Fatality rate of the sixth wave of epidemic centered on new coronavirus Omicron strain in Japan

Comparison with the fifth wave of epidemic centered on Delta strain

Abstract

Objective: To calculate the fatality rate of the new coronavirus wave 6 Omicron strain positive persons aged 40 years or older in Japan and compare this with the wave 5 Delta strain positive persons.

Method: In January 2022, 21,821 people over the age of 40 who were diagnosed with a new type of coronavirus infection (COVID-19) in 7 prefectures, 3 core cities, 3 public health centers and notified were selected based on the mutation epidemic situation in Japan at that time. They were considered to be Omicron strain positive and used as subjects. A passive follow-up method based on the death notification based on the Infectious Diseases Control Law was used to ascertain the fact of death. 16,320 people diagnosed with COVID-19 in August-September 2021 were considered to be Delta strain positive due to the mutation epidemic situation in Japan at that time, and compared with the fatality rate calculated by the same method.

Result: The 30-day fatality rates for Omicron strain positive patients were 0.026% in their 40s (95% confidence interval: 0.00% to 0.061%), 0.021% in their 50s (0.00% to 0.061%), 0.14% in their 60s (0.00% to 0.27%), 0.74% in their 70s (0.37% to 1.12%), 2.77% in their 80s (1.84% to 3.70%), and 5.18% in their 90s or older (3.38% to 6.99%). The age group ratios to the fatality rate of the Delta strain in age 40s to 80s were 0.21, 0.079, 0.18, 0.36 and 0.49, which were significantly lower than unity. The age-adjusted fatality rate ratio between positives with Omicron and Delta variant was 0.42(95% confidence interval:0.40-0.45), based on the Japanese population in 2020, showing significantly lower rate in Omicron variant.

Conclusion: The fatality rate of COVID-19 wave 6 Omicron strain-positive people aged 50 to 90 years in Japan was significantly lower than that of wave 5 Delta strain-positive people.