Japan's National Center for Global Health and Medicine says an increasing percentage of people are dying after developing moderate symptoms of COVID-19. It also says many of them had pre-existing conditions that worsened after they became infected.

The center analyzed the data of 2,861 COVID-19 patients who died after being admitted to hospitals across the country through late August.

It says 42 percent of those who died in the fifth wave in the summer of last year had developed serious symptoms. But the figure fell to 13 percent in the sixth wave that began earlier this year.

Only five percent of those who died in the seventh wave this summer had severe symptoms.

On the other hand, 57 percent of those who died in the fifth wave had moderate symptoms. The figure jumped to 83 percent in the sixth wave and 89 percent in the seventh wave.

The center says fewer COVID-19 patients have been developing serious pneumonia in Japan as the vaccination rate increases, while more have died because the coronavirus causes their pre-existing conditions to worsen.

Ohmagari Norio, the director of the Disease Control and Prevention Center, was in charge of the analysis.

He says the study shows elderly people with chronic diseases often weaken and die after the virus starts to affect their organs, even if they only have moderate symptoms.

He says that although some say the coronavirus is no longer life-threatening, most patients would not have died if they had not become infected.

Ohmagari says people should continue to take measures to prevent infection and get fully vaccinated to prevent them from developing serious symptoms.