In December 2019, an outbreak of pneumonia caused by a new type of coronavirus (SARS-CoV-2) infection was reported from Wuhan, Hubei Province, China, and the infection later came to be named as COVID-19. The virus spread rapidly around the world, ultimately infecting more than 200 million people, including more than one million people in Japan. Analysis of the data of accumulated cases showed that older people and people with underlying diseases tended to have more severe disease, providing some insight into the pathogenesis of COVID-19. With the widespread vaccination drive against COVID-19 starting from February 2021, the number of severely affected older people declined in Japan. However, emergence of the δ variant strain and issues with vaccine penetration in young people caused the number of infections and severe illness to increase in people aged 60 years or under. As of October 2021, however, the number of new cases was trending downward. Screening by antigen testing, polymerase chain reaction (PCR), and chest computed tomography (CT) are the main diagnostic methods for COVID-19. It is important to recognize that mild interstitial pneumonia-like opacities could appear on the chest CT even in younger patients. In this report, we describe a non-COVID-19-vaccinated patient admitted with asthma, who had tested negative twice for SARS-CoV-2 by PCR examination of a saliva specimen. Later, after visiting our hospital, the patient tested positive for COVID-19 antibodies.