Background: Vaccinated individuals infected with measles can develop nodular pneumonia. These cases can be misdiagnosed due to the absence of specific IgM and typical symptoms. An effective diagnostic tool is needed. Methods: During March 2016, adult inpatients in Yucheng People's Hospital were enrolled prospectively and included in the study. Patients were included if samples were obtained <= 14 days from the onset of fever. Measles virus was detected by RT-PCR of the oropharyngeal swab sample. Chest computed tomography scans and medical records were obtained. Oropharyngeal swabs and blood samples were collected for IgM and IgG testing, RT-PCR, and gene sequencing. Results: Sixteen patients were enrolled. Ten were found to have nodular pneumonia and were defined as the nodular group. The remaining six patients were defined as the control group. Measles-specific IgG titers in the nodular group were high (3618.3-5000 mIU/ml), while IgM titers were low (< 25 mIU/ml); IgG titers in the control group were low (241.4-2560.3 mIU/ml), while IgM titers were high (137-5000 mIU/ml). No obvious viral mutation was detected in the nodular group. Conclusions: Measles-associated nodular pneumonia was only evident in those patients with an IgG/IgM ratio > 20. In measles outbreaks, the IgG/IgM ratio may be useful to identify nodular pneumonia. (c) 2018 Published by Elsevier Ltd on behalf of International Society for Infectious Diseases.