Chinese researchers found that four patients would need to be treated with oseltamivir (Tamiflu) to prevent one patient developing pneumonia.
Treatment started within two days of symptom onset also reduced duration of fever and viral RNA shedding, the contagious period of infection.
The retrospective study examined data from 1,291 patients with confirmed 2009 H1N1 infection.
Over three quarters (983) of these patients were treated with oseltamivir at a median of three days after onset of symptoms.
There were 75 cases of pneumonia among 308 patients who did not receive antiviral treatment, or one in four patients.
This compared to just 35 cases among 647 patients who received oseltamivir, or one in 20 patients.
They also noted that 2009 H1N1 is shed for longer than seasonal influenza virus.
Authors concluded: ‘Oseltamivir treatment was identified as a significant protective factor against subsequent development of radiographic pneumonia, faster fever clearance times, and shorter viral RNA shedding.’
However, researchers said the retrospective nature of the study and the fact that not all patients underwent radiography means the results should be treated with caution.