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Background: The first serosurvey conducted in Chennai, India in July 2020 reported sudden acute respiratory syndrome coronavirus 2 (SARS-CoV-2) prevalence of 18.4%. The aim of this study was to estimate the seroprevalence in the month of October 2020. Methods: We conducted a survey in 153 streets covering 51 wards and all 15 zones of the city and enrolled from each street 40 individuals >= 10 y of age. We collected 3-5 ml of venous blood and tested for anti-nucleocapsid (N) immunoglobulin (IgG) antibodies using a SARS-CoV-2 IgG assay. We estimated the weighted seroprevalence of SARS-CoV-2 infection and adjusted for test characteristics. Results: Of the 6366 sera tested, 2052 were positive for anti-N IgG antibodies. The weighted seroprevalence after adjusting for test characteristics was 30.1% (95% confidence interval [CI] 24.7 to 36.1). There was wide variation in the seroprevalence between wards, ranging from 11.0% (95% CI 5.6 to 16.4) to 48.1% (95% CI 39.5 to 56.7). Conclusions: The seroprevalence of SARS-CoV-2 infection in Chennai nearly doubled between July and October 2020.

PurposeWe describe here a multicentric community-dwelling cohort of older adults (>60 years of age) established to estimate incidence, study risk factors, healthcare utilisation and economic burden associated with influenza and respiratory syncytial virus (RSV) in India.ParticipantsThe four sites of this cohort are in northern (Ballabgarh), southern (Chennai), eastern (Kolkata) and western (Pune) parts of India. We enrolled 5336 participants across 4220 households and began surveillance in July 2018 for viral respiratory infections with additional participants enrolled annually. Trained field workers collected data about individual-level and household-level risk factors at enrolment and quarterly assessed frailty and grip strength. Trained nurses surveilled weekly to identify acute respiratory infections (ARI) and clinically assessed individuals to diagnose acute lower respiratory infection (ALRI) as per protocol. Nasal and oropharyngeal swabs are collected from all ALRI cases and one-fifth of the other ARI cases for laboratory testing. Cost data of the episode are collected using the WHO approach for estimating the economic burden of seasonal influenza. Handheld tablets with Open Data Kit platform were used for data collection.Findings to dateThe attrition of 352 participants due to migration and deaths was offset by enrolling 680 new entrants in the second year. All four sites reported negligible influenza vaccination uptake (0.1%–0.4%), low health insurance coverage (0.4%–22%) and high tobacco use (19%–52%). Ballabgarh had the highest proportion (54.4%) of households in the richest wealth quintile, but reported high solid fuel use (92%). Frailty levels were highest in Kolkata (11.3%) and lowest in Pune (6.8%). The Chennai cohort had highest self-reported morbidity (90.1%).Future plansThe findings of this cohort will be used to inform prioritisation of strategies for influenza and RSV control for older adults in India. We also plan to conduct epidemiological studies of SARS-CoV-2 using this platform.