Bq

But those with the newer monovalent vaccine had 3.5x higher levels of antibodies against the XBB and BQ.1.1 strains after their booster vaccination.

On Friday, Tepco said seawater samples taken on Thursday afternoon showed radioactivity levels were well within safe limits, with a tritium concentration below 1,500 bq/L.

"We confirmed that the analysed value is equal to the calculated concentration and that the analysed value is below 1,500 bq/L," Tepco spokesman Keisuke Matsuo told a news conference.

Becquerels per litre - or bq/L - is a measure of radioactivity.

Ever since the COVID-19 pandemic began, there have been a long list of variants with confusing, jumbled names — from BA.5 to BQ.1 to XBB — which can feel like a bit of an alphabet soup.