the city’s population has COVID-19 antibodies in 81% of the sampled population. It is one of the highest rates ever. The study only measured antibodies, as there is no simple method to check for cell-mediated immunity or neutralizing antibodies. Some studies in other countries have shown that in addition to antibodies, there is a significant proportion of infected people who will not show antibodies but will have cell-mediated immunity.
Studies carried out in Ahmedabad during previous cycles of serosurveillance have shown that up to 30% of the population with a previous infection, which is not recent, do not present antibodies in these surveys. It is possible that the decline in antibodies and cell-mediated immunity may indicate that an additional 10-15% of people could be protected. Therefore, the proportion of people with some immune protection at the population level in Ahmedabad can be close to 90-95%.
On collective immunity
The Ahmedabad Municipal Corporation is one of the most progressive municipal corporations to use the evidence from serological surveys to plan and decide on the course of action. As part of the COVID-19 control work, in addition to routine testing, tracing and isolation work – which most cities or districts have done – the Ahmedabad municipal company has also invested in carrying out their own periodic serosurveillance. It is time to use the efforts of the city of Ahmedabad as a case study to revisit the discussion on the collective immunity threshold.
At the national level, the National Institute of Epidemiology (NIE) of the Indian Council of Medical Research (ICMR) conducted four rounds of serological surveys; the Scientific and Industrial Research Council (CSIR) also conducted a nationwide serological survey. States including Karnataka, Kerala, Haryana, West Bengal, Odisha, Maharashtra, Gujarat, Delhi, Tamil Nadu and Punjab have carried out serological surveys throughout the city. Sero-surveillance of cities shows the percentage of the population infected with the virus and have antibodies in a specific period of time. The threshold of immunity at the herd or population level is a local geographic phenomenon, in which the population or herd mixes with each other and spreads disease within them. Therefore, it is only reliable to estimate the level of immunity at the city or town level, since there are no significant amounts of migration occurring, changing the dynamics of transmission of the population.
For example, in Gujarat, two neighboring towns, Ahmedabad and Baroda, behave like two distinct communities or herds because there is not much movement and mixing between the inhabitants of the two towns compared to mixing within the same. city. So the two cities can have very different immunity levels – say 80% in one and 20% in the other. Thus, the disease could infect many people, as a greater proportion of susceptible people are present in the city with 20% immunity but do not spread much in the city with higher levels of protection, say immunity of 80%.
Some serologic surveys that combine smaller numbers from more than one study can be misleading. For example, suppose we are doing a joint study between two cities. In this case, we can estimate 55% of people immune, which infers that both cities have a high degree of vulnerability. Although a limited inference means that the disease will spread equally in the two cities, this is not entirely true as one city will have a lower burden in the future, with 80% of people having a form immune response. Each of the large towns or villages must carry out its own serosurveillance to estimate the level of protection and to estimate the proportion of the susceptible population in that town.
A nationally representative sample of serological surveys is useful in providing an overview of the situation. The NIE is to be commended for its efforts and for having conducted four rounds of serological surveys in the past 15 months. The efforts of the Ministry of Health and Family Welfare, the Government of India and the World Health Organization’s TB program on the ground and the technical partnership led by the NIE are worthy of credit. praise for any public health agency; comparable to best-in-class in a rigorous process, and peer-reviewed publications in high-impact journals.
Reviews | Serological surveys underestimate the strengthening of collective immunity
The results of these surveys showed that antibody levels rose from 0.7% in April 2020 to 67% in June 2021. There is also not much difference between the urban-rural and male groups. wife. A higher prevalence is observed in people over 45 years old, in vaccinated people and health workers. But you have to understand that these are all average figures from a study on 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 nationally aggregated data inspires the good work being done by NIE-ICMR scientists and other researchers in the collaboration, there is no reason to rejoice in showing unreasonable optimism. . We must be careful not to take the plunge in declaring that we are approaching the threshold of population immunity at the national level.
We may still have major outbreaks in specific geographic areas in the future, as seen in the UK, Israel and others. There will also be regional and state differences. Urban areas, as sampled by the ICMR study, showed a prevalence of 69% – slightly higher than in rural areas, which had a figure of 65%. Rural areas have a greater spread of the virus similar to urban areas, although rural areas are less congested with weaker social interactions. There is not much overcrowding in rural areas as seen in urban areas. Compared to the Indian urban sample of the ICMR, the HIV status of the city of Ahmedabad is higher at 81%. The time ahead will be a marker for how many cases will be detected in Ahmedabad and how the transmission dynamics and seroprevalence will change in the future. Therefore, the surveys in this city constitute a good case study for the country to examine and plan actions specific to the city.
Ahmedabad had a very bad second wave. A second similar wave was also observed in Delhi, Mumbai, Bengaluru and other major cities. If serosurveillance is done in these towns, we could see very high antibody levels. Urban local communities and state governments should initiate rapid and successive series of serological surveys.
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 made by Karnataka using the forces of the National AIDS Organization field team as well as technical supervision from several academic institutions in Bangalore, including the Public Health Foundation of India, the Indian Institute of Science, the Indian Statistical Institute, etc. These efforts can guide and inform decisions about how open city businesses, educational institutions and markets can be. Wider and faster immunization coverage is an additional and absolute necessity.
The distribution of vaccines, the intensification of the hospital response and the severity of future waves can be understood and addressed by the periodic prevalence of antibodies from serological surveys. Based on serosurveillance studies, such an evidence-based approach will be very useful in the decision-making process while unlocking cities and increasing economic activities. Until supply constraints are completely resolved, this will also help deploy scarce vaccine resources to places that need them most. In addition, serosurveillance is inexpensive – at a price of 500 per test, it will cost 25 lakh to test 5,000 people. This is not a major cost for a large city. We are constantly faced with a problem: Cities do not have mechanisms to use the expertise of evidence generation and analysis of public health professionals, including epidemiologists.
In summary, every major city with a population over 10 lakh should conduct a rapid serosurveillance survey and set up sentinel surveillance to confirm the levels of protection in the existing population and plan. It is time to use information on existing levels of antibodies in the population to guide evidence-based policy making. These efforts will help to understand and mitigate the risk of opening up the economy and society, and to decide on the priority of immunization. More than 40 cities have one million inhabitants and another 300 cities have populations between one lakh and 10 lakh. There is an urgent need to undertake serosurveillance studies to help guide the COVID-19 strategy across rural and urban India.
Dr Dileep Mavalankar is Director, Indian Institute of Public Health, Gandhinagar. Dr Giridhara R. Babu is Professor, Head of Life Course Epidemiology at the Public Health Foundation of India