Omicron is causing a huge wave of Covid infections in France and is now the dominant variant but Delta cases are still occurring - and at a stable rate - among the population.
Santé publique France stated in its most recent pandemic report that Omicron now accounts for 74% of Covid cases.
The proportion of Delta infections has fallen rapidly over the past month. On December 6, it was causing 98% of cases, by December 20 this had come down to approximately half, and now only a quarter of infections are linked to this variant.
However, Santé publique’s data suggests that Delta, although no longer dominant, is not disappearing.
On January 3, an estimated 42,000 Delta cases were detected in France according to extrapolated sequencing data. This figure is only slightly lower than the Delta-related case peak of 47,000, recorded on December 13, suggesting that it is continuing to infect a similar number of people.
“There is a cognitive bias because the Omicron variant is circulating so much that the number of Delta cases seems low,” epidemiologist Pascal Crépey told Franceinfo. “But actually we are still on a plateau.
“We don’t allow Omicron to spread without also allowing Delta to spread,” he added.
“Omicron is already the dominant variant, and will become even more widespread, in the same way as in South Africa and the UK, but that doesn’t mean that Delta will disappear straightaway,” said the University of Montpellier’s Mircea Sofonea.
“Delta will remain the main worry of hospital workers for some weeks yet, as patients who were infected before Omicron took over are still hospitalised,” he added.
“But the principal concern by the end of January will definitely be Omicron. A coexistence is possible, for example, if the Delta variant infects people immunised by Omicron more easily, normally if they have not had a booster vaccine dose.
However, “the advantage of one variant over another varies over time,” affected by the progress of vaccination campaigns and the decline of collective immunity following vaccine doses.
“If our behaviours continue to relax, there will always be a place for Delta,” Dr Crépey said. “The population’s level of immunisation is not sufficient to break the epidemic trajectory,” as Delta can still spread among unvaccinated groups, he added.
What will happen after this wave?
Leading epidemiologist Mahmoud Zureik has suggested that there are three possible outcomes to the current Covid wave. It could be our last significant Covid surge, although it is much more probable that the virus could enter the ranks of seasonal illnesses and re-emerge to different extents each year. Finally, it is also possible that once this Omicron wave has passed, Delta could take over once again as the dominant variant.
“The Delta variant has been there since last spring, and is still active,” Prof Zureik said. “This variant is relatively stable, and its mutations are not very significant, unlike Omicron, which is still unstable.
“In other words, Omicron has won the battle, as it is much more contagious, but it may not have won the war against Delta.”
When can we say that one variant has definitively replaced another?
There is no official threshold after which we can say that one Covid variant has replaced another.
“In theory, it is the extinction of the preceding variant, but we never know when this moment occurs,” said Prof Sofonea.
“Some might say that the variant has been replaced if it stabilises below 5% of the number of cases reported.
“Sometimes, this can take a long time, without having a significant effect on the epidemic.”