The figures are climbing
As of October 2026, circulation is rising again. Around 120 cases are reported each week in Switzerland and Liechtenstein, where most summer weeks saw between 10 and 20, according to Swiss press citing the Federal Office of Public Health. That is fewer than a year earlier: the first week of October 2025 recorded more than 500 cases. The variant XFG dominates the Swiss situation. In France, as of October 2026, Santé publique France reports active circulation of SARS-CoV-2, with a rise observed since August, a pattern already seen the previous year at the same time.
The virus changes, each descendant displaces the last
SARS-CoV-2 accumulates mutations at a rate similar to that observed in other RNA viruses, or perhaps slightly slower, according to literature reviews. Before December 2021, the count stood at 23 substitutions per year; after the arrival of Omicron, it rose to 45.
Each descendant lineage replaces its parent. The reason lies in the spike, the surface protein that binds to the cell: when its shape changes, the antibodies already produced by the population recognise it less well.
In late 2021, in the laboratory, on blood samples, Omicron reduced neutralisation by vaccine-induced antibodies by a factor of 22 compared with the original strain. That measurement, outside the body, does not measure a risk of reinfection. In 2025, the WHO assessed the immune escape of XFG as comparable to that of previous variants in the same family, based on a laboratory measurement on blood using pseudovirus; confidence remains low, because only one study exists.
Antibodies fade
The second clock runs in the blood. After vaccination, neutralising antibodies fall sharply during the first three months, then their decline slows. Six months after the second dose, the response is greatly diminished, according to an Israeli study of nearly 5,000 healthcare workers in 2021, before the arrival of Omicron.
Another Israeli study, conducted in summer 2021, compared groups vaccinated two months apart, observed over the same period: those vaccinated earlier contracted the infection more often than those vaccinated later, in every age group.
In Amsterdam, a small group followed for more than thirty-five years through blood analyses showed that reinfection by the same common-cold coronavirus, twelve months after a first infection, was frequent. These seasonal viruses are not SARS-CoV-2.
School does not set the signal
In France, a 2008 study measured that holidays reduce transmission of flu to children by 20 to 29%, and prevent 16 to 18% of seasonal flu cases. In Canada, the peak of asthma hospitalisations among schoolchildren occurs on average 17.7 days after the return to school.
No clear seasonal rhythm has been established to date for SARS-CoV-2, according to a 2023 surveillance finding. Transmission continues year-round in countries of the European Union and the European Economic Area, driven by the arrival of new variants. In France, in August 2023, emergency department visits for suspected Covid-19 were trending upward, across all ages combined, at low levels, with schools closed.
For SARS-CoV-2, surveillance shows that transmission is driven by new variants, and rises occur in summer, with schools closed. The two clocks, mutation and antibody waning, give the starting signal.