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The US Centers for Disease Control and Prevention estimates that the coronavirus subvariant JN.1 is now causing about 20% of new Covid-19 infections in this country, and it’s the fastest-growing strain of the virus. It’s already dominant in the Northeast, where it is estimated to cause about a third of new infections.

JN.1 is descended from BA.2.86, or Pirola, a subvariant that came to the world’s attention over the summer because of the large number of changes to its spike proteins: more than 30. Scientists feared that it was so mutated that it would completely escape the protection of vaccines and antibodies against Covid-19, perhaps sparking another tidal wave of illness the way the original Omicron variant did in 2021.

That never happened, but BA.2.86 hung around, growing very slowly in some countries, including the US. Some studies suggested that it never really took off because it may have lost some of its ability to infect our cells.

Enter JN.1, which is two generations removed – a granddaughter, so to speak – of BA.2.86. JN.1 has only one change to its spike protein compared with its ancestor, but that seems to have been enough to make it a fitter and faster virus.