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

Abstract

COVID-19 waves peaked at different times in different countries, but the extent to which national epidemics
were synchronized, and how synchrony changed as variants, vaccination and policies changed, is rarely
quantified with several complementary methods. To compare the timing, duration and magnitude of
epidemic waves in ten countries and to evaluate their synchronization using cross-correlation, wavelet
coherence, dynamic time warping (DTW), network analysis and time-varying dependence measures, with
a random-effects meta-analytic synthesis across countries. A protocol-driven synthesis of a ten-country
daily dataset (Australia, Brazil, France, Germany, India, Italy, Japan, South Africa, United Kingdom, United
States; 16,751 country-day records, 239 complete weeks, 12 January 2020 to 4 August 2024) was combined
with a narrative critical review of the methods. Weekly cases per million were used because the daily
series were reported in weekly batches. Four meta-analyses (k = 10) used restricted maximum likelihood
with Hartung-Knapp adjustment and prediction intervals. 45 waves were detected. The between-country
standard deviation of peak timing (τ ) ranged from 3.4 weeks for the Omicron BA.1/BA.2 wave to 9.5 weeks
for the Alpha/Beta/Delta period. Mean maximum cross-correlation was 0.72, with 22 of 45 pairs significant
after surrogate testing and false-discovery-rate control, and 32 pairs peaking at zero lag. Mean wavelet
coherence 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 peaks
were 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 for
weekly growth (r = 0.43, I² = 78%). Synchrony was partial and episodic rather than constant: strongest for
the 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 in
testing and reporting, so the findings describe reported epidemics rather than true infection incidence.