It’s taken scientists time to understand that the novel coronavirus that emerged in late 2019 in central China behaves differently than similar pathogens, producing missteps in the response. The list of known symptoms of Covid-19, the disease it causes, also has grown, complicating efforts to diagnose and treat it. And the virus has yet to give up all its mysteries, creating uncertainty about when the pandemic might end.
Unlike the coronavirus responsible for the 2002-2003 outbreak in Asia of severe acute respiratory syndrome, or SARS, this new one can spread via people who are infected but have yet to develop symptoms, or don’t at all. The U.S. Centers for Disease Control and Prevention estimates that 40% to 45% of SARS-CoV-2 infections occur without symptoms. A study by researchers at the Yale School of Public Health’s Center for Infectious Disease Modeling and Analysis found “silent” transmitters are responsible for more than half of the cases in Covid-19 outbreaks. What’s more, the new virus has a relatively long incubation period -- the time between infection and the appearance of symptoms -- enabling it to spread silently in a community before being detected. The interval is about five to six days compared with two days for the flu, which spreads the same way and is the most common cause of pandemics. The stealthy nature of the coronavirus wasn’t well understood at first and contributed to the staggered and uneven quality of the response.