A Community Based Serosurvey for SARS-CoV2 among Population in an Assembly Constituency of Urban Bangalore, Karnataka

Introduction

India reported the first case of COVID-19 on January 30, 2020. Since then, the pandemic has spread throughout the country.¹ There has been a steady increase in the number of cases reported and tests conducted per day. The first wave peaked in early September with 97,000 plus cases on a single day, on 12^th September.² Subsequently, we have seen a fall in the number of cases reported over the next few months. As of December 1^st, 2020, the number of new cases reported were 39,111, the cumulative laboratory-confirmed cases were 10,95,288 and the test positivity rate was 3.6. Wide variation existed between the states and between the districts.³

Knowledge about the spread of infection is essential to plan prevention and control strategies for COVID-19. Community-based seroepidemiological studies are required to measure the infection in an area. ICMR-NIV,Pune had conducted the first and second cross-sectional seroprevalence survey for SARS-CoV-2 infection among the adults in the general population and sociodemographic factors associated with SARS-CoV-2 infection in India during May and August, 2020.^4,5 The Institute had developed a completely indigenous IgG ELISA test for antibody detection for SARS-CoV-2 and was found to have sensitivity and specificity of 98.7% and 100% respectively.⁶ There are many kits now available in the market for IgG antibody estimation.  

Many studies are in the pipeline, especially in big cities to know the level of infection in the community to plan for further preventive and control measures. In this regard, the present study was conducted with the objectives to assess the seropositivity to SARS-CoV2 among the population in an urban community and to describe the demographic profile of the subjects of Urban Bangalore.

Materials and Methods

A descriptive, cross-sectional study was conducted among the population under an assembly constituency in Urban Bangalore in December 2020. A total of 118 subjects were selected due to the difficulty, constraints and feasibility, through convenient sampling. The inclusion criteria considered were that the subject should be more than 18 years of age, must be a resident of the area and willing to give informed consent. If the subject was less than 18, an assent form was obtained from the participant and informed consent from their parents were taken. The subjects who had already participated in similar previous studies and severely ill were excluded.

A pre-tested, semi-structured questionnaire on the demographic characteristics of the population was used for the collection of data. After taking the informed consent, 5ml blood was collected by a trained personal under aseptic precautions and serum was analysed for Total Immune antibodies (IgM, IgG and IgA) and IgG in an ICMR approved laboratory in Bangalore. The test done to measure reactivity was USA FDA approved, CE mark chemiluminescence immunoassay. Results are semi-quantitative and are expressed as qualitative statements (reactive/non-reactive). A Cut-off-Index (COI) of ≥ 1 is considered reactive and <1 as nonreactive.

Test inference: A reactive test result indicates that IgG antibodies to SARS-CoV2- were detected and they are likely to have or previously had COVID-19, following which they have developed an antibody response to the virus. A non-reactive result means that the IgG antibodies to the virus that causes COVID-19 were not found in the sample

Data was entered in MS Excel and analysed using Stata 12, openepi software. Normality of test was done using Shapiro wilk test. Results were expressed by frequencies, proportions with 95% Confidence interval (CI), median, Inter Quartile Range (IQR).

Results

Among the 118 subjects, 61 (54.46%, 95% CI: 42.69- 60.62) subjects were reactive (specific IgG for COVID-19) and 06 (5.1%, 95% CI: 2.08-10.27) subjects had TIA, which may be reactive for COVID-19. Overall, 67 (56.78%, 95% CI: 47.73-65.5) subjects were reactive to IgG and TIA as described in Table 1. 

The median IgG value among the reactive subjects was 5.42 [IQR: 3.05-7.92]. About 44.7% (95% CI: 33.21- 56.78) of the subjects were in the age group of 21-40 years as described in Graph-I. Thirty five (57.3% 95% CI: 44.76-69.3) subjects had IgG value > 5 in the blood and the maximum detectable IgG level was 12.3. The median IgG was 4.99 [IQR: 3.12- 5.73] in the age group of 61-80 years. 

Thirty four (50.75%) of the reactive subjects were males and 33 (49.25%) were females as described in Table 2. The median IgG value in females was 4.82 [IQR: 2.97- 7.01] and 5.78 [IQR: 4.79:7.70] was the median IgG value in males. The maximum IgG value observed in males was 11.7 and females was 12.3.

Twelve subjects had tested COVID-19 positive through RT-PCR test as described in Table 3. Four (33.3%) positive subjects were in the age group of 51-60 years. Eight (66.6%) subjects were males and 4 (33.4%) were females.

Discussion

India was the fifth country in the world to isolate COVID-19 virus strain in March 2020.⁷ Over time, the diagnosis of COVID-19 through real-time reverse transcription-polymerase chain reaction (RT-qPCR) and Rapid Antigen Test (RAT) was scaled up. The case reporting is based on the testing of suspect individuals by RTPCR. The testing criteria will miss asymptomatic and mild infections. Serosurvey at the national and state level to measure exposure to infection was conducted as early as May 2020.

Population- weighted seroprevalence was 0.73% [95% confidence interval (CI): 0.34-1.13] during the first round of ICMR survey done during May-June, 2020. The weighted and adjusted seroprevalence was 7.1% (6.2–8.2) in the second serosurvey done between August to September, 2020.^4,5 Seroprevalence was similar across age groups, sexes, and occupations. Seroprevalence was highest in urban slum areas followed by urban non-slum and rural areas.⁵

Over half the surveyed population had reactive antibody titres in the present study, which was quite high and could be due to the non-randomization in the selection of the study subjects. Nevertheless, the subjects were of the high-risk group or those who went outside home or travelled. There was significant protection seen in them.

Another survey done from June to August 2020 in 20 districts included the proportion of people who had COVID-19 infection recently and those currently having active SARS-CoV-2 infection. Adjusted seroprevalence across Karnataka was 46.7% (95% CI: 43.3-50.0), including 44.1% (95% CI: 40.0-48.2) in rural and 53.8% (95% CI: 48.4-59.2) in urban areas, which were similar to the observations of the present study. Plasma was separated and tested for IgG antibodies to the receptorbinding domain (RBD) of the SARS-CoV-2 virus using an ELISA test developed by Translational Health Science and Technology Institute, India.⁸ The values reported were similar to the observations in the present study.

According to the Government of Karnataka, the serosurvey estimated that around 27.3% of the state’s population have already had the infection as of 16 September, 2020. The survey included people from low to high-risk population. The seroprevalence of SARSCoV-2 IgG was 16.4% (95% CI: 15.1 - 17.7). The same survey found that 22% in Bengaluru had developed antibodies against the virus.⁹ This is lower than the estimates observed in the present study.

The difference in serosurvey values may also be due to different kits used in different studies. The serosurvey results inform about the direction of the pandemic and the control measures that can be suggested then.

Limitation: The population is too small for generalization 

Conclusion

A little more than fifty percent of the subjects had seroprotectivity to COVID-19. A larger study covering all the wards and involving all the sections of the society in Bangalore is suggested for generalization.  

Acknowledgements

The authors would like to thank the medical officers, ASHA works, swab collectors, lab technicians and all the UPHC staff of the study area for their invaluable support in the conduct of the study. The authors would also like to thank the ICMR approved laboratory for blood analysis.

Conflict of Interest

Nil.

Source of funding

Nil.