Beyond Simultaneous Waves: A Critical Review of COVID-19 Epidemic Synchronization and Different Country Transmission Dynamics

COVID-19 waves peaked at different times in different countries, but the extent to which national epidemicswere synchronized, and how synchrony changed as variants, vaccination and policies changed, is rarelyquantified with several complementary methods. To compare the timing, duration and magnitude ofepidemic waves in ten countries and to evaluate their synchronization using cross-correlation, waveletcoherence, dynamic time warping (DTW), network analysis and time-varying dependence measures, witha random-effects meta-analytic synthesis across countries. A protocol-driven synthesis of a ten-countrydaily dataset (Australia, Brazil, France, Germany, India, Italy, Japan, South Africa, United Kingdom, UnitedStates; 16,751 country-day records, 239 complete weeks, 12 January 2020 to 4 August 2024) was combinedwith a narrative critical review of the methods. Weekly cases per million were used because the dailyseries were reported in weekly batches. Four meta-analyses (k = 10) used restricted maximum likelihoodwith Hartung-Knapp adjustment and prediction intervals. 45 waves were detected. The between-countrystandard deviation of peak timing (τ ) ranged from 3.4 weeks for the Omicron BA.1/BA.2 wave to 9.5 weeksfor the Alpha/Beta/Delta period. Mean maximum cross-correlation was 0.72, with 22 of 45 pairs significantafter surrogate testing and false-discovery-rate control, and 32 pairs peaking at zero lag. Mean waveletcoherence was 0.43-0.46 across bands, but only 12-18% of the usable time-frequency area was significant.Phase synchrony rose from 0.22 before vaccination to 0.59 in the late period (null mean 0.28). Omicron peakswere 3.6 times earlier peaks (95% CI 1.7-7.6), and the case-fatality ratio was 0.17 times the pre-Omicron ratio(95% CI 0.11-0.27). Synchrony with the other nine countries was high for levels (r = 0.84) and moderate forweekly growth (r = 0.43, I² = 78%). Synchrony was partial and episodic rather than constant: strongest forthe Omicron wave and late-period dynamics, weakest during the first year and the Alpha/Beta/Delta period.Spatial proximity explained little of the pairwise synchrony. Case-based timing is affected by changes intesting and reporting, so the findings describe reported epidemics rather than true infection incidence.