The company said it has enrolled children aged six months to 24 months in the phase 1 study to assess the appropriate dose of its vaccine designed to help prevent respiratory syncytial virus (RSV) and parainfluenza virus type 3 infections (PIV-3).

The two viruses are among the major causes of lower respiratory tract infections and hospitalization in infants and young children, MedImmune said. A previous study of the vaccine in adults and non-susceptible older children showed the treatment is safe and well tolerated, the company said.

RSV is to blame for about 125,000 infant hospitalizations each year, while PIV-3 is second behind RSV as the top cause of viral pneumonia and bronchiolitis in young children and infants, leading to more than 3 million doctor visits and more than 30,000 U.S. hospitalizations annually, according to MedImmune.

The drug maker said it is also working with the National Institutes of Health to develop intranasal vaccines to prevent PIV types 1, 2 and human metapneumovirus.