Cumulative Excess Deaths in New Zealand in the COVID-19 Era: Biases from Ignoring Changes in Population Growth Rates

Accurate data on health and economic outcomes are needed to evaluate policy responses to COVID-19. A potentially comprehensive health indicator is excess deaths, which shows the gap between all-cause deaths and deaths to be expected under normal circumstances. New Zealand's public health community has seized upon an excess deaths series that seemingly shows negative cumulative excess mortality in the first three years of COVID-19 - in other words, fewer deaths than expected. This is a flawed measure because it ignores changes in population growth. There was a rapid rise in deaths in New Zealand in the 2015-19 period, due to immigration-driven population growth rates of two percent per annum. This growth came almost to a standstill after the border closed in March 2020 so methods of extrapolating from the past to predict future deaths, to ascertain if actual deaths exceed the projection, must take account of this sharp change in population growth rates. Rather than New Zealand being unique, in having negative cumulative excess deaths in the COVID-19 era, as claimed by public health commentators, cumulative deaths are about four percent above expected deaths once population changes are accounted for. Several developed countries had better outcomes according to this indicator.