Background “Pneumonia Prevention No.1” belongs to ‘traditional Chinese medicine prescription for prevention of viral pneumonia and influenza’ was urgently formulated by Notice on Printing the Novel Coronavirus Diagnosis and Treatment Scheme for COVID-19 (Trial Version 3) and Traditional Chinese Medicine Prevention and Treatment Scheme for COVID-19 in Hubei Province (Trial). Because the prescription drug has the bidirectional regulation function of human immune function, moderate improvement of immune function can effectively resist virus invasion, while excessive immune function will produce immune overresponse. Excessive immune response will aggravate the condition of patients with COVID-19, resulting in the death of severe patients. Methods Twenty medical workers aged 20–60 years old, who had no immune disease, no current disease and healthy physical examination, were selected as participants. The participants took Hubei “Pneumonia Prevention No.1” decoction, one dosage each day, twice a day, for 7 consecutive days. With the before-after control method, blood samples were collected from the median cubital veins before and after medication. Immunoglobulin IgA, IgG and IgM were measured by immunoturbidimetry, and T lymphocyte subsets CD3, CD4, CD8 and CD4/CD8 were measured by flow cytometry. The changes of indexes before and after medication were compared with SPPS 13.0 statistical software. The data were expressed by (mean ± standard deviation). T -test was adopted, and P < 0.05 was considered statistically significant (P < 0.05). Results The results of this study show that in healthy participants, the immunoglobulin and T lymphocyte subsets did not differ significantly before and after drug administration (P > 0.05). Conclusion Under normal drug administration circumstances, “Pneumonia Prevention No. 1” had no significant regulating effect on the immune system in a healthy population and did not increase the immune system capacity beyond a reasonable range. It is safe to be used as a prophylactic measure in healthy populations.