The government says rapid declines in the efficacy of the third COVID-19 vaccine shots have prompted its decision to quickly launch a fourth round of vaccinations. 

The state task force on COVID-19 vaccination relayed the stance on Monday, saying the effectiveness of the first booster shots in nursing home patients aged 60 and older dissipated by half or more 12 weeks after their inoculation. 

According to authorities, the third vaccine increased neutralizing antibody titers until the ninth or tenth week, but they fell to half their initial levels from the 12th week for the delta variant and even to one-third of initial levels for the omicron strain. 

They echoed similar studies in the United Kingdom, which showed that the effectiveness of the third vaccine doses in preventing the omicron began falling three months later and plummeted to 20 to 40 percent of initial levels 15 weeks later. 

On Monday, the government announced a decision to administer a fourth vaccination for some one-point-eight million people who are either nursing home patients or workers or those with weak immune systems. 

They are the two highest-risk groups categorized by health authorities, who view those aged 75 and older and those aged 60 to 74 as the next vulnerable groups to the pandemic.