Seasonal Variation in Reporting of Venous Thromboembolism in a Spontaneous Adverse Event Reporting System: Analysis...

Venous thromboembolism (VTE) is a major cardiovascular disorder associated with high morbidity and mortality. Several epidemiological studies have reported seasonal variations in VTE, with higher rates during winter. However, it remains unclear whether reports of VTE adverse events in spontaneous reporting systems exhibit similar seasonal patterns.

A retrospective pharmacovigilance study was conducted using data from the Japanese Adverse Drug Event Report (JADER) database. Reports of VTE, including deep vein thrombosis and pulmonary embolism, were identified from the adverse event records of the JADER database. Monthly reporting rates were calculated by dividing the number of VTE reports by the total number of adverse event reports. Seasonal variations were evaluated using negative binomial regression models with monthly indicator variables and harmonic seasonal terms. The association between monthly VTE reporting rates and the mean monthly temperature in Tokyo was assessed using Pearson's correlation analysis.

VTE adverse event reports showed a clear seasonal pattern. In the harmonic seasonal model, the estimated peak occurred in December and the nadir in July, with a peak-to-nadir rate ratio of 1.29. The harmonic seasonal model showed a better fit than the monthly indicator model (Akaike information criterion: 1696.8 vs 1712.6). Monthly VTE reporting rates were inversely correlated with mean monthly temperature in Tokyo (Pearson's r = -0.769, p = 0.003).

VTE adverse event reports in the JADER database showed a clear seasonal pattern, with higher and lower reporting rates in winter and summer, respectively. Seasonal and environmental factors may be associated with seasonal variation in VTE reporting. It should be noted, however, that spontaneous reporting data cannot be used to estimate the absolute incidence of VTE or to establish causal relationships between drugs and adverse events. Further studies using complementary data sources are required to confirm these findings.