Exploring the Big Data Paradox for various estimands using vaccination data from the global COVID-19 Trends and Impact Survey (CTIS).

Selection bias poses a substantial challenge to valid statistical inference in nonprobability samples. This study compared estimates of the first-dose COVID-19 vaccination rates among Indian adults in 2021 from a large nonprobability sample, the COVID-19 Trends and Impact Survey (CTIS), and a small probability survey, the Center for Voting Options and Trends in Election Research (CVoter), against national benchmark data from the COVID Vaccine Intelligence Network. Notably, CTIS exhibits a larger estimation error on average (0.37) compared to CVoter (0.14). Additionally, we explored the accuracy (regarding mean squared error) of CTIS in estimating successive differences (over time) and subgroup differences (for females versus males) in mean vaccine uptakes. Compared to the overall vaccination rates, targeting these alternative estimands comparing differences or relative differences in two means increased the effective sample size. These results suggest that the Big Data Paradox can manifest in countries beyond the United States and may not apply equally to every estimand of interest.

COVID mortality in India: National survey data and health facility deaths.

India's national COVID death totals remain undetermined. Using an independent nationally representative survey of 0.14 million (M) adults, we compared COVID mortality during the 2020 and 2021 viral waves to expected all-cause mortality. COVID constituted 29% (95%CI 28-31%) of deaths from June 2020-July 2021, corresponding to 3.2M (3.1-3.4) deaths, of which 2.7M (2.6-2.9) occurred in April-July 2021 (when COVID doubled all-cause mortality). A sub-survey of 57,000 adults showed similar temporal increases in mortality with COVID and non-COVID deaths peaking similarly. Two government data sources found that, when compared to pre-pandemic periods, all-cause mortality was 27% (23-32%) higher in 0.2M health facilities and 26% (21-31%) higher in civil registration deaths in ten states; both increases occurred mostly in 2021. The analyses find that India's cumulative COVID deaths by September 2021 were 6-7 times higher than reported officially.