Once a proportion of individuals in a population become immune due to prior infection or vaccination, epidemiologists use the term effective reproduction number, called Re or Rt, to measure the transmissibility of the virus. The Re of the omicron variant has been estimated to be 2.5 times higher than the delta variant. This increased transmissibility most likely helped omicron out-compete delta to become the dominant variant.
The larger question, then, is what is driving the evolution of omicron sublineages? The answer to that is a well-known process called natural selection. Natural selection is an evolutionary process where traits that give a species a reproductive advantage continue to be passed down to the next generation, while traits that don’t are phased out through competition. As SARS-CoV-2 continues to circulate, natural selection will favor mutations that give the virus the greatest survival advantage.
BA.5 will not be the end game. As the virus continues to circulate, this evolutionary trend will likely lead to the emergence of more transmissible variants that are capable of immune escape.
While it is difficult to predict what variants will arrive next, we researchers cannot rule out the possibility that some of these variants could lead to increased disease severity and higher hospitalization rates.