The panel sketched out six scenarios of how the Omicron phase of the pandemic could play out.

The first three, which modelers advising the government say are the most likely, are that Omicron does show a transmission advantage over Delta and that it can partially evade the immunity conferred by vaccination or prior infection, broadening the pool of people who can be infected. Those advantages mean Omicron would ultimately outcompete Delta, just as Delta beat back earlier variants such as Alpha.

“The only way that one infection drives another to extinction is when they compete for the same resources—susceptible people,” said Matt Keeling, director of the Zeeman Institute for Systems Biology and Infectious Disease Epidemiology Research at the University of Warwick.

The difference between the three scenarios is in how severe it is in terms of how many infected people need to be hospitalized. Though scientists are optimistic that vaccines will continue to shield most people from severe illness with Omicron, the downbeat prognosis is that even if the number of admissions tied to each Omicron case is lower than with Delta, the variant’s other advantages mean the scale of the outbreak could still lead to a surge in serious illness, stretching the capacity of the state-run National Health Service to cope. Greater severity of illness would mean a deadlier wave.