Study Compares Excess Mortality in Massachusetts During Delta and Initial Omicron Periods
In a recent study published in the Journal of the American Medical Association (JAMA)researchers estimated excess mortality in Massachusetts during the coronavirus disease 2019 (COVID-19) Delta and early Omicron waves.
The COVID-19 pandemic caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has resulted in excess mortality, i.e. more deaths than expected over a given period.
The study and the conclusions
In the current study, researchers compared excess deaths in Massachusetts, a high-vaccination state, during the Delta wave and the start of the Omicron period. They applied autoregressive integrated moving average (ARIMA) models to United States census populations (2014-2019). Seasonal ARIMA (sARIMA) models were applied to Massachusetts Department of Health all-cause mortality statistics between January 5, 2015 and February 8, 2020.
The researchers projected the age-stratified weekly population and expected deaths in the state from February 9, 2020 through February 20, 2022, with particular emphasis on the Delta wave (June 28, 2021 – December 5, 2021), the Delta-Omicron transition period (December 6 – 26, 2021) and the start of the Omicron period (December 27, 2021 – February 20, 2022). The boundaries of these periods were defined based on the dominance of the SARS-CoV-2 variant in regional wastewater.
The state’s approximate population is 6.9 million, and the expected deaths for population decline due to pandemic-related excess mortality have been corrected. 95% confidence intervals (CI) for expected deaths were calculated using population covariates. The authors defined excess mortality as the difference between observed deaths and the point estimate of expected deaths determined by the sARIMA models.
The researchers noted that 1,975 excess deaths occurred during the Delta period, or 27,265 deaths were recorded for 25,290 expected deaths. During the Omicron period, excess deaths were 2,294 (observed: 12,231; expected: 9,937) and 753 (observed: 4,242; expected: 3,489) during the Delta-Omicron transition period.
In terms of excess deaths, we found that Omicron was actually much worse for Massachusetts than Delta.
The SARS-CoV-2 Omicron to Delta incident rate ratio for excess deaths was 3.34 per week. Stratified by age, excess deaths were statistically significant for all adults. The observed/expected excess mortality ratio was higher in adults during the Omicron wave than during the Delta period.
The authors noted that excess all-cause mortality during the eight-week Omicron period was higher in Massachusetts than during the 23 weeks of the Delta wave. Although older age groups had numerically higher excess mortality, all adult age groups had excess mortality. It has been reported that SARS-CoV-2 Omicron infection usually results in milder disease. Assuming the case, the observed results could reflect a higher mortality product in Massachusetts, i.e. a lower infection fatality rate (IFR) multiplied by a higher rate of COVID-19 infection. .
This could mean that highly contagious variants, even if they cause relatively milder disease, can still cause substantial excess mortality, even in a highly vaccinated population.
Use of preliminary data could limit study results, although death reporting was nearly 99% complete in the state during the study period. A minority of deaths during the Omicron period may have been due to Delta variant infection. Nonetheless, these findings indicated that highly contagious SARS-CoV-2 variants like Omicron could lead to excess deaths, including in populations with high and increasing vaccination rates.