The Omicron BA.4 and BA.5 subvariants of SARS-CoV-2 have proven to be stealthier at evading people’s immune defences than all of their predecessors.

But recent research shows that previous infection with an older variant (such as Alpha, Beta or Delta) offers some protection against reinfection with BA.4 or BA.5, and that a prior Omicron infection is substantially more effective. That was the conclusion of a study that evaluated all of Qatar’s COVID-19 cases since the wave of BA.4 and BA.5 infections began.

The work, which was posted on the medRxiv preprint server on 12 July and has not yet been peer reviewed, feeds into broader research on “how different immunities combine with each other”, says study co-author Laith Abu-Raddad, an infectious-disease epidemiologist at Weill Cornell Medicine-Qatar in Doha.