In December 2019, a cluster of cases of acute respiratory illness, novel coronavirus-infected pneumonia, occurred in Wuhan, Hubei Province, China. The false-negative nasopharyngeal swabs for SARS-CoV-2 caused 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 has rarely been addressed previously. This study evaluated two clusters of COVID-19 in six patients, four of whom (66.7%) tested negative for RNA of SARS-CoV-2 on RT-PCR of nasopharyngeal swabs. All epidemiological, clinical, and laboratory data were collected. The first cluster was a nosocomial infection of four health care providers in early January. One case resulted in a sequential familial cluster of infection. All patients either self-quarantined at home or were admitted to hospital for isolated treatment. All recovered and were anti-SARS-CoV-2 IgG- and/or IgM-positive (100%) for serological detection of SARS-CoV-2 at the recovery stage. Our study provides a cautionary warning that negative results for 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 diagnosis of COVID-19.