Vinceti M, Filippini T, Rothman KJ, Di Federico S, Orsini N. The association between first and second wave COVID-19 mortality in Italy. BMC Public Health. 2021 Nov 11;21(1):2069.

BACKGROUND: The relation between the magnitude of successive waves of the COVID-19 outbreak within the same communities could be useful in predicting the scope of new outbreaks.

METHODS: We investigated the extent to which COVID-19 mortality in Italy during the second wave was related to first wave mortality within the same provinces. We compared data on province-specific COVID-19 2020 mortality in two time periods, corresponding to the first wave (February 24-June 30, 2020) and to the second wave (September 1-December 31, 2020), using cubic spline regression.

RESULTS: For provinces with the lowest crude mortality rate in the first wave (February-June), i.e. < 22 cases/100,000/month, mortality in the second wave (September-December) was positively associated with mortality during the first wave. In provinces with mortality greater than 22/100,000/month during the first wave, higher mortality in the first wave was associated with a lower second wave mortality. Results were similar when the analysis was censored at October 2020, before the implementation of region-specific measures against the outbreak. Neither vaccination nor variant spread had any role during the study period.

CONCLUSIONS: These findings indicate that provinces with the most severe initial COVID-19 outbreaks, as assessed through mortality data, faced milder second waves.

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