THROUGHOUT the pandemic, SARS-CoV-2 has proved full of surprises, most of them nasty. Initially regarded as a respiratory virus, we now know it infects other organ systems, and can linger for months. It disproportionally kills people from poor and ethnic minority backgrounds and also men, for reasons that still aren’t fully understood. It doesn’t seem to be suppressed by warm weather or climates.

But the latest surprise is a nice one. Initial fears that the virus would fail to raise immune memory – the lengthy, sometimes lifelong, protection that we get from exposure to many viruses …