In December 2019, a cluster of cases of acute respiratory illness, novel coronavirusinfected pneumonia, occurred in Wuhan, Hubei Province, China. The false-negative nasopharyngeal swabs of SARS-CoV-2 caused the delayed diagnosis of COVID-19 which hindered the prevention and control of the pandemic. The transmission risk of SARS-CoV-2 in negative nasopharyngeal swabs cases were little addressed previously. This study evaluated two clusters of COVID-19 in six patients. Four of six (66.7%) showed negative RNA of SARS-CoV-2 by nasopharyngeal swabs. All epidemiological, clinical and laboratory information was collected. The first cluster was a nosocomial infection of four health care providers at early January. One of them made sequential familial cluster of infection. All patients received either selfquarantined at home or were admitted to hospital for isolated treatment. All recovered and had anti-SARS-CoV-2 IgG and/or IgM positive (100%) for serological detection of SARS-CoV-2 at recovery stage. Our study provides a cautionary warning that negative results of nasopharyngeal swabs of suspected SARS-CoV-2 infection can increase the risk of nosocomial infection among health care providers. Serologic detection for anti-SARS-CoV-2 IgG and/or IgM is an important test in the assistant diagnosis of COVID-19.