key: cord-1020544-eohxf3t6
authors: Chen, Yi-Chu; Chow-In Ko, Patrick; Lee, Wen-Chung; Lien, Wan-Ching
title: Ecological fallacy in COVID-19 epidemiological inference: influenza vaccination rate as an example
date: 2021-03-19
journal: J Formos Med Assoc
DOI: 10.1016/j.jfma.2021.03.011
sha: 49999396cb9c02d2ec5edb7ef6d4795f3aa850bc
doc_id: 1020544
cord_uid: eohxf3t6

nan

To the editor While the effective vaccines against are not yet broadly available, recent studies have suggested that prior vaccination to tuberculosis and influenza would confer some protection against COVID-19. 1, 2 We conducted an ecological study to evaluate the association between the influenza vaccine coverage percentage and cumulative incidence rate, cumulative mortality rate, and case fatality risk in the United States of America (USA).

The data of the cumulative number of COVID-19 cases and deaths in each state from Feb 1 st to Apr 30 th was obtained from the Centers for Disease Control and Prevention (CDC) of the USA. The total population, population density, average temperature and humidity, and the influenza vaccine coverage percentage of each state were collected from the USA Census Bureau, the National Climatic Data Center and the USA CDC, respectively.

The cumulative incidence and mortality of COVID-19 of each state were identified as the numbers of cumulative cases and deaths divided by the population. The case fatality risk was calculated as the ratio of the cumulative number of deaths to the cumulative number of confirmed COVID-19 cases. All the variables were categorized into categorical variables.

Poisson regression analysis was used to assess the adjusted rate ratio (RR) J o u r n a l P r e -p r o o f and 95% confidence intervals (CIs) on the cumulative incidence and the mortality rate. Logistic regression analysis was used to evaluate the adjusted odds ratio (OR) and 95% CIs for the case-fatality risk. Statistical analyses were performed using SAS statistical software (version 9.4; SAS Institute, Cary, NC, USA).

A p-value of less than 0.05 was considered statistically significant. Table 1) .

The results showed that the influenza vaccine coverage percentage over 40% had a potential protective effect against the COVID-19 pandemic in the USA. However, a fallacy in COVID-19 epidemiological inference may arise from an ecological study such as this. The data in this study is at the population level that the aggregate-level correlation and the individual-level correlation may differ greatly or even in opposite signs. 1, 3 Also, receiving the influenza vaccine would be a surrogate for race disparities or socioeconomic status. For example, underserved African-Americans had a lower rate of influenza vaccination. 4 Asians had a relatively higher rate of vaccination 5 

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