The mystery of why Covid-19 seems to be becoming milder
When virologists took their first peek at XEC, the Covid-19 variant which started to become dominant in the autumn of 2024, the early signs were ominous.
The latest descendant of the Omicron variant of SARS-CoV-2, XEC had arisen through recombination, a process where two other variants had forged their genetic material together. Tests seemed to indicate that this would easily allow it to evade the immune protection offered by past infections or the latest iterations of the Covid-19 vaccines, based on the older JN.1 and KP.2 variants.
"The spike protein is quite different from previous variants, so it was quite easy to assume that XEC has the potential to evade immunity induced by JN.1 infection," says Kei Sato, a virology professor at the University of Tokyo, who carried out one of the first studies of XEC, published in December 2024.
In the US, infectious disease specialists braced themselves for an immediate surge in hospitalisations in the wake of the Thanksgiving holiday weekend. But it didn't happen. Surveillance testing carried out by measuring Covid in wastewater samples across major cities indicated that XEC was definitely infecting people. However, the numbers of people actually ending up in hospital was considerably less than previous winters. According to CDC data, the rate of hospitalisations at the start of Dec 2023 was 6.1 per 100,000 people. During the equivalent week in December 2024, that had fallen to two per 100,000 people.
