Because no one had ever studied SARS-CoV-2 before, scientists came with their own assumptions about how it would adapt. Many were guided by experiences with another RNA virus that causes respiratory infections: influenza. “We just didn’t have much information about other respiratory viruses that could cause pandemics,” says Hodcroft.

Influenza spreads mainly through the acquisition of mutations that allow it to evade people’s immunity. Because no one had ever been infected with SARS-CoV-2 before 2019, many scientists didn’t expect to see much viral change until after there was substantial pressure placed on it by people’s immune systems, either through infections or better yet, vaccination.

The emergence of faster-transmitting, deadlier variants of SARS-CoV-2, such as Alpha and Delta, obliterated some early assumptions. Even by early 2020, SARS-CoV-2 had picked up a single amino-acid change that substantially boosted its spread. Many others would follow.