1. How is this virus different?
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.
Read more: Why ‘Silent Spreaders’ Make Coronavirus Hard to Beat: QuickTake
2. Why wasn’t it contained?
On Jan. 23, China imposed the most extensive quarantine in known history in Hubei province, where the outbreak began in the capital Wuhan, an industrial city of 11 million. By then, however, the virus had been seeded in other places. Starting in early February, many countries introduced travel bans but not before the virus had reached around the world. Governments issued stay-at-home orders and mandated “social distancing” to “flatten the curve” of new infections. But the economic toll propelled reopenings that in many places brought a surge in new cases, sometimes followed by new movement restrictions. It took months for some governments to recommend or mandate that people wear masks in public to counter the virus’s silent spread, and many still haven’t done so.
Read more: Mask or No Mask? And Which Kind? What the Experts Say: QuickTake
3. What’s the biggest mystery about the virus?
One major question is whether those who get the coronavirus emerge with immunity. Generally speaking, infections prompt the body to develop antibodies that protect against reinfection, although there are notable exceptions such as HIV and malaria. By mid-year a slew of antibody test kits were available, including some that could be taken at home. Many tests, however, weren’t reliable. What’s more, researchers still don’t know whether the presence of antibodies means someone has immunity, or how long that protection might last. With the main coronaviruses that cause the common cold, immunity generally doesn’t last very long. In Hong Kong, a man tested positive for the coronavirus in late August after recovering from a different strain in April, in what scientists said was the first documented case of reinfection. The man had no symptoms the second time, however, suggesting his immune system provided some protection, according to doctors. Dozens of reinfections have since been reported, according to a tracker maintained by