Study compares excess mortality in Massachusetts during the Delta and initial Omicron periods

By Tarun Sai LomteReviewed by Danielle Ellis, B.Sc.May 24 2022

In a recent study published in the Journal of 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 severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has resulted in excess mortality, i.e., more deaths than expected over a given time.

The study and findings

In the current study, researchers compared excess deaths in Massachusetts, a state with a high vaccination rate, during the Delta wave and the early Omicron period. They applied autoregressive integrated moving average (ARIMA) models to the United States (US) census populations (2014 – 2019). Seasonal ARIMA (sARIMA) models were applied to all-cause mortality statistics from the Health Department of Massachusetts between January 5, 2015, and February 8, 2020.

The researchers projected age-stratified weekly population and expected deaths in the state from February 9, 2020, to February 20, 2022, with a particular emphasis on the Delta wave (June 28, 2021 - December 5, 2021), Delta-Omicron transition period (December 6 – 26, 2021), and early Omicron period (27 December 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 approximate population of the state is 6.9 million people, and the expected deaths for the decreased population due to pandemic-related excess deaths were corrected. The 95% confidence intervals (CIs) for expected deaths were calculated using population covariates. The authors defined excess deaths as the difference between observed deaths and the point-estimate of expected deaths determined by sARIMA models.

The researchers noted that 1975 excess deaths occurred during the Delta period, i.e., 27,265 deaths were recorded for 25,290 expected deaths. During the Omicron period, excess deaths were 2294 (observed: 12,231; expected: 9937) and 753 (observed: 4242; expected: 3489) during the Delta-Omicron transition period.

In terms of excess death, 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. When stratified by age, excess deaths were statistically significant across all adults. The observed-to-expected excess mortality ratio was higher in adults during the Omicron wave than in the Delta period.

Conclusions

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 higher excess deaths numerically, all adult age groups had excess mortality. It has been reported that infection with SARS-CoV-2 Omicron generally results in milder disease. Assuming the case, the observed findings might reflect a higher mortality product in Massachusetts, i.e., 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 illness, can still lead to substantial excess mortality, even in a highly vaccinated population.”

The use of preliminary data might limit the study findings, albeit reporting of deaths was almost 99% complete in the state during the study period. A minority of fatalities during the Omicron period might have been due to infection with the Delta variant. Nonetheless, these findings indicated that the highly contagious SARS-CoV-2 variants like Omicron could lead to excess deaths, including in populations with high and increasing vaccination rates.