A Descriptive Study of Community based Seroprevalence of SARS-CoV2 antibodies among adult population in Mumbai, India using an ELISA based assay

Introduction

As of May 2022, the Indian Ministry of Health and Family Welfare reported over 42 million COVID-19 cases since the beginning of the pandemic of which close to 8 million were reported in the state of Maharashtra. Although there are significant regional variations, India’s COVID-19 disease trend was marked by three distinct waves. The first, second and third waves occurred respectively between July and November 2020, March and May 2021 and December 2021 and February 2022. While reported cases are a good indicator of disease trends, they generally significantly underestimate the true burden of disease due to the high number of asymptomatic and mildly symptomatic cases. In order to overcome this challenge, WHO and others have recommended population- based sero-epidemiological studies to generate data and inform pandemic response policies. These surveys also can give us an estimation of the proportion of the population still susceptible to the infection as it is assumed that antibodies provide immunity. The protective role of antibodies against SARS-CoV-2 is unknown, but these antibodies are usually a reasonable correlate of antiviral immunity, and anti–receptor- binding domain antibody levels correspond to plasma viral neutralizing activity.

In Mumbai, a city of over 12 million residents in Maharashtra, the health authorities of the Municipal Corporation of Greater Mumbai (MCGM) have conducted regular SARS CoV-2 serosurveys since the beginning of the pandemic. The first survey, conducted in July 2020, found a prevalence of 57% in slums and 16% in non-slums areas. This first study showed the significant difference in virus transmission between slums-where over half of Mumbai dwellers live - and non-slum areas [1]. The second serosurvey, conducted in August, 2020, estimated the prevalence at around 44.9% in slums and 17.5% in non-slums. The third round, conducted in March 2021, showed a further reduction in the gap between slums and non-slum areas with an estimated seroprevalence of respectively 41.6% and 28.5%. An additional survey was conducted in the paediatric population in June 2021, immediately after the second wave. The study estimated a seroprevalence of 51.42%. The COVID-19 vaccination campaign in India started in January 20221 for priority groups and opened to all adults by May 2021. As of May 2022, 72% of the population is partially vaccinated and 62% is fully vaccinated.

Results

A total of 8,701 participants were recruited between 12th August, 2021 and 8th September, 2021 in municipal dispensaries and general practitioners’ practices in all 24 wards of Mumbai. After data cleaning and results consolidation, 8,674 people were included in the analysis.

More than half of study participants were recruited in municipal dispensaries (4,608 of 8,674, 53%) while the rest were recruited in private practices. Over 20% (1,799 of 8,674) of participants reported living in a kacca house - these are generally made of easily available material like mud, straw or stones and are typically owned or occupied by the lowest socio-economic classes. The remaining participants lived in pucca houses made of steel, cement or bricks that form a concrete structure. The majority of participants reported having a separate toilet (4,933 of 8,674, 56%) while the rest shared a common toilet with their building or neighbors.

Most study participants were partially or fully vaccinated (5,660 of 8,674, 65%; 3,014 unvaccinated, 35%). Among vaccinated individuals, 95% reported being vaccinated with Covishield and 4% with Covaxin. Among unvaccinated individuals, the most common reasons for not being vaccinated were unavailability of COVID-19 vaccine (51%), fear of side effects (14%) and technical difficulties to book a vaccine slot (11%). Participant recruitment occurred in healthcare facilities by an external team and our sample included a large number of healthcare workers. A total of 1,867 (22%) healthcare workers were included in the study.

The prevalence of SARS CoV-2 antibodies in the population was 86.6% with, for the first time, only a marginal difference between slums and non-slum residents. Using residence type based on the recruitment location (municipal dispensaries for slum residents and general practitioners for non-slum residents), the difference between slum and non- slum is not significant (87.0% vs. 86.2%; RR: 1.009, CI: 0.992- 1.026). The seroprevalence was also higher in women than in men (88.3% vs. 85.1%; CI 95%: 0.94-0.97). The younger (18-44 years old) and older population (60 years and above) had a lower seroprevalence than the middle-aged group (45- 59 years old). The overall COVID-19 vaccination coverage was 65% across the city. It was similar across gender but it was lower for the younger age group (59%) than for the “45- 59” and “60 and above” age group (at respectively 72% and 70%). Major gaps in vaccination coverage were observed by location. Slum dwellers had vaccination coverage of 58% while it was 74% for non-slum dwellers. The vaccination coverage was 69% for Hindus, 27% for Muslims, 74% for Christians, and 88% for others.

Discussion

This study represents the first serosurvey conducted in Mumbai after the second wave and the launch of the vaccination campaign in the general population. It shows a high level of seropositivity in the community. In particular, this is the first survey to show a similar seroprevalence in slum and non-slum population. Serosurveys conducted at the same period in Delhi showed a similar seropositivity of 89.5% [2].

Due to recruitment biases, we over-recruited health care workers in the study. Our study shows a similar prevalence rate in the health-care worker and non-health care worker population. While they are generally considered to be more at risk, this could be due to higher vaccination rates in this population or better protection against the virus through effective and regular use of personal protective equipment. The seroprevalence is higher in women; this was already the case in previous studies and requires further investigation. Some possibilities would be that women tend to stay indoors most times and this represents a higher infection risk. Our findings on vaccination coverage highlight major inequalities in vaccination. The proportion of vaccination in non-slum areas is significantly higher than in slum areas. This could be due to the difficulties to get an appointment using the government application, Co-WIN that initially was the only medium to access the vaccination. In addition, research on routine immunisation in India (Ministry of Health and Family Welfare, 2017) [3] shows the poorest quintile has a lower vaccination rate. This is further enhanced for the COVID-19 vaccination campaign with a low 27% coverage in the Muslim community, well below the average of 65%.

Conclusion

Our findings show the high level of virus transmission and uptake of the vaccination campaign has significantly increased the levels of seropositivity in Mumbai. While the disparities between slums and non-slum areas have reduced, this research has highlighted some pockets of lower seroprevalence that should be the target of public health interventions. This research provides insightful understanding of the potential impact of future waves as well as transmission dynamics in urban settings.

Conflicts of Interest

We declare no conflict of interest

Acknowledgement

Ms. Sofia Imad, Ms. Meenaz Munshi and Ms. Nikita Kwatra from IDFC Foundation for training of the field workers and execution of the field survey. Acknowledgment of technical help: Mr. Kishore Kumar and Mr. Purushottam Gowda from ConcilX Digital.

Funding

This research was funded by ATE Chandra Foundation and ACT Grants.