At least 41 cases of the variant have been detected in England.
The concern is that a K417N mutation may be more resistant to jabs already rolled out to tens of millions of people in the UK.
Public health chiefs have put in place “enhanced” contact tracing, fast testing and isolation measures for areas where Delta plus infections are identified.
The Prime Minister’s official spokesman said: “Public Health England set out last week the small number of cases, I think it’s 41 in total, that have been identified in England.
“PHE have said additional measures are already in place where cases of this variant are detected, that includes enhanced contact tracing, rapid testing and isolation.
“As other countries are, PHE is working to better understand the significance of the K417N in relation to Delta.”
Asked if it was known how effective vaccines were against the Delta plus mutation, he added: “Not at this stage.
“We have not got trial results of that.
“It’s something that we are monitoring carefully and there is work going on both here and internationally on this.
“Obviously we know that double vaccination has been proved to be effective against all existing variants that have been sequenced.”