High ratio of measles-specific IgG/IgM associated with nodular pneumonia in vaccinated individuals

•Vaccinated individuals with measles present with nodular pneumonia.•Varying amounts of nodules were observed in the chest computed tomography scans of the cases.•A high ratio of measles-specific IgG/IgM was found to be associated with nodular pneumonia. 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. 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. 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 (3618.3–5000mIU/ml), while IgM titers were low (20. In measles outbreaks, the IgG/IgM ratio may be useful to identify nodular pneumonia.