Photo/Illutration An electron microscope image of the Omicron variant of the novel coronavirus (Provided by the National Institute of Infectious Diseases)

The highly transmissible Omicron variant of the novel coronavirus, even though its symptoms are considered less severe than the Delta strain, is proving to have a higher fatality rate than seasonal influenza, a preliminary analysis suggests.

The finding was submitted to a March 2 meeting of a health ministry advisory board by Hitoshi Oshitani, a professor of virology at Tohoku University in Sendai, Miyagi Prefecture.

Thirteen other experts affixed their signatures to the report, including Shigeru Omi, chairman of the government’s expert panel on the COVID-19 pandemic, and Takaji Wakita, who chairs the advisory board and is head of the National Institute of Infectious Diseases. 

They calculated the fatality rate of COVID-19 during January and February of this year when the Omicron variant was particularly virulent and came up with a figure of 0.13 percent based on reported deaths.

Multiple methods were used to calculate the fatality rates of seasonal flu. They used an index called "excess mortality," which refers to the number of deaths from all causes above the norm during a particular period. They also relied on patients’ information, or "receipt data."

The fatality rates of seasonal influenza based on excess mortality figures in 2018 and 2019 ranged from 0.01 to 0.05 percent. The fatality rate of seasonal influenza based on receipt data during the same period came to 0.09 percent.

They also calculated the fatality rate from COVID-19 based on excess mortality data from January 2020 to October 2021, before the Omicron variant took hold and began to spread. The figure they came up with was 4.25 percent.

However, the experts stressed it is difficult to accurately compare fatality rates of the novel coronavirus and seasonal influenza. This is because data on the number of people with flu and patients infected with the novel coronavirus are collected differently.

Every hospital in Japan is required to notify the government of each case of novel coronavirus infection, including asymptomatic ones, whereas only around 5,000 hospitals are asked to do so with regard to the number of seasonal influenza patients.

Another problem is that patients who die after catching the novel coronavirus are listed as COVID-19 fatalities even when the direct cause of death is not COVID-19.

Nonetheless, the preliminary analysis could prove important in discussions on the need to implement pre-emergency measures, which limit people’s movements and behavior, and other steps to contain the public health crisis.

One of the requirements under government guidelines to declare pre-emergency curbs is that the rate of occurrence of serious COVID-19 symptoms, such as pneumonia, is significantly higher than that of seasonal influenza.

During the meeting, Hiroshi Nishiura, a professor of environmental hygienics at Kyoto University, presented an estimate on the spread of the BA.2 Omicron subvariant, which is believed to be even more contagious.

He estimated that 74 percent of new coronavirus patients in Tokyo will be infected with the BA.2 strain by April 1.

Nishiura based his figure on the effective reproduction number in Denmark, where BA.2 is currently spreading, as well as the number of BA.2 cases detected in Tokyo in February. The panel urged caution about the subvariant, even though it said BA.2 doesn’t appear to be taking over the first known variant of Omicron, named BA.1.

The panel also noted that new cases of the novel coronavirus are only gradually decreasing, despite the number of fresh cases in the week just passed reaching 364.29 per 100,000 population, or 84 percent of the figure in the previous week.

It is calling on the public to be extra vigilant in taking precautions against infection ahead of a period when many seasonal events are scheduled.

(This article was written by Kai Ichino and Yuki Edamatsu.)