https://www.nejm.org/doi/full/10.1056/NEJMc2001468 A woman from Shanghai traveled to Germany for a business trip between January 19 and January 22. She had no symptoms of coronavirus during this time and only became sick on her flight back to China. She was diagnosed with COVID-19 on January 26. But on January 24, a businessman who met with the woman on January 20 and 21 developed symptoms of coronavirus. He went back to work on January 27 and later tested positive for COVID-19. “We discovered that shedding of potentially infectious virus may occur in persons who have no fever and no signs or only minor signs of infection,” the report’s authors concluded.
https://www.nejm.org/doi/full/10.1056/NEJMc2001899 two individuals who had recently returned to Germany from Wuhan, China, appeared to be asymptomatic, but tested positive for COVID-19 when they were given a throat swab. "We discovered that shedding of potentially infectious virus may occur in persons who have no fever and no signs or only minor signs of infection,"
https://science.sciencemag.org/content/early/2020/03/13/scie... A majority of cases in China were transmitted by “undocumented infections” in the period before the country placed travel restrictions on Hubei province on January 23. “This high proportion of undocumented infections, many of whom were likely not severely symptomatic, appears to have facilitated the rapid spread of the virus throughout China"
https://www.medrxiv.org/content/10.1101/2020.03.05.20031815v... a significant number of infections can be attributed to people who had not yet developed symptoms
https://www.nejm.org/doi/full/10.1056/NEJMp2003762 Bill Gates "COVID-19 is transmitted quite efficiently," he wrote. "There is also strong evidence that it can be transmitted by people who are just mildly ill or even pre-symptomatic. That means COVID-19 will be much harder to contain than the Middle East respiratory syndrome or severe acute respiratory syndrome (SARS), which were spread much less efficiently and only by symptomatic people."