Japanese researchers say the country's COVID-19 vaccination program has greatly helped reduce the numbers of infections and fatalities.

A Kyoto University research group led by Professor Nishiura Hiroshi published their evaluation in the international science journal, Scientific Reports.

Using a mathematical model, the researchers simulated the numbers of coronavirus infections and deaths in the absence of vaccination for a period of 10 months from February 2021, when the inoculation began.

The simulation was based on the assumption that the actual number of infections, including asymptomatic cases, was four times greater than confirmed cases.

The researchers also considered people's movements, the characteristics of coronavirus variants, and other factors to draw up a transmission model.

The researchers say that, in the absence of vaccination, the cumulative numbers of infections and deaths during the study period would have been 63.3 million and 364,000, respectively.

The actual numbers of infections and the number of deaths were, respectively, 4.7 million and 10,000.

They say the high effectiveness of the vaccines was likely due to herd immunity achieved by the vaccination rollout as it can protect unvaccinated people from the virus.

Professor Nishiura says that using this method, authorities can implement a vaccination program based on real-time observations of vaccines' effectiveness.

Inaba Hisashi, a project professor at the Tokyo Gakugei University, says it is impossible to actually observe how the infection situation would have been without the vaccines.

But he says the simulation using a mathematical model suggests that the vaccination program had significant, indirect effects.

Inaba says there are no established methods to assess the actual effectiveness of vaccination at population level. He says researchers need to continue to explore whether there are any other factors they should take into consideration.