An omicron subvariant appears to be even more contagious than the original fast-spreading strain, U.K. health authorities said, though vaccine booster shots remain an effective shield. 

Data from contact tracing showed the subvariant, BA.2, spread more frequently in households, the U.K. Health Security Agency said on Friday. Its rate of transmission among household contacts was 13.4%, compared to 10.3% for omicron. Though the data is a good indication of how transmissible the subvariant is, the agency cautioned that it's preliminary and could fluctuate. 

Covid vaccines, in particular booster doses, were just as effective against BA.2, the agency said. By 25 weeks or more after the second dose, vaccines blocked 13% of cases -- a rate that rose to 70% two weeks after a booster.